I have been spending a lot of time lately writing up a formal paper on high redshift galaxies, so haven’t had much time to write here. The paper is a lot more involved than I told you so, but yeah, I did. Repeatedly. I do have a start on a post on self-interacting dark matter that I hope eventually to get back to. Today, I want to give a quick note about the MHONGOOSE survey. But first, a non-commercial interruption.
Triton Station joins Rogue Scholar
In internet news, Triton Station has joined Rogue Scholar. The blog itself hasn’t moved; Rogue Scholar is a community of science blogs. It provides some important capabilities, including full-text search, long-term archiving, DOIs, and metadata. The DOIs (Digital Object Identifiers) were of particular interest to me, as they have become the standard for identifying unique articles in regular academic journals now that these have mostly (entirely?) gone on-line. I had not envisioned ever citing this blog in a refereed journal, but a DOI makes it possible to do so. Any scientists who find a post useful are welcome to make use of this feature. I’m inclined to follow the example of JCAP and make the format volume, page be yearmonth, date (YYMM, DD), which comes out to Triton Station (2022), 2201, 03 in the standard astronomy journal format. I do not anticipate continuing to publish in the twenty second century, so no need for YYYYMM, Y2K experience notwithstanding.
For everyone interested in science, Rogue Scholar is a great place to find new blogs.
MHONGOOSE
In science news, the MHONGOOSE collaboration has released its big survey summary paper. Many survey science papers are in the pipeline. Congratulations to all involved, especially PI Erwin de Blok.
I was involved in the early phases of the MHONGOOSE project, helping to select the sample of target galaxies (it is really important to cover the full dynamic range of galaxy properties, dwarf to giant) and define the aspirational target sensitivity. HI observations often taper off below a column density of 1020 hydrogen atoms per cm2 (about 1 solar mass per square parsec). With work, one can get down to a few times 1019 cm-2. We want to go much deeper to see how much farther out the atomic gas extends. It was already known to go further out than the stars, but how far? Is there a hard edge, or just a continuous fall off?
We also hope to detect new dwarf galaxies that are low surface brightness in HI. There could, in theory, be zillions of such things lurking in all the dark matter subhalos that are predicted to exist around big galaxies. Irrespective of theory, are there HI gas-rich galaxies that are entirely devoid of stars? Do such things exist? People have been looking for them a long time, and there are now many examples of galaxies that are well over 95% gas, but there always seem to be at least a few stars associated with them. Is this always true? If we have cases that are 98, 99% gas, why not 100%? Do galaxies with gas always manage to turn at least a little of it into stars? They do have a Hubble time to work on it, so it is also a question why there is so much gas still around in these cases.
And… a lot of other things, but I don’t want to be here all day. So just a few quick highlights from the main survey paper. First, the obligatory sensitivity diagram. This shows how deep the survey reaches (lower column density) as a function of resolution (beam size). You want to see deeply and you want to resolve what you see, so ideally both of these numbers would be small. MHONGOOSE undercuts existing surveys, and is unlikely to be bettered until the full SKA comes on-line, which is still a long way off.
Sensitivity versus resolution in HI surveys.
And here are a couple of individual galaxy observations:
Optical images and the HI moment zero, one, and two maps. The moment zero map of the intensity of 21 cm radiation tells us where the atomic gas is, and how much of it there is. The moment one map is the velocity field from which we can construct a rotation curve. The second moment measures the velocity dispersion of the gas.
These are beautiful data. The spiral arms appear in the HI as well as in starlight, and continue in HI to larger radii. The outer edge of the HI disk is pretty hard; there doesn’t seem to be a lot of extra gas at low column densities extending indefinitely into the great beyond. I’m particular struck by the velocity dispersion of NGC 1566 tracking the spiral structure: this means the spiral arms have mass, and any stirring caused by star formation is localized to the spirals where much of the star formation goes on. That’s natural, but the surroundings seem relatively unperturbed: feedback is happening locally, but not globally. The velocity field of NGC 5068 has a big twist in the zero velocity contour (the thick line dividing the red receding side from the blue approaching side); this is a signature of non-circular motion, probably caused in this case by the visible bar. These are two-dimensional examples of Renzo’s rule (Sancisi’s Law), in which features in the visible mass distribution correspond to features in the kinematics.
I’ll end with a quick peak at the environments around some MHONGOOSE target galaxies:
Fields where additional galaxies (in blue) are present around the central target.
This is nifty on many levels. First, some (presumptively satellite) dwarf galaxies are detected. That in itself is a treat to me: once upon a time, Renzo Sancisi asked me to smooth the bejeepers out of the LSB galaxy data cubes to look for satellites. After much work, we found nada. Nothing. Zilch. It turns out that LSB galaxies are among the most isolated galaxy types in the universe. So that we detect some things here is gratifying, even in targets that are not LSBs.
Second, there are not a lot of new detections. The halos of big galaxies are not swimming in heretofore unseen swarms of low column density gas clouds. There can always be more at sensitivities yet unreached, but the data sure don’t encourage that perspective. MHONGOOSE is sensitive to very low mass gas clouds. The exact limit is distance-dependent, but a million solar masses of atomic gas should be readily visible. That’s a tiny amount by extragalactic standards, about one globular cluster’s worth of material. There’s just not a lot there.
Disappointing as the absence of zillions of new detections may be discovery-wise, it does teach us some important lessons. Empirically, galaxies look like island universes in gas as well as stars. There may be a few outlying galaxies, but they are not embedded in an obvious cosmic network of ephemeral cold gas. Nor are there thousands of unseen satellites/subhalos suddenly becoming visible – at least not in atomic gas. Theorists can of course imagine other things, but we observers can only measure one thing at a time, as instrumentation and telescope availability allows. This is a big step forward.
We will return to our usual programming shortly. But first, a few words on the eclipse experiencelast Monday. It. Was. Awesome.
That’s a few words, so Mission Accomplished.
That’s really all I had planned to say. However, I find I am still giddy from this momentous event, so will share my experience of the day, such as words can humbly convey.
Prelude
We had good weather here in Cleveland, with the temperature reaching the upper 60s Fahrenheit. It was cloudy in the morning and many people were concerned about the prospects to see the eclipse. I was not. Having spent a lifetime as an observer with many nights at mountaintop observatories wishing for clouds to go away, and obsessively refreshing satellite maps to try to judge when they might do so, I knew there was no point in fretting about it at this juncture. Either it would clear, or clear not.
To indulge in a little superstition, I was more concerned that the date of the eclipse coincided with the home opener for the Cleveland Guardians. Opening day is always a happy, celebratory time, with people jamming the ballpark to enjoy the return of baseball and mark the coming halcyon days of summer. The weather here on opening day inevitably seems to repay that optimism with cold, clouds, and various forms of precipitation. Opening day weather is always miserable. I cannot think of a single home opener in the past quarter century for which I wanted to be in the stadium, and quite a few for which I was grateful not to have been. In local experience, a few inches or more of snow is as likely than a nice day.
A typical April in Cleveland. This was April 21, 2021.
This is a recurring theme.
April 7, 2017, seven years and a day before the eclipse.
I could go on – I have lots of epic snow pictures from Aprils past. We got a full foot of snow one Easter. But April also brings daffodils, so not all hope is extinguished.
Given this sterling record of opaque skies and outright blizzards, many people traveled to Texas to see the eclipse. The climate statistics for clear skies there are better there than here, and indeed, better than most other places along the path of totality, making it the destination of choice for serious eclipse chasers. But weather is notoriously fickle: climate is what you expect; weather is what you get. The day of the eclipse it was cloudy in Texas. You make your bets and you roll the dice.
The Build Up
Here in Cleveland, the early clouds had cleared to a brilliant blue by noon. At this time there was a special lunch followed by a panel discussion that I served on, together with Prof. Paul Iverson, an expert on ancient culture and the Antikythera Mechanism, a remarkably advanced analog computer for accurately predicting planetary motions including eclipses, Prof. Aviva Rothman, an historian of the Scientific Revolution and Kepler in particular, and Prof. Chris Zorman, an engineer involved in conducting radio experiments on the reaction of the ionosphere to the passage of the moon’s shadow. I opened by describing what was happening physically, and closed with a description of what to expect to see. There were good questions form the audience; perhaps my favorite being if we could extend the eclipse experience by chasing it in a plane. Yes, but the shadow sweeps past at over a thousand miles per hour, so to keep up you’d have to go supersonic, so totality could only be extended for however long your fuel could last. Attendance was great but limited* to the largest ballroom in the Tinkham Veale University Center; I’m told it filled within minutes of registration being opened. Duh – I had been trying to impress the enormity of this event on the powers that be on campus for years without success. Most people seem incapable of thinking that far ahead. Still, they did eventually get on it, and did a good job organizing everything, albeit at a predictably desperate clip for the last few weeks. The campus event turned out well, if for a rather smaller audience than demand might have had it.
People gather on Freiberger Field to witness the eclipse. The little picket fence for the VIPs can be seen, with the Tinkham Veale University Center behind.
By design, the panel ended right as the partial eclipse started. Freiberger Field next door to Tinkham Veale had been designated for eclipse viewing, complete with portapotties and, bemusingly, a little fenced-off area$ for us VIPs who had been at the lunch. I preferred to watch it with my colleagues and students who had set up a small telescope with a projection screen nearby.
Students with one of the department’s portable telescopes. The partial eclipse has progressed far enough to begin to give things a sepia tone. Note the high-tech cardboard aperture reducer that Dr. Bill Janesh rigged for persistent observation of the sun, which can overheat optical elements.
Many of our students are wearing the t-shirt I designed to commemorate the occasion.
Commemorative eclipse t-shirt. The location and time is given in words and again as latitude, longitude, and Julian date. Trivia points for those who can identify the inspiration of the font in the middle lines. (Not the Case Astronomy part – I did that free hand.)
First, though, I walked back to my office to drop off the sports coat I had donned for the panel, as it had become downright warm in the sun. It would take a bit over an hour to reach totality, so I had plenty of time to walk across campus and back. It also gave me something to do besides mill about in anticipation: I looked up occasionally on the walk over, checking the progress of the moon through eclipse glasses; it was casually devouring the sun one bite at a time as I casually crossed campus.
I grabbed a bundle of eclipse glasses from my office. There were plenty at Freiberger, but we had started stocking up on them before that had been arranged, and what else were we going to use them for? As soon as I stepped outside, I encountered a couple of students who needed them. Immediately after that, a visitor from Pittsburgh who was originally form Armenia asked where he could get paper supplies to make a pinhole camera. I handed him a pair of eclipse glasses and pointed him towards the nearby FedEx, with directions on to the campus bookstore should that prove helpful. By this time, I could tell that the light was starting to dim.
On the way back, the weather worsened. Some murk started to roll in, and for a bit it looked like it might become completely opaque. But the clouds remained limited to cirrus clouds that amounted to only a thin veil, and which provided a rainbow halo completely encircling the sun. A few commercial jetliners left long, fat contrails+ whose shadows could be seen cast on the cirrus at lower elevation.
Rainbow halo around the sun in cirrus clouds. A few contrails cast shadows on the clouds. The sun is more than half obscured by the moon at this point, but my phone’s camera can only see that it is still really bright.
At this point, it started to cool off. One could viscerally feel the effect of the shade cast by the moon. The temperature dropped 7oF, then rebounded some afterwards. I did not regret having abandoned my jacket – it was still a pleasant spring day – but probably would have put it back on for a bit if I still had it with me. I could hear some mild grumbles in the crowd that they wanted one. You could definitely feel the difference as a mild breeze picked up.
