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.

4 thoughts on “Some rethinking

  1. Stacy, I am waiting for your full report. Some things I am interested in whether these were discussed at the meeting or not.
    o Can things like Baryonic-Tully Fisher, Renzo’s rule, the fact that asymptotic centripetal accelerations of a variety of systems is constant(something written a lot about) be explained via Lambda cdm? More importantly , are people taking these observational results seriously. (I get the opposite impression from talking to people who are not aware of these intriguing observational hints
    o Last few years there has been a spurt of DM models with large DeBroglie wavelength (Different people have called these different names (such as
    wave dark matter, Flavor-mixed dark Matter, psi-Dark matter etc). Were these models discussed at this meeting and is this the new trend in dark matter business? or no one taking these ideas seriously?
    o After the LIGO detection is primordial black holes been re-considered?

    Kudos to Jim. Incidentally Stanley Deser (who overlapped as a postdoc with Einstein) at IAS, wrote a paper this month discussing the upper limit on graviton mass. Amazing.


  2. I’m sure I won’t be writing anything like a full report on the meeting. However, I’m happy to address the points you raise. Indeed, I hope to do so repeatedly in this forum. You pose many more legitimate questions than I can answer right now. One blog at a time!

    Can LCDM explain the observed acceleration-scale related phenomena in galaxies? Depends on who you ask. I’d say no and yes. No: it is not natural for a particular physical scale to appear in scale free CDM. Yes: there is sufficient freedom in galaxy formation models with feedback to fit many of the observations.

    If you find such equivocation unsatisfactory, so do I. But it does encapsulate where the field is at.

    There is a spectrum of how seriously people take these issues. Many professional cosmologists appear to remain completely clueless about them. Others are in outright reality denial. Many seem to be taking the problems seriously and are attempting to address them (often with feedback models). Still others consider them to be fundamental, and are exploring other explanations, some of which you allude to.

    A few of the newer dark matter models were discussed at the meeting, but not any of the specific ones you mention. The field has become rather fragmented. One idea that was discussed was dark matter as a superfluid, an idea advanced by Justin Khoury. This is one of a class of models that tries to address the observed galaxy phenomenology while preserving the perceived success of LCDM.

    There was also discussion of primordial black holes as the dark matter. Mark Kamionkowski pointed out that if you built CDM halos with 30 solar mass black holes (perfectly acceptable to cosmological data, provided you make them quickly so as to sequester the baryons before BBN) and integrated the predicted halo mass spectrum down to tiny halos (500 solar masses), then you could produce about the right number of binaries through random collisions to give the LIGO signal. While this is a really cool idea, and I’m glad someone has worked it out, they did pose the title of their paper as a question: “Did LIGO detect dark matter?” Titles of astrophysical papers posed as questions are almost always answered in the negative.


    1. Thanks, Stacy. Btw I have one more pedantic question (sorry if this is too dumb). But what exactly is a “dark matter halo” in our real universe? (not in simulations which I understand) More specifically what is the volume (in space density) of a dark matter halo of a given mass (assuming DM mass is 100 GeV, which of course we do not know, but let’s pick an example). Or what is the average separation between two dark matter particles in a given DM halo? what determines the boundary of a DM halo (or is there an upper limit on no of DM particles in a given DM halo can have)? Also what is the average separation between 2 dark matter haloes?
      Sorry if these are too naive questions. But I have never seen these addressed or know of a paper where these things are discussed.


  3. There is no well defined boundary to dark matter halos. They seem to just keep going.
    The separation between particles, on the other hand, is well defined by the mass density required to fit observations. Locally that density is around 0.009 solar mass per square parsec. For a 100 GeV WIMP, there are a few hundred passing through your body at any given moment, so separations of order cm. The density declines steeply with radius and is thought to just fade into the background average.


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