Some rethinking

So I’m back from this small, convivial meeting. Many thanks to hosts Priya Natarajan and Doug Finkbeiner for putting the program together. I find it especially useful when scientists working on the same problem from different fields come together in this fashion.  It provides fresh perspective.

I had wondered whether we were capable of genuine rethinking. The opening dinner brought up a wide ranging discussion of cartoon characters (you had to be there), which put me in mind of Lucy van Pelt’s quote from A Charlie Brown Christmas:


“The mere fact that you realize you need help indicates that you are not too far gone.”

This could be said of theories as well as people.  The predictable range of responses were on display – some of us really are too far gone – but I was encouraged that this was not typical, at least at this small gathering.

What I learned was that particle physics is complicated. Not that I didn’t know this, but in the context of dark matter models, things are rarely as clear cut as they are portrayed. For example, the constraints on dark matter from experiments at the LHC are often stated as hard limits, but these are based on very particular assumptions about how dark matter particles might be produced there. Since we don’t really know what the dark matter is (or even if it is really a particle and not some scalar field or GKW – God Knows What), there are a multiplicity of possibilities that are not quite so neatly described. Consequently, the hard limits are rarely that hard, once one drops the assumption of classic WIMP dark matter.

This is both good and bad. Good, in that there is indeed some rethinking to be done. Bad, in the sense that we might step into a bottomless pit. Which I suspect we’ve done already. We’ve already passed the natural cross section for WIMPs. Twice. The original prediction of 10-39 cm2 was falsified ages ago. The next natural cross section of 10-44 cm2 was crossed more recently. I was not alone in asking, when do we know to stop?

The next natural threshold is apparently 10-49 cm2. Around that level, there are second order loop processes that are unavoidable in any WIMP-like scenario. Or so the experts said. Something has to show up there. If not, we need something genuinely new. So that is when to stop with the current approach.

What `genuinely new’ might be is another matter. There was some encouraging rethinking on this point. But it still struck me as confined within traditional disciplinary boundaries. “We’re particle physicists, so we’ll make up a new particle.” I suspect we need to think outside this box.

Let me interrupt this rant to give a shout out to Jim Peebles, who showed up for this meeting on the eve of his 81st birthday. Still sharp as ever, he had lots of spot on questions for all the participants. Best of all, he gave a classic talk, to the effect of “yes, yes, we’ve solved all these large scale problems (many thanks to him!), but what about galaxies?” He showed actual pictures of all the bright, nearby galaxies listed by Tully, and went into some detail about how these did not really look much like what you’d expect in ΛCDM. A great theoretical cosmologist who looks at actual data and takes it seriously. The field could use more like him.

Rethinking the Dark Matter Paradigm

I travel to Cambridge, MA tomorrow to participate in the workshop Rethinking the Dark Matter Paradigm (I had nothing to do with the choice of title). I went to college at MIT in the ’80s, so is a bit back to the future for me in space as well as time. There is a lot to rethink, or nothing at all, depending on who you ask. I’m curious to see if any of us are willing to think beyond I was right all along!

One of the compelling notions that emerged in the ’80s was non-baryonic dark matter. Baryons are the massive particles (protons & neutrons) of which normal stuff is made. It was well established by that time that the light elements were produced in the early universe by Big Bang Nucleosynthesis (BBN). It became clear in the ’80s that the mass density of normal stuff produced by BBN did not add up to the mass we needed to explain a whole host of astronomical observations, in both cosmology and galaxy dynamics. In short, Einstein’s General Relativity plus the baryons we could see did not suffice to explain the universe.

There were two obvious paths forward. Modify Einstein’s theory, or invoke unseen non-baryonic matter. The latter course seems by far the more plausible. No one had a compelling reason to challenge Einstein’s highly successful theory. On the other hand, there were plenty of reasons in particle physics to imagine new particles outside the standard model, particularly in the hypothesized supersymmetric sector.

It was quickly realized that large scale structure would only grow if this new stuff were composed of slow moving, non-relativistic particles – a condition summarized as dynamically “cold.” Hence Cold Dark Matter (CDM) was born. Weakly Interacting Massive Particles (WIMPs) from supersymmetry were a good candidate to be the CDM.

Thus began the marriage of astronomy and particle physics, two fields divided by a common interest in dark matter and cosmology. The heated embrace of the honeymoon has long since worn off, to the point that some of us are ready to rethink the whole paradigm.

This is no small step. Though I’ve come to doubt the existence of CDM, I still feel very comfortable with it.  First love, and all. More importantly, it has been the one essential item in cosmology that has remained unchanged through the turbulent ’90s and on to today. But that is a longer story that will take many posts to tell.

For now, we’ll go see how much rethinking we’re willing to do.

Hello, World

Tell me, O muse, of that ingenious scientist who travelled far and wide after he had falsified the famous cosmology of Ptolemy. Many paradigms did he visit, and many were the theories with whose manners and customs he was acquainted; moreover he suffered much from the two body problem while trying to save the soul of science and raise his family safely at home. Tell me about all these things, O daughter of Jove, from whatsoever source you may know them.

Those familiar with the Classics will recognize the above text as a paraphrase of the opening lines of Homer’s Odyssey. It seems a fitting start to this blog, as my career in science has been a long voyage beset with many storms, complete with monsters worthy of mythology, both great and petty. I have, by chance of circumstance as much as choice of will, led an epic life.

This is a bold claim. Whether it is an accurate depiction of the stories I have to tell, I leave for others to judge. But I do have stories to tell. Many stories, such that it is impossible that they should all come out. Yet they clamor to be heard, and I find myself compelled to begin to tell them, at long last overcoming my discretion and better wisdom. Though in truth I have been at it since 1997, just not in blog form.

These words may sound odd as the preamble to a science blog. The reigning stereotype of a scientist is that of a dry arbiter of facts. This could not be more opposite the truth. We are passionate about the science we do. We care about the paradigms we develop, often much too deeply. We want them to be accurate depictions of the Truth, and all too often convince ourselves that they are.

This blog will cover many topics in science. Primarily it will focus on my own subjects of cosmology, astronomy, and astrophysics. These words mean different things to difference people, so the sociology of science will also be a frequent topic, as will the philosophy and history of science.

There is a tendency to oversimplify the history of science in order to satisfy the human need for a compelling story told in a short time. This serves an essential function: both our patience and our lifespans are finite. It is impossible to relate all the lessons of the past in their full detail. Yet sometimes the oversimplification inverts the truth, and scientists are as susceptible to this human foible as anyone.

I have no plan for how the stories will progress. They boil up, wanting to be told. I expect they will tumble out piecemeal, unstuck in time and devoid of linearity. Sometimes I will discuss current events. Sometimes I will relate what seem to be ancient anecdotes. In no circumstance will I dumb it down. Indeed, one thing I expect to do is write brief summaries of refereed science papers, which even scientists rarely manage read.

I am a practicing scientist. I am not a science journalist. I will attempt to be clear, but I am not trying to reach a mass audience nor explain things to the lowest common denominator. I make this distinction because I am what historians would call an original source.  I am not a reporter of science: I do it.