The partial eclipse as totality nears, as seen projected by the small telescope seen above.
Partial Eclipses Past
Partial eclipses are just that: partial. In the 2014 my younger daughter and I went to the roof of a parking structure to watch one that reached about 30% coverage. And indeed, it looked like a clean bite had been taken out of the sun. But if you didn’t know when to look (and have appropriate eye protection), you wouldn’t notice. The sun was still plenty bright, and there was no perceptible change in the environment. Indeed, we noticed that people didn’t notice. We had come prepared with welder’s glass, and offered a glimpse to passers by. No one took us up on it. Indeed, every single one gave us a wide berth as obviously crazy people.
On 21 August 2017, there was a major eclipse for which the path of totality passed several hundred miles to our south. We saw about 80% coverage in Cleveland on that occasion. I figured that people who were serious about it would have left town to see it. However, there had been a lot of hype about this eclipse, so I expected that, come the day of, a lot of folks would be calling up us astronomers asking what’s up.
In anticipation, we (the CWRU Department of Astronomy) had stocked up on eclipse glasses. The day of, we sallied forth to entertain those who found a sudden interest in astronomical events. This included many people from both on and off campus, but especially new freshmen – it happened during Discovery Week, which is freshmen orientation here. On that occasion, I had difficult persuading the people running orientation that they needed to account for the eclipse in their scheduling. They were having none of it: they had a very busy schedule, it was important the the freshmen attend all orientation events, and if we wanted to host an astronomical event, we should schedule it sometime else. Eventually I had to appeal to the provost, pointing out that students would have heard of the eclipse, so were likely to walk out of whatever orientation event was running at the time, so it would be better to embrace the event than pretend it wasn’t happening. So we did.
Dr. Paul Harding (left) and Prof. Chris Mihos (right) help the gathered crowd witness the partial eclipse of 2017. Not pictured: Charley Knox, who had opened the 9″ refracting telescope bequeathed to us by Warner & Swasey to visitors. Perched atop a campus building, a line to use it promptly formeddown five flights of stairs and out onto the quad.
The weather in August 2017 was clear and hot. While my colleagues operated the telescopes, I ran around handing out eclipse glasses and playing carnival barker. This included announcing the time of maximum coverage, which this time was enough to cause a perceptible dimming. It was weird – it wasn’t like a cloud blocking the sun; indeed, the sky was completely clear. Everything just seemed… tuned down. Nature stilled. The light gave everything a sepia tone; sort of a golden hour from above rather than from the horizon.
At this point, anything with a small hole acted as a pinhole camera to project an image of the partial eclipse. A colander works quite well for this. Heck, even the leaves of the trees got into it.
Images of the 2017 partial eclipse cast by the leaves of the trees acting as pinhole cameras.
We were lucky with the weather. We were hot and dehydrated, but we got through it all. After we had packed up but before I could even walk back to the department, storm clouds gathered and the heavens opened with torrential rain: a classic summer thunderstorm. I was happy to wait it out in Tinkham Veale, quite exhausted. I realized then that we couldn’t pull off a similar event for a full eclipse, which would have exponentially more interest. The partial eclipse was all we could manage, and the department is half the size now that it was then#.
Totality Approaches
The light level at the maximum of the 2017 partial eclipse returns us to the 2024 eclipse. We had reached a point that was uncharted territory for me. With some help from a filter, the phone camera could now kinda sorta make out that something was happening.
As totality neared, even a phone camera could discern that the sun was no longer round.
The light obtained the same weird, bright-yet-dim sepia tone I recalled from 2017. It continued to darken, and began to look like sunset on the horizon, only all 360o around. Then the umbral shadow swept in, the cirrus clouds above marking its path. We were in a giant dark shadow, with daylight perceptible at a distance all around us. But for us, it got dark.
I watched the last limb of the sun disappear behind the moon through the eclipse shades, the thin horns of light contracting rapidly. It broke into segments, atmospheric seeing warring with lunar topography. When I could see no more, I took them off just in time to see the diamond ring effect just as the sun disappeared entirely. The total eclipse had arrived.
Totality
I’m pretty jaded. I’ve worked at major observatories in Arizona and New Mexico, in the Chilean Andes, on La Palma in the Canary islands. I’ve traveled the world debating deep matters of cosmology and philosophy with renowned scientists from all over, each brilliant in their own way, some the most admirable people you could hope to meet; others, not so much. I’ve seen partial lunar eclipses, total lunar eclipses, the 2004 and 2012 transits^ of Venus, and partial solar eclipses. At this point, I’m very hard to impress. But I had never a total solar eclipse.
I was gobsmacked.
Totality in Cleveland lasted for 3 minutes and 49 seconds. Venus is visible below the sun/moon. Jupiter was also visible on the opposite side from the sun; some people spotted Saturn near the horizon. But the true star of the show was the solar corona.
Words really can’t do justice to a full eclipse. Totality is just stunning. The disk of the moon completely covers the body of the sun, and lurks there for a few minutes. This natural occultation experiment reveals the solar corona. Always there but never otherwise seen, the corona shimmers like a phantasm of white silk around the dark circle of the moon; it is so mesmerizing you’d think it overdone if it were a special effect. I was enthused to make out the small, pink glow of a solar prominence near the bottom of the disk from our perspective: a band of plasma entrained in the magnetic field of sunspots like cosmic iron filings that glow in the pinkish Balmer line of hydrogen. Venus and Jupiter were easily visible; some folks saw Saturn as well. Saturn was over towards the horizon; I didn’t look that far aside for 3 minutes and 49 seconds. Pictures really don’t do it justice. They seem ill-suited to illustrate the extend of the corona without overexposing the prominences.
You literally had to see it to appreciate it.
Fade Out
People cheered as totality started, and again as it came to an end. Daylight returned, albeit the weird dim sepia light of the partial eclipse. What had seemed stunning in its own right a few short minutes before now seemed almost mundane. We talked and milled about and shared a general sense of well-being stemming from bearing common witness to a remarkable event that is both phenomenally rare and stunningly beautiful, a shared feeling that reminds me of Melville’s words:
Oh! my dear fellow beings, why should we longer cherish any social acerbities, or know the slightest ill-humor or envy!
Melville, in Moby Dick
As the light trended back to normal, we decided to pay a visit to the rooftop telescope, where Bill and Charley had watched the eclipse. We were joined by roving groups of astronomy students and alumni, and found Charley in the dome of the 9″ with a projector in place, the brass of the eyepiece warm to the touch.
Charley Knox in his element. The moon is receding, but still blocking a portion of the sun.
There was a communal feeling of satisfaction and general well-being that I can describe no better than totality itself. Classes had been cancelled for the day, and rightly so – nothing could be more educational, nothing could match this experience, and there was no going back inside afterwards.
As the moon passed away, one could see sunspots in the projection from the 9″. That was true during the 2017 partial eclipse as well; I share an image from that time as it shows the sunspots most clearly:
The sun with sunspots, regions of magnetic disturbance that appear dark against the surface of the sun by virtue of being slightly less hot than the surrounding surface. The moon recedes at lower right.
For perspective, recall the spectacular coincidences that make eclipse observations possible. The sun is vastly larger than the moon, but also farther away. Yet they appear very nearly the same angular size in the sky, with the greater distance to the sun relative to the moon almost exactly right to balance the greater size of the sun relative to the moon. It didn’t have to be that way. Indeed, it seems phenomenally unlikely that it should be so. That it is so makes total eclipses extraordinarily rare, as the point of the conical shadow of the moon only just reaches the surface of the earth, so only a small spot is in eclipse at a given time. Indeed, the slight eccentricity of the moon’s orbit means that sometimes the point of the umbra doesn’t even reach the surface, and we get an annular eclipse in which the sun is mostly but not quite fully covered. We were lucky to get nearly four minutes of totality, but the small size of the shadow cast by the moon on the Earth by itself guarantees that eclipses are rare. Add in that the moon’s orbit is tipped about 5 degrees to the plane of the ecliptic (the orbit of the Earth around the sun) and that none of the relevant periods (day, lunar month, year) are integer multiples of one another means that the perfect alignment (syzygy) required for an eclipse rarely repeats over the same spot. But it does happen, and we humans noticed it – by the time of the ancient Babylonians, the lengthy periods on which eclipses were likely to repeat were known – they lacked sufficiently accurate data to predict exactly when and where an eclipse would occur, but they knew when it was eclipse season – a sort of astronomical weather forecast: scattered clouds with a chance of eclipses. These events made a big impression on us; it would have taken careful observations conveyed over many generations to work this out.
Eclipses on planet Earth are quite remarkable. We could have had a bigger moon or a smaller moon or lots of moons or no moon at all. But we got a moon that is exactly the right size at exactly the right distance to almost exactly cover the disk of the sun, and reveal to the human eye the corona that is otherwise lost in the glare of the solar photosphere. This coincidence in space is remarkable enough, but it is also a coincidence in time. The moon helps raise the tides on the Earth, and the tides pull back against the moon. The net effect is a slow transfer of angular momentum from the spin of the Earth to the orbit of the moon. As a consequence, the moon is slowly getting farther away (a few cm/year) and the length of the day is gradually getting longer, having been about 22 hours a mere 600 million years ago, around the time of the Cambrian explosion when multicellular life proliferated. Consequently, the moon would have been a bit closer and appeared somewhat larger on the sky for early land animals; dinosaurs would have seen somewhat more frequent eclipses of longer duration, but would have had a worse view of the corona and prominences, as the larger moon would have blocked more of the emission from near the surface of the sun.
The coincidences that make our current eclipse experience are rather special in both time and space. Make of that what you will.
*There had been so many preparatory emails that the precise location of the discussion panel was lost in the hectic babble. I remembered it was in Tinkham Veale, which is big, but not so big that I was worried about finding the right room. That would surely be easier than finding it in the enormous email thread. When I arrived, I figured the most likely location was the ballroom on the second floor, and indeed, I found the stairs blocked by a sign 2ND FLOOR CLOSED FOR PRIVATE EVENT. Bypassing this, I was greeted by enhanced campus security and a person who asked my name. Scrolling a handheld device, she got that concerned look officious people get when you’re not on the list. She politely checked the spelling of my name, checked again, then apologized that I wasn’t on the list. As this was going on, I realized this must be a list for people who registered to hear the panel, so I said “I’m the astronomer ON the panel.” Her eyes got big. “Oh!” she said. “Come right in…”
So, the moral of that story is that you can always talk your way into an exclusive event by claiming to be an astronomer – provided, of course, that it is a very specialized subset of exclusive events about astronomy.
$I found it bemusing because it was just a tiny picket fence set up in the midst of a much larger field. There was nothing special or meritorious about the location, so it was just exclusionary, which is a thing I’m generally against.
+Contrails like this are usually a bad sign for observational astronomy, being a harbinger of bad seeing as well as high humidity. In this case, it was just part of the show – and a very small part at that. Mostly I pitied the fools who had paid to confine themselves inside a metal tube at 10 km altitude while the most amazing of celestial events was going on.
#In 2017, the academic staff of the Department of Astronomy consisted of five faculty and one research scientist. By 2019 attrition had reduced us to three faculty. That no hires have been made since then is a long story of administrative incompetence and malfeasance.
^I almost missed the 2004 transit, which was conveniently observable in Europe but which we nearly over by the time the sun rose in the U.S. Not only did one have to get up at the literal crack of daen, but that meant the sun was on the eastern horizon. The only way I could find to witness it was to hold a pair of binoculars at a window in our attic and project the image onto the wall.
