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.