The 2012 transit was more friendly to observers in North America, occurring mid-day. I set up a small telescope in front of my house; the neighbors took turns holding the projection screen for all to see. Many stayed for hours to follow the gradual progress of Venus against the face of the sun.
Venus appears as the small, dark circle at the top left of the disk of the sun during its 2012 transit.
Perhaps the most compelling astronomical phenomenon accessible to a naked-eye observer is a total eclipse of the sun. These rare events have always fascinated us, and often terrified us. It is abnormal and disturbing for the sun to be blotted from the sky!
A solar eclipse will occur on Monday, 8 April 2024. A partial eclipse will be visible from nearly every part of North America. The path of totality will sweep from Mexico through Texas, the Midwest, New England, and across the maritime provinces of Canada. If you are anywhere where this event is visible, go out, don a pair of eclipse glasses, and look up. This is especially true in the path of totality. Partial eclipse are cool. Total eclipses are so much more that they have inspired science, art, and literature, with descriptions frequently evincing the deep emotion of profound religious experience*.
The American Astronomical Society has posted lots of useful information, including a map of the path of totality and advice about proper eclipse glasses. These are super-cheap, but that doesn’t preclude bad actors from selling ineffective versions. Simple rule of thumb: don’t look straight at the sun. A proper pair of eclipse glasses enable you to comfortably do so. If it hurts, stop+: close your eyes and look away. Listen to the messages from your pain receptors.
If you can get to the path of totality, it is worth doing so. Expect crowds and plan accordingly. This is a draw of epic proportions, and for many will be the only practical opportunity of their lifetime. Totality is brief, only a few minutes, so be sure to be in the right place at the right time$.
The AAS provides a good list of the phenomena to expect. Most of the action is around and during totality. The partial eclipse is a long (hour+) build up to the brief main show (a few minutes of totality). In addition to seeing the corona, the diamond ring and Baily’s Beads effects, this should be a good time to see solar prominences as the sun is nearing the maximum in its eleven year sunspot cycle. What we will see is unknown, as this is the solar analog of weather phenomena. The forecast calls for a high chance of prominences, but that doesn’t guarantee they’ll show.
One last thing I’ll note is that all the planets are relatively close to the sun on the sky at present, and some might be visible during the eclipse. Venus and Jupiter will be most prominent and easy to spot. Uranus and Neptune, not so much. The others maybe. Also present is Comet 12P/Pons-Brooks (aka the devil comet) in the vicinity of Jupiter. It is quite a temporal coincidence for this comet with a 71 year period to be in the inner solar system during this eclipse. It is unlikely to put on much of a show: comets are notoriously fickle, and the odds are that it will be invisible to the naked eye. But it is there, so keep a weather eye out, just in case.
All the planets and even a comet will be in the sky during the eclipse.
Now go forth this Monday and witness one of nature’s greatest marvels.
*There are many myths and monsters associated with eclipses. Until the light pollution of recent times, the motions of the sky were very much in our faces. People cared deeply about these things. They were well aware of more than the daily rising and setting of the sun. The phases of the moon, the patterns in the stars, and the wandering of the planets was obvious to everyone who looked up. People learned long ago to keep close track of these events, even those as rare as eclipses. Some of the earliest tablets unearthed from ancient Babylon are elaborate tables of eclipse seasons recognizing lengthy periods like the roughly 18 year Saros cycle. One doesn’t just up and write down this sort of knowledge on a whim one day, as it requires centuries of careful observation and record keeping to recognize the recurrence of events with such long periods, especially for solar eclipses that do not visit exactly the same spot every exeligmos cycle. I suspect there was a strong oral tradition of astronomical record keeping for long ages before we learned to write. Astronomy is the oldest science: this was important knowledge to acquire, preserve, and pass on.
The ancients managed to deduce cycles of eclipse seasons, so they could forecast the chance for eclipses, but only with the same precision as a weather forecast: there is a chance of rain, but we can’t be sure exactly when and where. Now we have measured planetary motions accurately enough and understand the geometry of what is going on so we can forecast exactly when and where eclipses will occur. This is a staggering achievement of human intellect and communal effort.
+There are a lot of misconceptions about the dangers of eclipse viewing. Looking straight at the sun is uncomfortable and dangerous at any time. The only thing special about a total eclipse is that it becomes truly dark for a few minutes, and your pupils start to expand to adapt to the darkness. Consequently, the most dangerous moment is at the end of totality, when your eyes have grown wide and the sun suddenly reappears. Be sure to don your eclipse glasses or look away right before the sun reappears; you don’t want to look straight into the sun at that moment.
Time and Date is a great resource for getting the timing of the eclipse for your specific location, accurate to within a few seconds.
$As with any astronomical observation, no guarantee is made that the skies will be clear of clouds. I have spent many a night at observatories wishing for the sky to clear and obsessively refreshing the satellite maps to discern when it might do so. It doesn’t help – it’s almost as if nature doesn’t care that we want to witness one of its greatest displays. So my advice is to go where you can and don’t sweat the weather forecast. Either the sky cooperates or it doesn’t.
I’ve agreed to serve on a discussion panel about the eclipse on campus, so I’ll be here in Cleveland. We are right in the path of totality, but the weather statistics here are… not good. To make matters worse for the superstitious, April 8 is also the home opener for the Cleveland Guardians. Opening day is always a joyous time with a packed stadium, but the weather is inevitably miserable. Nevertheless, all we need is a brief opening in the clouds at just the right time. At an observatory we would call a that a sucker hole – a gap in the clouds big enough to get the inexperienced observer to run around prepping the instrument and the telescope, an intense amount of work, to open up and observe just in time for the clouds to cover up the sky again. Come Monday, I’ll happily accept a well-timed sucker hole.
In 1986, I was a grad student at Princeton, working in the atomic physics lab of Will Happer. It was at a department colloquium that I first heard a science talk that raised serious concerns about our use of fossil fuels potentially impacting the climate. This was not received well.
People asked all sorts of questions, with much of the discussion revolving around feedback effects. Perhaps warmer weather from CO2 will result in higher humidity, making more clouds*, and reflecting more sunlight into space. It does not. What about ice cover? This is actually a positive feedback – as the globe warms, ice coverage is replaced by darker surfaces, leading to more absorption of the incident solar radiation. And so on.
I thought the speaker did a creditable job of answering the concerns raised, repeatedly making the point that most feedback effects would make things worse, not better. It was entirely new to me at the time; I didn’t have any context to judge the relative merits of the discussion. Prof. Happer is one of those remarkable people who seems to know a lot about everything. So, as I related before, I asked him. His immediate and harsh retort was
“We can’t turn off the wheels of industry, and go back to living like cavemen.”
I relate this story again because the same language comes up today in a story I saw in the Guardian:
The president of Cop28, Sultan Al Jaber, has claimed there is “no science” indicating that a phase-out of fossil fuels is needed** to restrict global heating to 1.5C, the Guardian and the Centre for Climate Reporting can reveal.
Al Jaber also said a phase-out of fossil fuels would not allow sustainable development “unless you want to take the world back into caves”.
This is exactly the same solution aversion that Happer displayed, using exactly the same language. It doesn’t address the actual question. It leaps ahead to the worst conceivable consequence, doesn’t like it, and so reverts to reality denial: We don’t want that to happen, so the evidence must be wrong!
We humans excel at reality-denial. It is not helpful. Rather than starting to deal with the problem of climate change thirty years ago – the science was already crystal clear by then – we’ve dug ourselves a much deeper hole. That’s not to say we should abandon all hope and revert to living in caves, but we do need to take serious and rapid steps to reform the ways in which we generate power. It is doing more of the same that risks sending us back into caves.
The quoted reaction to this assertion in the story is predictably tepid. The quote in the Guardian is “The comments were `incredibly concerning’ and `verging on climate denial’, scientists said.” As a scientist who is not directly involved with dealing with these people, let me be more blunt:
ARE YOU FUCKING KIDDING ME?
Al Jaber’s attitude isn’t verging on climate denial, it is the archetype of climate denial. Literally the same thing that climate deniers said in the 1980s. It expresses an attitude that was clearly wrong and dangerously backwards by the early 1990s. And this guy is the president of COP28? I say again
ARE YOU FUCKING KIDDING ME?
And who is this guy? The Guardian reports “Al Jaber is also the chief executive of the United Arab Emirates’ state oil company, Adnoc, which many observers see as a serious conflict of interest.” A conflict of interest? Really? Do you think? Again I say
ARE YOU FUCKING KIDDING ME?
This is obviously a conflict of interest, of the worst sort. His personal wealth, and the sovereign wealth of his nation, is entirely based on the production and sale of fossil fuels. Mitigating climate change means reducing our consumption of fossil fuels, which is a direct threat to the economic interests he represents. Talk about putting the fox in charge of the hen house.
I am impressed by how the moneyed interests have managed to slither their way into positions of consequence on discussions in which they have an obvious conflict. I guess money always finds a way in. But can we please stop being so polite that we fail to call out obvious bullshit wherever it crops up? It seems to be spreading at the rate of made-up conspiracy nonsense on that site formerly known as Twitter. We should stop putting up with it already.
*Ironically, SO2 pollution from ocean-going vessels does have this effect, and as these emissions have been cleaned up, we can see the effect in global temperatures. This is not to advocate for SO2 pollution! though injecting aerosols like SO2 into the stratosphere is one of geoengineering approaches that gets discussed. Before going down that path, the obvious first step is to stop pouring petrol on the fire by continuing to add CO2 to the atmosphere.
**We’re already committed to 1.5C. I see no conceivable way that we can curb emissions fast enough to avoid that. So I guess this statement is true, from a certain point of view – that of a liar. A less misleading statement would be that a phase-out of fossil fuels is necessary to prevent things from getting much worse than forecast for the 1.5C threshold.
Taking a break from galaxies and cosmology, I’d like to post a little praise of NASA for safely returning a piece of an asteroid to Earth.
One of the amazing things to me about astronomy & astrophysics is that we have learned how to decipher the composition of distant stars and gas clouds by observing their spectra. I worked on this early in my career and retain an interest in the cosmic abundance of the elements. Fun fact: though often overlooked because it is a boring noble gas that doesn’t bind chemically into any common molecules or minerals, neon is number 5 on the list of most common elements, which goes hydrogen, helium, oxygen, carbon, neon. Nitrogen is number 6 by number, but iron supplants it if we weight by mass – there are more nitrogen atoms by number in the sun but the iron weighs more because of the greater mass of each atom. The order of the first five remains the same by either accounting.
Amazing as it is that we can do this, it can only be accomplished by passive observation. What we’d really like to do is get samples of the remote universe to analyze in the laboratory where precision is much higher and we can better control for systematic effects. Of course we can’t travel to stars and nebulae that are many light-years distant, let alone return from there. But we can do it within the solar system, which is amazing enough. The Apollo astronauts brought back rocks from the moon that helped determine the age of the solar system (4.568 billion years, give or take a million), and the period of “late heavy bombardment” when most big lunar craters were formed – a mere 3.9 billion years ago. This in turn calibrates crater densities; counting craters on other solar system bodies lets us gauge the age of a surface. Lots of craters means old; few craters means something interesting had to happen to cover up all the craters that formed during heavy bombardment. It’s not like all those early meteoroids were dodging the Earth while hammering the moon; it’s just that the Earth has covered it up since.
One of the most interesting things scientifically are samples of pristine material – the stuff from which the solar system formed. The Earth is a remarkably active planet geologically, which means that its rocks are always getting remade by erosion, subduction, and volcanism. They’re about as far from pristine as a rock can get. The closest we expect we can get are the comets and asteroids orbiting safely away from the big planets that have a complex history of their own.
Hence the idea for a mission that could return a sample from a remote asteroid. This is what OSIRIS-REx has now accomplished. It is worth pausing to reflect what an amazing feat this is.
We’ve only had the capacity to launch things beyond the atmosphere of our planet for 66 years. Though satellite launches are now relatively common, the most frequent destination is low earth orbit. That’s only a couple thousand kilometers, which is about a third of an Earth radius, so still pretty close. It is a distance that planes traverse horizontally all the time, if only at an altitude of 10 km or so. It’s just not that far on an interplanetary scale.
Deep space missions that leave Earth’s gravity well are harder and much less common. Those that go out to an asteroid, grab a piece, and return are even harder. It’s one thing to shoot something off a rocket so hard it never comes back. It’s quite another to do that and then turn around and come back at a time and place of our choosing. That’s a remarkable feat of celestial navigation and rocket engineering. Oh, and pause on the way to graze an asteroid, grab a sample, and store it for safe return.
Safe is key here. If one wants a pristine sample of the early solar system, you not only need to go to deep space to collect it, but you have to keep it safe through the rigors of reentry, collect it, and get it to your lab unsullied by terrestrial contaminants. Lots that can go wrong. The spacecraft has to endure the heat of reentry, suffer no leaks, and land gently in a spot where the sample can be retrieved. This all went well for OSIRIS-REx. It doesn’t always work so well.
Genesis was another sample return mission. Launched in 2001, it collected particles from the solar wind – a good way to get a measure of the composition of the sun. It did this for several years before returning 19 years ago to the month, to the same landing area as OSIRIS-REx. As it happened, I had just flown to Tucson to observe at Kitt Peak, and found myself having breakfast in the La Quinta next to the airport before renting a car to drive up the mountain. The landing was on the TV there, so it was breakfast and a show.
Only the show didn’t go so well. A helicopter was supposed to snag the capsule as it drifted at the end of its parachute to ensure no contamination from the ground. Through some amazing camera work, they showed a fairly zoomed-in image of the return capsule as it hurtled from the sky. Spinning, spinning, spinning… it looked out of control. Shouldn’t the parachute have deployed by now? Maybe not – that’s often done at fairly low altitude where the air is thick enough to bite. So I watched, spinning, spinning, as seconds stretched into minutes, spinning, spinning, surely the parachute will deploy any moment now, spinning, spinning, any moment now, spinning, spinning, really, any moment now, spinning, spinning, SMACK! into the ground.
Genesis did not experience a gentle landing. Photo credit: USAF, public domain.
The parachute failed to deploy. Apparently Lockheed Martin installed it backwards, a mistake for which I’m sure they were well remunerated. This is but one of the hazards of space travel.
So it was with a little trepidation that I watched the return of OSIRIS-REx this morning. There was again some amazing camera work. First we saw the blaze of reentry, then after that faded the capsule itself emerged, becoming visible while still at high altitude. Spinning, spinning.
As I was watching on NASA TV, it was announced that the order to deploy the parachute had been issued. Spinning, spinning. Good. Spinning, spinning. No parachute. Was there a time delay on that order? Still seemed high to be deploying a chute, but it was hard to judge the altitude from watching a small spinning blob on TV. Spinning, spinning. I am old and jaded, so I didn’t feel nervous – yet. Spinning, spinning. Only a tiny bit of anxiety. Spinning, spinning. Then it was announced that the parachute was scheduled to deploy at 49 minutes past the hour – still two minutes away. Spinning, spinning. Then, at 48 minutes past the hour, the parachute deployed. I was so enthused to see it that I didn’t worry that it had come a bit early – better than too late! Apparently it deployed at an altitude of 20,000 feet when it wasn’t supposed to deploy until 5,000. So that went wrong, but only a tiny bit wrong – it came gently to rest on the ground near the edge of the target ellipse – i.e., within the error bars.
Osiris-Rex return capsule where it landed in Utah. Screen shot from NASA TV. As in, I took a picture of the TV with my phone.
I had written most of the post below the line before an exchange with a senior colleague who accused me of asking us to abandon General Relativity (GR). Anyone who read the last post knows that this is the opposite of true. So how does this happen?
Much of the field is mired in bad ideas that seemed like good ideas in the 1980s. There has been some progress, but the idea that MOND is an abandonment of GR I recognize as a misconception from that time. It arose because the initial MOND hypothesis suggested modifying the law of inertia without showing a clear path to how this might be consistent with GR. GR was built on the Equivalence Principle (EP), the equivalence1 of gravitational charge with inertial mass. The original MOND hypothesis directly contradicted that, so it was a fair concern in 1983. It was not by 19842. I was still an undergraduate then, so I don’t know the sociology, but I get the impression that most of the community wrote MOND off at this point and never gave it further thought.
I guess this is why I still encounter people with this attitude, that someone is trying to rob them of GR. It’s feels like we’re always starting at square one, like there has been zero progress in forty years. I hope it isn’t that bad, but I admit my patience is wearing thin.
I’m trying to help you. Don’t waste you’re entire career chasing phantoms.
What MOND does ask us to abandon is the Strong Equivalence Principle. Not the Weak EP, nor even the Einstein EP. Just the Strong EP. That’s a much more limited ask that abandoning all of GR. Indeed, all flavors of EP are subject to experimental test. The Weak EP has been repeatedly validated, but there is nothing about MOND that implies platinum would fall differently from titanium. Experimental tests of the Strong EP are less favorable.
I understand that MOND seems impossible. It also keeps having its predictions come true. This combination is what makes it important. The history of science is chock full of ideas that were initially rejected as impossible or absurd, going all the way back to heliocentrism. The greater the cognitive dissonance, the more important the result.
Continuing the previous discussion of UT, where do we go from here? If we accept that maybe we have all these problems in cosmology because we’re piling on auxiliary hypotheses to continue to be able to approximate UT with FLRW, what now?
I don’t know.
It’s hard to accept that we don’t understand something we thought we understood. Scientists hate revisiting issues that seem settled. Feels like a waste of time. It also feels like a waste of time continuing to add epicycles to a zombie theory, be it LCDM or MOND or the phoenix universe or tired light or whatever fantasy reality you favor. So, painful as it may be, one has find a little humility to step back and take account of what we know empirically independent of the interpretive veneer of theory.
Still, to give one pertinent example, BBN only works if the expansion rate is as expected during the epoch of radiation domination. So whatever is going on has to converge to that early on. This is hardly surprising for UT since it was stipulated to contain GR in the relevant limit, but we don’t actually know how it does so until we work out what UT is – a tall order that we can’t expect to accomplish overnight, or even over the course of many decades without a critical mass of scientists thinking about it (and not being vilified by other scientists for doing so).
Another example is that the cosmological principle – that the universe is homogeneous and isotropic – is observed to be true in the CMB. The temperature is the same all over the sky to one part in 100,000. That’s isotropy. The temperature is tightly coupled to the density, so if the temperature is the same everywhere, so is the density. That’s homogeneity. So both of the assumptions made by the cosmological principle are corroborated by observations of the CMB.
The cosmological principle is extremely useful for solving the equations of GR as applied to the whole universe. If the universe has a uniform density on average, then the solution is straightforward (though it is rather tedious to work through to the Friedmann equation). If the universe is not homogeneous and isotropic, then it becomes a nightmare to solve the equations. One needs to know where everything was for all of time.
Starting from the uniform condition of the CMB, it is straightforward to show that the assumption of homogeneity and isotropy should persist on large scales up to the present day. “Small” things like galaxies go nonlinear and collapse, but huge volumes containing billions of galaxies should remain in the linear regime and these small-scale variations average out. One cubic Gigaparsec will have the same average density as the next as the next, so the cosmological principle continues to hold today.
Anyone spot the rub? I said homogeneity and isotropy should persist. This statement assumes GR. Perhaps it doesn’t hold in UT?
This aspect of cosmology is so deeply embedded in everything that we do in the field that it was only recently that I realized it might not hold absolutely – and I’ve been actively contemplating such a possibility for a long time. Shouldn’t have taken me so long. Felten (1984) realized right away that a MONDian universe would depart from isotropy by late times. I read that paper long ago but didn’t grasp the significance of that statement. I did absorb that in the absence of a cosmological constant (which no one believed in at the time), the universe would inevitably recollapse, regardless of what the density was. This seems like an elegant solution to the flatness/coincidence problem that obsessed cosmologists at the time. There is no special value of the mass density that provides an over/under line demarcating eternal expansion from eventual recollapse, so there is no coincidence problem. All naive MOND cosmologies share the same ultimate fate, so it doesn’t matter what we observe for the mass density.
MOND departs from isotropy for the same reason it forms structure fast: it is inherently non-linear. As well as predicting that big galaxies would form by z=10, Sanders (1998) correctly anticipated the size of the largest structures collapsing today (things like the local supercluster Laniakea) and the scale of homogeneity (a few hundred Mpc if there is a cosmological constant). Pretty much everyone who looked into it came to similar conclusions.
But MOND and cosmology, as we know it in the absence of UT, are incompatible. Where LCDM encompasses both cosmology and the dynamics of bound systems (dark matter halos3), MOND addresses the dynamics of low acceleration systems (the most common examples being individual galaxies) but says nothing about cosmology. So how do we proceed?
For starters, we have to admit our ignorance. From there, one has to assume some expanding background – that much is well established – and ask what happens to particles responding to a MONDian force-law in this background, starting from the very nearly uniform initial condition indicated by the CMB. From that simple starting point, it turns out one can get a long way without knowing the details of the cosmic expansion history or the metric that so obsess cosmologists. These are interesting things, to be sure, but they are aspects of UT we don’t know and can manage without to some finite extent.
For one, the thermal history of the universe is pretty much the same with or without dark matter, with or without a cosmological constant. Without dark matter, structure can’t get going until after thermal decoupling (when the matter is free to diverge thermally from the temperature of the background radiation). After that happens, around z = 200, the baryons suddenly find themselves in the low acceleration regime, newly free to respond to the nonlinear force of MOND, and structure starts forming fast, with the consequences previously elaborated.
But what about the expansion history? The geometry? The big questions of cosmology?
Again, I don’t know. MOND is a dynamical theory that extends Newton. It doesn’t address these questions. Hence the need for UT.
I’ve encountered people who refuse to acknowledge4 that MOND gets predictions like z=10 galaxies right without a proper theory for cosmology. That attitude puts the cart before the horse. One doesn’t look for UT unless well motivated. That one is able to correctly predict 25 years in advance something that comes as a huge surprise to cosmologists today is the motivation. Indeed, the degree of surprise and the longevity of the prediction amplify the motivation: if this doesn’t get your attention, what possibly could?
There is no guarantee that our first attempt at UT (or our second or third or fourth) will work out. It is possible that in the search for UT, one comes up with a theory that fails to do what was successfully predicted by the more primitive theory. That just lets you know you’ve taken a wrong turn. It does not mean that a correct UT doesn’t exist, or that the initial prediction was some impossible fluke.
One candidate theory for UT is bimetric MOND. This appears to justify the assumptions made by Sanders’s early work, and provide a basis for a relativistic theory that leads to rapid structure formation. Whether it can also fit the acoustic power spectrum of the CMB as well as LCDM and AeST has yet to be seen. These things take time and effort. What they really need is a critical mass of people working on the problem – a community that enjoys the support of other scientists and funding institutions like NSF. Until we have that5, progress will remain grudgingly slow.
1The equivalence of gravitational charge and inertial mass means that the m in F=GMm/d2 is identically the same as the m in F=ma. Modified gravity changes the former; modified inertia the latter.
2Bekenstein & Milgrom (1984) showed how a modification of Newtonian gravity could avoid the non-conservation issues suffered by the original hypothesis of modified inertia. They also outlined a path towards a generally covariant theory that Bekenstein pursued for the rest of his life. That he never managed to obtain a completely satisfactory version is often cited as evidence that it can’t be done, since he was widely acknowledged as one of the smartest people in the field. One wonders why he persisted if, as these detractors would have us believe, the smart thing to do was not even try.
4I have entirely lost patience with this attitude. If a phenomena is correctly predicted in advance in the literature, we are obliged as scientists to take it seriously+. Pretending that it is not meaningful in the absence of UT is just an avoidance strategy: an excuse to ignore inconvenient facts.
+I’ve heard eminent scientists describe MOND’s predictive ability as “magic.” This also seems like an avoidance strategy. I, for one, do not believe in magic. That it works as well as it does – that it works at all – must be telling us something about the natural world, not the supernatural.
5There does exist a large and active community of astroparticle physicists trying to come up with theories for what the dark matter could be. That’s good: that’s what needs to happen, and we should exhaust all possibilities. We should do the same for new dynamical theories.
Kuhn noted that as paradigms reach their breaking point, there is a divergence of opinions between scientists about what the important evidence is, or what even counts as evidence. This has come to pass in the debate over whether dark matter or modified gravity is a better interpretation of the acceleration discrepancy problem. It sometimes feels like we’re speaking about different topics in a different language. That’s why I split the diagram version of the dark matter tree as I did:
Evidence indicating acceleration discrepancies in the universe and various flavors of hypothesized solutions.
Astroparticle physicists seem to be well-informed about the cosmological evidence (top) and favor solutions in the particle sector (left). As more of these people entered the field in the ’00s and began attending conferences where we overlapped, I recognized gaping holes in their knowledge about the dynamical evidence (bottom) and related hypotheses (right). This was part of my motivation to develop an evidence-based course1 on dark matter, to try to fill in the gaps in essential knowledge that were obviously being missed in the typical graduate physics curriculum. Though popular on my campus, not everyone in the field has the opportunity to take this course. It seems that the chasm has continued to grow, though not for lack of attempts at communication.
Part of the problem is a phase difference: many of the questions that concern astroparticle physicists (structure formation is a big one) were addressed 20 years ago in MOND. There is also a difference in texture: dark matter rarely predicts things but always explains them, even if it doesn’t. MOND often nails some predictions but leaves other things unexplained – just a complete blank. So they’re asking questions that are either way behind the curve or as-yet unanswerable. Progress rarely follows a smooth progression in linear time.
I have become aware of a common construction among many advocates of dark matter to criticize “MOND people.” First, I don’t know what a “MOND person” is. I am a scientist who works on a number of topics, among them both dark matter and MOND. I imagine the latter makes me a “MOND person,” though I still don’t really know what that means. It seems to be a generic straw man. Users of this term consistently paint such a luridly ridiculous picture of what MOND people do or do not do that I don’t recognize it as a legitimate depiction of myself or of any of the people I’ve met who work on MOND. I am left to wonder, who are these “MOND people”? They sound very bad. Are there any here in the room with us?
I am under no illusions as to what these people likely say when I am out of ear shot. Someone recently pointed me to a comment on Peter Woit’s blog that I would not have come across on my own. I am specifically named. Here is a screen shot:
This concisely pinpoints where the field2 is at, both right and wrong. Let’s break it down.
let me just remind everyone that the primary reason to believe in the phenomenon of cold dark matter is the very high precision with which we measure the CMB power spectrum, especially modes beyond the second acoustic peak
This is correct, but it is not the original reason to believe in CDM. The history of the subject matters, as we already believed in CDM quite firmly before any modes of the acoustic power spectrum of the CMB were measured. The original reasons to believe in cold dark matter were (1) that the measured, gravitating mass density exceeds the mass density of baryons as indicated by BBN, so there is stuff out there with mass that is not normal matter, and (2) large scale structure has grown by a factor of 105 from the very smooth initial condition indicated initially by the nondetection of fluctuations in the CMB, while normal matter (with normal gravity) can only get us a factor of 103 (there were upper limits excluding this before there was a detection). Structure formation additionally imposes the requirement that whatever the dark matter is moves slowly (hence “cold”) and does not interact via electromagnetism in order to evade making too big an impact on the fluctuations in the CMB (hence the need, again, for something non-baryonic).
When cold dark matter became accepted as the dominant paradigm, fluctuations in the CMB had not yet been measured. The absence of observable fluctuations at a larger level sufficed to indicate the need for CDM. This, together with Ωm > Ωb from BBN (which seemed the better of the two arguments at the time), sufficed to convince me, along with most everyone else who was interested in the problem, that the answer had3 to be CDM.
This all happened before the first fluctuations were observed by COBE in 1992. By that time, we already believed firmly in CDM. The COBE observations caused initial confusion and great consternation – it was too much! We actually had a prediction from then-standard SCDM, and it had predicted an even lower level of fluctuations than what COBE observed. This did not cause us (including me) to doubt CDM (thought there was one suggestion that it might be due to self-interacting dark matter); it seemed a mere puzzle to accommodate, not an anomaly. And accommodate it we did: the power in the large scale fluctuations observed by COBE is part of how we got LCDM, albeit only a modest part. A lot of younger scientists seem to have been taught that the power spectrum is some incredibly successful prediction of CDM when in fact it has surprised us at nearly every turn.
As I’ve related here before, it wasn’t until the end of the century that CMB observations became precise enough to provide a test that might distinguish between CDM and MOND. That test initially came out in favor of MOND – or at least in favor of the absence of dark matter: No-CDM, which I had suggested as a proxy for MOND. Cosmologists and dark matter advocates consistently omit this part of the history of the subject.
I had hoped that cosmologists would experience the same surprise and doubt and reevaluation that I had experienced when MOND cropped up in my own data when it cropped up in theirs. Instead, they went into denial, ignoring the successful prediction of the first-to-second peak amplitude ratio, or, worse, making up stories that it hadn’t happened. Indeed, the amplitude of the second peak was so surprising that the first paper to measure it omitted mention of it entirely. Just didn’t talk about it, let alone admit that “Gee, this crazy prediction came true!” as I had with MOND in LSB galaxies. Consequently, I decided that it was better to spend my time working on topics where progress could be made. This is why most of my work on the CMB predates “modes beyond the second peak” just as our strong belief in CDM also predated that evidence. Indeed, communal belief in CDM was undimmed when the modes defining the second peak were observed, despite the No-CDM proxy for MOND being the only hypothesis to correctly predict it quantitatively a priori.
That said, I agree with clayton’s assessment that
CDM thinks [the second and third peak] should be about the same
That this is the best evidence now is both correct and a much weaker argument than it is made out to be. It sounds really strong, because a formal fit to the CMB data require a dark matter component at extremely high confidence – something approaching 100 sigma. This analysis assumes that dark matter exist. It does not contemplate that something else might cause the same effect, so all it really does, yet again, is demonstrate that General Relativity cannot explain cosmology when restricted to the material entities we concretely know to exist.
Given the timing, the third peak was not a strong element of my original prediction, as we did not yet have either a first or second peak. We hadn’t yet clearly observed peaks at all, so what I was doing was pretty far-sighted, but I wasn’t thinking that far ahead. However, the natural prediction for the No-CDM picture I was considering was indeed that the third peak should be lower than the second, as I’ve discussed before.
The No-CDM model (blue line) that correctly predicted the amplitude of the second peak fails to predict that of the third. Data from the Planck satellite; model line from McGaugh (2004); figure from McGaugh (2015).
In contrast, in CDM, the acoustic power spectrum of the CMB can do a wide variety of things:
Acoustic power spectra calculated for the CMB for a variety of cosmic parameters. From Dodelson & Hu (2002).
Given the diversity of possibilities illustrated here, there was never any doubt that a model could be fit to the data, provided that oscillations were observed as expected in any of the theories under consideration here. Consequently, I do not find fits to the data, though excellent, to be anywhere near as impressive as commonly portrayed. What does impress me is consistency with independent data.
What impresses me even more are a priori predictions. These are the gold standard of the scientific method. That’s why I worked my younger self’s tail off to make a prediction for the second peak before the data came out. In order to make a clean test, you need to know what both theories predict, so I did this for both LCDM and No-CDM. Here are the peak ratios predicted before there were data to constrain them, together with the data that came after:
The ratio of the first-to-second (left) and second-to-third peak (right) amplitude ratio in LCDM (red) and No-CDM (blue) as predicted by Ostriker & Steinhardt (1995) and McGaugh (1999). Subsequent data as labeled.
The left hand panel shows the predicted amplitude ratio of the first-to-second peak, A1:2. This is the primary quantity that I predicted for both paradigms. There is a clear distinction between the predicted bands. I was not unique in my prediction for LCDM; the same thing can be seen in other contemporaneous models. All contemporaneous models. I was the only one who was not surprised by the data when they came in, as I was the only one who had considered the model that got the prediction right: No-CDM.
The same No-CDM model fails to correctly predict the second-to-third peak ratio, A2:3. It is, in fact, way off, while LCDM is consistent with A2:3, just as Clayton says. This is a strong argument against No-CDM, because No-CDM makes a clear and unequivocal prediction that it gets wrong. Clayton calls this
a stone-cold, qualitative, crystal clear prediction of CDM
which is true. It is also qualitative, so I call it weak sauce. LCDM could be made to fit a very large range of A2:3, but it had already got A1:2 wrong. We had to adjust the baryon densityoutside the allowed range in order to make it consistent with the CMB data. The generous upper limit that LCDM might conceivably have predicted in advance of the CMB data was A1:2 < 2.06, which is still clearly less than observed. For the first years of the century, the attitude was that BBN had been close, but not quite right – preference being given to the value needed to fit the CMB. Nowadays, BBN and the CMB are said to be in great concordance, but this is only true if one restricts oneself to deuterium measurements obtained after the “right” answer was known from the CMB. Prior to that, practically all of the measurements for all of the important isotopes of the light elements, deuterium, helium, and lithium, all concurred that the baryon density Ωbh2 < 0.02, with the consensus value being Ωbh2 = 0.0125 ± 0.0005. This is barely half the value subsequently required to fit the CMB (Ωbh2 = 0.0224 ± 0.0001). But what’s a factor of two among cosmologists? (In this case, 4 sigma.)
Taking the data at face value, the original prediction of LCDM was falsified by the second peak. But, no problem, we can move the goal posts, in this case by increasing the baryon density. The successful prediction of the third peak only comes after the goal posts have been moved to accommodate the second peak. Citing only the comparable size of third peak to the second while not acknowledging that the second was too small elides the critical fact that No-CDM got something right, a priori, that LCDM did not. No-CDM failed only after LCDM had already failed. The difference is that I acknowledge its failure while cosmologists elide this inconvenient detail. Perhaps the second peak amplitude is a fluke, but it was a unique prediction that was exactly nailed and remains true in all subsequent data. That’s a pretty remarkable fluke4.
LCDM wins ugly here by virtue of its flexibility. It has greater freedom to fit the data – any of the models in the figure of Dodelson & Hu will do. In contrast. No-CDM is the single blue line in my figure above, and nothing else. Plausible variations in the baryon density make hardly any difference: A1:2 has to have the value that was subsequently observed, and no other. It passed that test with flying colors. It flunked the subsequent test posed by A2:3. For LCDM this isn’t even a test, it is an exercise in fitting the data with a model that has enough parameters5 to do so.
In those days, when No-CDM was the only correct a priori prediction, I would point out to cosmologists that it had got A1:2 right when I got the chance (which was rarely: I was invited to plenty of conferences in those days, but none on the CMB). The typical reaction was usually outright denial6 though sometimes it warranted a dismissive “That’s not a MOND prediction.” The latter is a fair criticism. No-CDM is just General Relativity without CDM. It represented MOND as a proxy under the ansatz that MOND effects had not yet manifested in a way that affected the CMB. I expected that this ansatz would fail at some point, and discussed some of the ways that this should happen. One that’s relevant today is that galaxies form early in MOND, so reionization happens early, and the amplitude of gravitational lensing effects is amplified. There is evidence for both of these now. What I did not anticipate was a departure from a damping spectrum around L=600 (between the second and third peaks). That’s a clear deviation from the prediction, which falsifies the ansatz but not MOND itself. After all, they were correct in noting that this wasn’t a MOND prediction per se, just a proxy. MOND, like Newtonian dynamics before it, is relativity adjacent, but not itself a relativistic theory. Neither can explain the CMB on their own. If you find that an unsatisfactory answer, imagine how I feel.
The same people who complained then that No-CDM wasn’t a real MOND prediction now want to hold MOND to the No-CDM predicted power spectrum and nothing else. First it was the second peak isn’t a real MOND prediction! then when the third peak was observed it became no way MOND can do this! This isn’t just hypocritical, it is bad science. The obvious way to proceed would be to build on the theory that had the greater, if incomplete, predictive success. Instead, the reaction has consistently been to cherry-pick the subset of facts that precludes the need for serious rethinking.
This brings us to sociology, so let’s examine some more of what Clayton has to say:
Any talk I’ve ever seen by McGaugh (or more exotic modified gravity people like Verlinde) elides this fact, and they evade the questions when I put my hand up to ask. I have invited McGaugh to a conference before specifically to discuss this point, and he just doesn’t want to.
There is so much to unpack here, I hardly know where to start. By saying I “elide this fact” about the qualitatively equality of the second and third peak, Clayton is basically accusing me of lying by omission. This is pretty rich coming from a community that consistently elides the history I relate above, and never addresses the question raised by MOND’s predictive power.
Intellectual honesty is very important to me – being honest that MOND predicted what I saw in low surface brightness where my own prediction was wrong is what got me into this mess in the first place. It would have been vastly more convenient to pretend that I never heard of MOND (at first I hadn’t7) and act like that never happened. That would be an lie of omission. It would be a large lie, a lie that denies an important aspect of how the world works (what we’re supposed to uncover through science), the sort of lie that cleric Paul Gerhardt may have had in mind when he said
When a man lies, he murders some part of the world.
Clayton is, in essence, accusing me of exactly that by failing to mention the CMB in talks he has seen. That might be true – I give a lot of talks. He hasn’t been to most of them, and I usually talk about things I’ve done more recently than 2004. I’ve commented explicitly on this complaint before –
There’s only so much you can address in a half hour talk. [This is a recurring problem. No matter what I say, there always seems to be someone who asks “why didn’t you address X?” where X is usually that person’s pet topic. Usually I could do so, but not in the time allotted.]
– so you may appreciate my exasperation at being accused of dishonesty by someone whose complaint is so predictable that I’ve complained before about people who make this complaint. I’m only human – I can’t cover all subjects for all audiences every time all the time. Moreover, I do tend to choose to discuss subjects that may be news to an audience, not simply reprise the greatest hits they want to hear. Clayton obviously knows about the third peak; he doesn’t need to hear about it from me. This is the scientific equivalent of shouting Freebird!at a concert.
It isn’t like I haven’t talked about it. I have been rigorously honest about the CMB, and certainly have not omitted mention of the third peak. Here is a comment from February 2003 when the third peak was only tentatively detected:
Page et al. (2003) do not offer a WMAP measurement of the third peak. They do quote a compilation of other experiments by Wang et al. (2003). Taking this number at face value, the second to third peak amplitude ratio is A2:3 = 1.03 +/- 0.20. The LCDM expectation value for this quantity was 1.1, while the No-CDM expectation was 1.9. By this measure, LCDM is clearly preferable, in contradiction to the better measured first-to-second peak ratio.
the Boomerang data and the last credible point in the 3-year WMAP data both have power that is clearly in excess of the no-CDM prediction. The most natural interpretation of this observation is forcing by a mass component that does not interact with photons, such as non-baryonic cold dark matter.
There are lots like this, including my review for CJP and this talk given at KITP where I had been asked to explicitly take the side of MOND in a debate format for an audience of largely particle physicists. The CMB, including the third peak, appears on the fourth slide, which is right up front, not being elided at all. In the first slide, I tried to encapsulate the attitudes of both sides:
I did the same at a meeting in Stony Brook where I got a weird vibe from the audience; they seemed to think I was lying about the history of the second peak that I recount above. It will be hard to agree on an interpretation if we can’t agree on documented historical facts.
More recently, this image appears on slide 9 of this lecture from the cosmology course I just taught (Fall 2022):
I recognize this slide from talks I’ve given over the past five plus years; this class is the most recent place I’ve used it, not the first. On some occasions I wrote “The 3rd peak is the best evidence for CDM.” I do not recall which all talks I used this in; many of them were likely colloquia for physics departments where one has more time to cover things than in a typical conference talk. Regardless, these apparently were not the talks that Clayton attended. Rather than it being the case that I never address this subject, the more conservative interpretation of the experience he relates would be that I happened not to address it in the small subset of talks that he happened to attend.
But do go off, dude: tell everyone how I never address this issue and evade questions about it.
I have been extraordinarily patient with this sort of thing, but I confess to a great deal of exasperation at the perpetual whataboutism that many scientists engage in. It is used reflexively to shut down discussion of alternatives: dark matter has to be right for this reason (here the CMB); nothing else matters (galaxy dynamics), so we should forbid discussion of MOND. Even if dark matter proves to be correct, the CMB is being used an excuse to not address the question of the century: why does MOND get so many predictions right? Any scientist with a decent physical intuition who takes the time to rub two brain cells together in contemplation of this question will realize that there is something important going on that simply invoking dark matter does not address.
In fairness to McGaugh, he pointed out some very interesting features of galactic DM distributions that do deserve answers. But it turns out that there are a plurality of possibilities, from complex DM physics (self interactions) to unmodelable SM physics (stellar feedback, galaxy-galaxy interactions). There are no such alternatives to CDM to explain the CMB power spectrum.
Thanks. This is nice, and why I say it would be easier to just pretend to never have heard of MOND. Indeed, this succinctly describes the trajectory I was on before I became aware of MOND. I would prefer to be recognized for my own work – of whichthereisplenty – than an association with a theory that is not my own – an association that is born of honestly reporting a surprising observation. I find my reception to be more favorable if I just talk about the data, but what is the point of taking data if we don’t test the hypotheses?
I have gone to great extremes to consider all the possibilities. There is not a plurality of viable possibilities; most of these things do not work. The specific ideas that are cited here are known not work. SIDM apears to work because it has more free parameters than are required to describe the data. This is a common failing of dark matter models that simply fit some functional form to observed rotation curves. They can be made to fit the data, but they cannot be used to predict the way MOND can.
Feedback is even worse. Never mind the details of specific feedback models, and think about what is being said here: the observations are to be explained by “unmodelable [standard model] physics.” This is a way of saying that dark matter claims to explain the phenomena while declining to make a prediction. Don’t worry – it’ll work out! How can that be considered better than or even equivalent to MOND when many of the problems we invoke feedback to solve are caused by the predictions of MOND coming true? We’re just invoking unmodelable physics as a deus ex machina to make dark matter models look like something they are not. Are physicists straight-up asserting that it is better to have a theory that is unmodelable than one that makes predictions that come true?
Returning to the CMB, are there no “alternatives to CDM to explain the CMB power spectrum”? I certainly do not know how to explain the third peak with the No-CDM ansatz. For that we need a relativistic theory, like Beklenstein‘s TeVeS. This initially seemed promising, as it solved the long-standing problem of gravitational lensing in MOND. However, it quickly became clear that it did not work for the CMB. Nevertheless, I learned from this that there could be more to the CMB oscillations than allowed by the simple No-CDM ansatz. The scalar field (an entity theorists love to introduce) in TeVeS-like theories could play a role analogous to cold dark matter in the oscillation equations. That means that what I thought was a killer argument against MOND – the exact same argument Clayton is making – is not as absolute as I had thought.
Writing down a new relativistic theory is not trivial. It is not what I do. I am an observational astronomer. I only play at theory when I can’t get telescope time.
Comic from the Far Side by Gary Larson.
So in the mid-00’s, I decided to let theorists do theory and started the first steps in what would ultimately become the SPARC database (it took a decade and a lot of effort by Jim Schombert and Federico Lelli in addition to myself). On the theoretical side, it also took a long time to make progress because it is a hard problem. Thanks to work by Skordis & Zlosnik on a theory they [now] call AeST8, it is possible to fit the acoustic power spectrum of the CMB:
I consider this to be a demonstration, not necessarily the last word on the correct theory, but hopefully an iteration towards one. The point here is that it is possible to fit the CMB. That’s all that matters for our current discussion: contrary to the steady insistence of cosmologists over the past 15 years, CDM is not the only way to fit the CMB. There may be other possibilities that we have yet to figure out. Perhaps even a plurality of possibilities. This is hard work and to make progress we need a critical mass of people contributing to the effort, not shouting rubbish from the peanut gallery.
As I’ve done before, I like to take the language used in favor of dark matter, and see if it also fits when I put on a MOND hat:
As a galaxy dynamicist, let me just remind everyone that the primary reason to believe in MOND as a physical theory and not some curious dark matter phenomenology is the very high precision with which MOND predicts, a priori, the dynamics of low-acceleration systems, especially low surface brightness galaxies whose kinematics were practically unknown at the time of its inception. There is a stone-cold, quantitative, crystal clear prediction of MOND that the kinematics of galaxies follows uniquely from their observed baryon distributions. This is something CDM profoundly and irremediably gets wrong: it predicts that the dark matter halo should have a central cusp9 that is not observed, and makes no prediction at all for the baryon distribution, let alone does it account for the detailed correspondence between bumps and wiggles in the baryon distribution and those in rotation curves. This is observed over and over again in hundreds upon hundreds of galaxies, each of which has its own unique mass distribution so that each and every individual case provides a distinct, independent test of the hypothesized force law. In contrast, CDM does not even attempt a comparable prediction: rather than enabling the real-world application to predict that this specific galaxy will have this particular rotation curve, it can only refer to the statistical properties of galaxy-like objects formed in numerical simulations that resemble real galaxies only in the abstract, and can never be used to directly predict the kinematics of a real galaxy in advance of the observation – an ability that has been demonstrated repeatedly by MOND. The simple fact that the simple formula of MOND is so repeatably correct in mapping what we see to what we get is to me the most convincing way to see that we need a grander theory that contains MOND and exactly MOND in the low acceleration limit, irrespective of the physical mechanism by which this is achieved.
That is stronger language than I would ordinarily permit myself. I do so entirely to show the danger of being so darn sure. I actually agree with clayton’s perspective in his quote; I’m just showing what it looks like if we adopt the same attitude with a different perspective. The problems pointed out for each theory are genuine, and the supposed solutions are not obviously viable (in either case). Sometimes I feel like we’re up the proverbial creek without a paddle. I do not know what the right answer is, and you should be skeptical of anyone who is sure that he does. Being sure is the sure road to stagnation.
1It may surprise some advocates of dark matter that I barely touch on MOND in this course, only getting to it at the end of the semester, if at all. It really is evidence-based, with a focus on the dynamical evidence as there is a lot more to this than seems to be appreciated by most physicists*. We also teach a course on cosmology, where students get the material that physicists seem to be more familiar with.
*I once had a colleague who was is a physics department ask how to deal with opposition to developing a course on galaxy dynamics. Apparently, some of the physicists there thought it was not a rigorous subject worthy of an entire semester course – an attitude that is all too common. I suggested that she pointedly drop the textbook of Binney & Tremaine on their desks. She reported back that this technique proved effective.
2I do not know who clayton is; that screen name does not suffice as an identifier. He claims to have been in contact with me at some point, which is certainly possible: I talk to a lot of people about these issues. He is welcome to contact me again, though he may wish to consider opening with an apology.
3One of the hardest realizations I ever had as a scientist was that both of the reasons (1) and (2) that I believed to absolutely require CDM assumed that gravity was normal. If one drops that assumption, as one must to contemplate MOND, then these reasons don’t require CDM so much as they highlight that something is very wrong with the universe. That something could be MOND instead of CDM, both of which are in the category of who ordered that?
4In the early days (late ’90s) when I first started asking why MOND gets any predictions right, one of the people I asked was Joe Silk. He dismissed the rotation curve fits of MOND as a fluke. There were 80 galaxies that had been fit at the time, which seemed like a lot of flukes. I mention this because one of the persistent myths of the subject is that MOND is somehow guaranteed to magically fit rotation curves. Erwin de Blok and I explicitly showed that this was not true in a 1998 paper.
5I sometimes hear cosmologists speak in awe of the thousands of observed CMB modes that are fit by half a dozen LCDM parameters. This is impressive, but we’re fitting a damped and driven oscillation – those thousands of modes are not all physically independent. Moreover, as can be seen in the figure from Dodelson & Hu, some free parameters provide more flexibility than others: there is plenty of flexibility in a model with dark matter to fit the CMB data. Only with the Planck data do minortensions arise, the reaction to which is generally to add more free parameters, like decoupling the primordial helium abundance from that of deuterium, which is anathema to standard BBN so is sometimes portrayed as exciting, potentially new physics.
For some reason, I never hear the same people speak in equal awe of the hundreds of galaxy rotation curves that can be fit by MOND with a universal acceleration scale and a single physical free parameter, the mass-to-light ratio. Such fits are over-constrained, and every single galaxy is an independent test. Indeed, MOND can predict rotation curves parameter-free in cases where gas dominates so that the stellar mass-to-light ratio is irrelevant.
How should we weigh the relative merit of these very different lines of evidence?
6On a number of memorable occasions, people shouted “No you didn’t!” On smaller number of those occasions (exactly two), they bothered to look up the prediction in the literature and then wrote to apologize and agree that I had indeed predicted that.
7If you read this paper, part of what you will see is me being confused about how low surface brightness galaxies could adhere so tightly to the Tully-Fisher relation. They should not. In retrospect, one can see that this was a MOND prediction coming true, but at the time I didn’t know about that; all I could see was that the result made no sense in the conventional dark matter picture.
Some while after we published that paper, Bob Sanders, who was at the same institute as my collaborators, related to me that Milgrom had written to him and asked “Do you know these guys?”
8Initially they had called it RelMOND, or just RMOND. AeST stands for Aether-Scalar-Tensor, and is clearly a step along the lines that Bekenstein made with TeVeS.
In addition to fitting the CMB, AeST retains the virtues of TeVeS in terms of providing a lensing signal consistent with the kinematics. However, it is not obvious that it works in detail – Tobias Mistele has a brand new paper testing it, and it doesn’t look good at extremely low accelerations. With that caveat, it significantly outperforms extant dark matter models.
There is an oft-repeated fallacy that comes up any time a MOND-related theory has a problem: “MOND doesn’t work therefore it has to be dark matter.” This only ever seems to hold when you don’t bother to check what dark matter predicts. In this case, we should but don’t detect the edge of dark matter halos at higher accelerations than where AeST runs into trouble.
9Another question I’ve posed for over a quarter century now is what would falsify CDM? The first person to give a straight answer to this question was Simon White, who said that cusps in dark matter halos were an ironclad prediction; they had to be there. Many years later, it is clear that they are not, but does anyone still believe this is an ironclad prediction? If it is, then CDM is already falsified. If it is not, then what would be? It seems like the paradigm can fit any surprising result, no matter how unlikely a priori. This is not a strength, it is a weakness. We can, and do, add epicycle upon epicycle to save the phenomenon. This has been my concern for CDM for a long time now: not that it gets some predictions wrong, but that it can apparently never get a prediction so wrong that we can’t patch it up, so we can never come to doubt it if it happens to be wrong.
That’s the question of the year, and perhaps of the century. I’ve been asking it since before this century began, and I have yet to hear a satisfactory answer. Most of the relevant scientific community has aggressively failed to engage with it. Even if MOND is wrong for [insert favorite reason], this does not relieve us of the burden to understand why it gets many predictions right – predictions that have repeatedly come as a surprise to the community that has declined to engage, preferring to ignore the elephant in the room.
It is not good enough to explain MOND phenomenology post facto with some contrived LCDM model. That’s mostly1 what is on offer, being born of the attitude that we’re sure LCDM is right, so somehow MOND phenomenology must emerge from it. We could just as [un]reasonably adopt the attitude that MOND is correct, so surely LCDM phenomenology happens as a result of trying to fit the standard cosmological model to some deeper, subtly different theory.
A basic tenet of the scientific method is that if a theory has its predictions come true, we are obliged to acknowledge its efficacy. This is how we know when to change our minds. This holds even if we don’t like said theory – especially if we don’t like it.
That was my experience with MOND. It correctly predicted the kinematics of the low surface brightness galaxies I was interested in. Dark matter did not. The data falsified all the models available at the time, including my own dark matter-based hypothesis. The only successful a priori predictions were those made by Milgrom. So what am I to conclude2 from this? That he was wrong?
I understand the reluctance to engage. It really ticked me off that my own model was falsified. How could this stupid theory of Milgrom’s do better for my galaxies? Indeed, how could it get anything right? I had no answer to this, nor does the wider community. It is not for lack of trying on my part; I’ve spent a lot of time3 building conventional dark matter models. They don’t work. Most of the models made by others that I’ve seen are just variations on models I had already considered and rejected as obviously unworkable. They might look workable from one angle, but they inevitably fail from some other, solving one problem at the expense of another.
Predictive success does not guarantee that a theory is right, but it does make it better than competing theories that fail for the same prediction. This is where MOND and LCDM are difficult to compare, as the relevant data are largely incommensurate. Where one is eloquent, the other tends to be muddled. However, it has been my experience that MOND more frequently reproduces the successes of dark matter than vice-versa. I expect this statement comes as a surprise to some, as it certainly did to me (see the comment line of astro-ph/9801102). The people who say the opposite clearly haven’t bothered to check2 as I have, or even to give MOND a real chance. If you come to a problem sure you know the answer, no data will change your mind. Hence:
A challenge: What would falsify the existence of dark matter?
If LCDM is a scientific theory, it should be falsifiable4. Dark matter, by itself, is a concept, not a theory: mass that is invisible. So how can we tell if it’s not there? Once we have convinced ourselves that the universe is full of invisible stuff that we can’t see or (so far) detect any other way, how do we disabuse ourselves of this notion, should it happen to be wrong? If it is correct, we can in principle find it in the lab, so its existence can be confirmed. But is it falsifiable? How?
That is my challenge to the dark matter community: what would convince you that the dark matter picture is wrong? Answers will vary, as it is up to each individual to decide for themself how to answer. But there has to be an answer. To leave this basic question unaddressed is to abandon the scientific method.
I’ll go first. Starting in 1985 when I was first presented evidence in a class taught by Scott Tremaine, I was as much of a believer in dark matter as anyone. I was even a vigorous advocate, for a time. What convinced me to first doubt the dark matter picture was the fine-tuning I had to engage in to salvage it. It was only after that experience that I realized that the problems I was encountering were caused by the data doing what MOND had predicted – something that really shouldn’t happen if dark matter is running the show. But the MOND part came after; I had already become dubious about dark matter in its own context.
Falsifiability is a question every scientist who works on dark matter needs to face. What would cause you to doubt the existence of dark matter? Nothing is not a scientific answer. Neither is it correct to assert that the evidence for dark matter is already overwhelming. That is a misstatement: the evidence for acceleration discrepancies is overwhelming, but these can be interpreted as evidence for either dark matter or MOND.
This important thing is to establish criteria by which you would change your mind. I changed my mind before: I am no longer convinced that the solution the acceleration discrepancy has to be non-baryonic dark matter. I will change my mind again if the evidence warrants. Let me state, yet again, what would cause me to doubt that MOND is a critical element of said solution. There are lots of possibilities, as MOND is readily falsifiable. Three important ones are:
MOND getting a fundamental prediction wrong;
Detecting dark matter;
Answering the question of the year.
None of these have happened yet. Just shouting MOND is falsified already! doesn’t make it so: the evidence has to be both clear and satisfactory. For example,
MOND might be falsified by cluster data, but it’s apparent failure is not fundamental. There is a residual missing mass problem in the richest clusters, but there’s nothing in MOND that says we have to have detected all the baryons by now. Indeed, LCDM doesn’t fare better, just differently, with both theories suffering a missing baryon problem. The chief difference is that we’re willing to give LCDM endless mulligans but MOND none at all. Where the problem for MOND in clusters comes up all the time, the analogous problem in LCDM is barely discussed, and is not even recognized as a problem.
A detection of dark matter would certainly help. To be satisfactory, it can’t be an isolated signal in a lone experiment that no one else can reproduce. If a new particle is detected, its properties have to be correct (e.g, it has the right mass density, etc.). As always, we must be wary of some standard model event masquerading as dark matter. WIMP detectors will soon reach the neutrino background accumulated from all the nuclear emissions of stars over the course of cosmic history, at which time they will start detecting weakly interacting particles as intended: neutrinos. Those aren’t the dark matter, but what are the odds that the first of those neutrino detections will be eagerly misinterpreted as dark matter?
Finally, the question of the year: why does MOND get any prediction right? To provide a satisfactory answer to this, one must come up with a physical model that provides a compelling explanation for the phenomena and has the same ability as MOND to make novel predictions. Just building a post-hoc model to match the data, which is the most common approach, doesn’t provide a satisfactory, let alone a compelling, explanation for the phenomenon, and provides no predictive power at all. If it did, we could have predicted MOND-like phenomenology and wouldn’t have to build these models after the fact.
So far, none of these three things have been clearly satisfied. The greatest danger to MOND comes from MOND itself: the residual mass discrepancy in clusters, the tension in Galactic data (some of which favor MOND, other of which don’t), and the apparent absence of dark matter in some galaxies. While these are real problems, they are also of the scale that is expected in the normal course of science: there are always tensions and misleading tidbits of information; I personally worry the most about the Galactic data. But even if my first point is satisfied and MOND fails on its own merits, that does not make dark matter better.
A large segment of the scientific community seems to suffer a common logical fallacy: any problem with MOND is seen as a success for dark matter. That’s silly. One has to evaluate the predictions of dark matter for the same observation to see how it fares. My experience has been that observations that are problematic for MOND are also problematic for dark matter. The latter often survives by not making a prediction at all, which is hardly a point in its favor.
Other situations are just plain weird. For example, it is popular these days to cite the absence of dark matter in some ultradiffuse galaxies as a challenge to MOND, which they are. But neither does it make sense to have galaxies without dark matter in a universe made of dark matter. Such a situation can be arranged, but the circumstances are rather contrived and usually involve some non-equilibrium dynamics. That’s fine; that can happen on rare occasions, but disequilibrium situations can happen in MOND too (the claims of falsification inevitably assume equilibrium). We can’t have it both ways, permitting special circumstances for one theory but not for the other. Worse, some examples of galaxies that are claimed to be devoid of dark matter are as much a problem for LCDM as for MOND. A disk galaxy devoid of either can’t happen; we need something to stabilize disks.
So where do we go from here? Who knows! There are fundamental questions that remain unanswered, and that’s a good thing. There is real science yet to be done. We can make progress if we stick to the scientific method. There is more to be done than measuring cosmological parameters to the sixth place of decimals. But we have to start by setting standards for falsification. If there is no observation or experimental result that would disabuse you of your current belief system, then that belief system is more akin to religion than to science.
1There are a few ideas, like superfluid dark matter, that try to automatically produce MOND phenomenology. This is what needs to happen. It isn’t clear yet whether these ideas work, but reproducing the MOND phenomenology naturally is a minimum standard that has to be met for a model to be viable. Run of the mill CDM models that invoke feedback do not meet this standard. They can always be made to reproduce the data once observed, but not to predict it in advance as MOND does.
2There is a common refrain that “MOND fits rotation curves and nothing else.” This is a myth, plain and simple. A good, old-fashioned falsehood sustained by the echo chamber effect. (That’s what I heard!) Seriously: if you are a scientist who thinks this, what is your source? Did it come from a review of MOND, or from idle chit-chat? How many MOND papers have you read? What do you actually know about it? Ignorance is not a strong position from which to draw a scientific conclusion.
3Like most of the community, I have invested considerably more effort in dark matter than in MOND. Where I differ from much of the galaxy formation community* is in admitting when those efforts fail. There is a temptation to slap some lipstick on the dark matter pig and claim success just to go along to get along, but what is the point of science if that is what we do when we encounter an inconvenient result? For me, MOND has been an incredibly inconvenient result. I would love to be able to falsify it, but so far intellectual honesty forbids.
*There is a widespread ethos of toxic positivity in the galaxy formation literature, which habitually puts a more positive spin on results than is objectively warranted. I’m aware of at least one prominent school where students are taught “to be optimistic” and omit mention of caveats that might detract from the a model’s reception. This is effective in a careerist sense, but antithetical to the scientific endeavor.
4The word “falsification” carries a lot of philosophical baggage that I don’t care to get into here. The point is that there must be a way to tell if a theory is wrong. If there is not, we might as well be debating the number of angels that can dance on the head of a pin.
I would like to write something positive to close out the year. Apparently, it is not in my nature, as I am finding it difficult to do so. I try not to say anything if I can’t say anything nice, and as a consequence I have said little here for weeks at a time.
Still, there are good things that happened this year. JWST launched a year ago. The predictions I made for it at that time have since been realized. There have been some bumps along the way, with some of the photometric redshifts for very high z galaxies turning out to be wrong. They have not all turned out to be wrong, and the current consensus seems to be converging towards acceptance of there existing a good number of relatively bright galaxies at z > 10. Some of these have been ‘confirmed’ by spectroscopy.
I remain skeptical of some of the spectra as well as the photometric redshifts. There isn’t much spectrum to see at these rest frame ultraviolet wavelengths. There aren’t a lot of obvious, distinctive features in the spectra that make for definitive line identifications, and the universe is rather opaque to the UV photons blueward of the Lyman break. Here is an example from the JADES survey:
Images and spectra of z > 10 galaxy candidates from JADES. [Image Credits: NASA, ESA, CSA, M. Zamani (ESA/Webb), Leah Hustak (STScI); Science Credits: Brant Robertson (UC Santa Cruz), S. Tacchella (Cambridge), E. Curtis-Lake (UOH), S. Carniani (Scuola Normale Superiore), JADES Collaboration]
Despite the lack of distinctive spectral lines, there is a clear shape that is ramping up towards the blue until hitting a sharp edge. This is consistent with the spectrum of a star forming galaxy with young stars that make a lot of UV light: the upward bend is expected for such a population, and hard to explain otherwise. The edge is cause by opacity: intervening gas and dust gobbles up those photons, few of which are likely to even escape their host galaxy, much less survive the billions of light-years to be traversed between there-then and here-now. So I concur that the most obvious interpretation of these spectra is that of high-z galaxies even if we don’t have the satisfaction of seeing blatantly obvious emission lines like C IV or Mg II (ionized species of carbon and magnesium that are frequently seen in the spectra of quasars). [The obscure nomenclature dates back to nineteenth century laboratory spectroscopy. Mg I is neutral, Mg II singly ionized, C IV triply ionized.]
Even if we seem headed towards consensus on the reality of big galaxies at high redshift, the same cannot yet be said about their interpretation. This certainly came as a huge surprise to astronomers not me. The obvious interpretation is the theory that predicted this observation in advance, no?
Apparently not. Another predictable phenomenon is that people will self-gaslight themselves into believing that this was expected all along. I have been watching in real time as the community makes the transition from “there is nothing above redshift 7” (the prediction of LCDM contemporary with Bob Sanders’s MOND prediction that galaxy mass objects form by z=10) to “this was unexpected!” and is genuinely problematic to “Nah, we’re good.” This is the same trajectory I’ve seen the community take with the cusp-core problem, the missing satellite problem, the RAR, the existence of massive clusters of galaxies at surprisingly high redshift, etc., etc. A theory is only good to the extent that its predictions are not malleable enough to be made to fit any observation.
As I was trying to explain on twitter that individually high mass galaxies had not been expected in LCDM, someone popped into my feed to assert that they had multiple simulations with galaxies that massive. That certainly had not been the case all along, so this just tells me that LCDM doesn’t really make a prediction here that can’t be fudged (crank up the star formation efficiency!). This is worse than no prediction at all: you can never know that you’re wrong, as you can fix any failing. Worse, it has been my experience that there is always someone willing to play the role of fixer, usually some ambitious young person eager to gain credit for saving the most favored theory. It works – I can point to many Ivy league careers that followed this approach. They don’t even have to work hard at it, as the community is predisposed to believe what they want to hear.
These are all reasons why predictions made in advance of the relevant observation are the most valuable.
That MOND has consistently predicted, in advance, results that were surprising to LCDM is a fact that the community apparently remains unaware of. Communication is inefficient, so for a long time I thought this sufficed as an explanation. That is no longer the case; the only explanation that fits the sociological observations is that the ignorance is willful.
“It is difficult to get a man to understand something, when his salary depends on his not understanding it.”
Upton Sinclair
We have been spoiled. The last 400 years has given us the impression that science progresses steadily and irresistibly forward. This is in no way guaranteed. Science progresses in fits and starts; it only looks continuous when the highlights are viewed in retrospective soft focus. Progress can halt and even regress, as happened abruptly with the many engineering feats of the Romans with the fall of their empire. Science is a human endeavor subject to human folly, and we might just as easily have a thousand years of belief in invisible mass as we did in epicycles.
Despite all this, I remain guardedly optimistic that we can and will progress. I don’t know what the right answer is. The first step is to let go of being sure that we do.
I’ll end with a quote pointed out to me by David Merritt that seems to apply today as it did centuries ago:
“The scepticism of that generation was the most uncompromising that the world has known; for it did not even trouble to deny: it simply ignored. It presented a blank wall of perfect indifference alike to the mysteries of the universe and to the solutions of them.”
It has been two months since my last post. Sorry for the extended silence, but I do have a real job. It is not coincidental that my last post precedes the start of the semester. It has been the best of semesters, but mostly the worst of semesters.
On the positive side, I’m teaching our upper level cosmology course. The students are great, really interested and interactive. Interest has always run high, going back to the first time I taught it (in 1999) as a graduate course at the University of Maryland. Aficionados of web history may marvel at the old course website, which was one of the first of its kind, as was the class – prior to that, graduate level cosmology was often taught as part of extragalactic astronomy. Being a new member of the faculty, it was an obvious gap to fill. I also remember with bemusement receiving Mike A’Hearn (comet expert and PI of Deep Impact) as an envoy from the serious-minded planetary scientists, who wondered if there was enough legitimate substance to the historically flaky subject of cosmology to teach a full three credit graduate course on the subject. Being both an expert and a skeptic, it was easy to reassure him: yes.
That class was large for a graduate level course, being taken in equal numbers by both astronomy and physics students. The astronomers were shocked and horrified that I went so deeply into the background theory to frame the course from the outset, and frequently asked “what’s a metric?” while the physicists loved that part. When we got to observational constraints, you could see the astronomers’ eyes glaze – not the distance scale again – while the physicists desperately asked “what’s a distance modulus?” This dichotomy persists.
This semester’s course is the largest it has ever been, up 70% from previous already-large enrollments. This is consistent with the explosive growth of the field. Interest in the field has never been higher. The number of astronomy majors has doubled over the past decade, having doubled already in the preceding decade.
That’s the good news. The bad news is that over the past four years, our department has been allowed to whither. In 2018, we were the smallest astronomy department in the country, with five tenured professors and an observatory manager who functioned as research faculty. The inevitable retirements that we had warned our administration were coming arrived, and we were allowed to fall off the demographic cliff (a common problem here and at many institutions). Despite the clear demand and the depth, breadth, and diversity of the available talent pool, the only faculty hire we have made in the past decade was an instructor (a rank that differs from a professor in having no research obligations), so now we are a department of two tenured professors and one instructor. I thought we were already small! It boggles the mind when you realize that the three of us are obliged to cover literally the entire universe in our curriculum.
Though always a small department, we managed. Now we don’t manage so much as cling to the edge of the cliff by our fingernails. We can barely cover the required courses for our majors. During the peak of concern about the Covid pandemic, we Chairs were asked to provide a plan for covering courses should one or some of our faculty become ill for an extended period. What a joke. The only “plan” I could offer was “don’t get sick.”
We did at least get along, which is not the case with faculty in all departments. The only minor tension we sometimes encountered was the distribution of research students. A Capstone (basically a senior thesis) is required here, and some faculty wound up with a higher supervisory load than others. That is baked-in now, as we have fewer faculty but more students to supervise.
We have reached a breaking point. The only way to address the problems we face is to hire new faculty. So the solution proffered by the dean is to merge our department into Physics.
Regardless of any other pros and cons, a merger does nothing to address the fundamental problem: we need astronomers to teach the astronomy curriculum. We need astronomers to conduct astronomy research, and to have a critical mass for a viable research community. In short, we need astronomers to do astronomy.
I have been Chair of the CWRU Department of Astronomy for over seven years now. Prof. Mihos served in this capacity for six years before that. No sane faculty member wants to be Chair; it is a service obligation we take on because there are tasks that need doing to serve our students and enable our research. Though necessary, these tasks are a drain on the person doing them, and detract from our ability to help our students and conduct research. Having sustained the department for this long to be told we needn’t have bothered is a deep and profound betrayal. I did not come here to turn out the lights.
Triton Station 