I’m just back from the World Science Festival where I participated in the discussion shaking up the dark universe. It was an interesting experience, and mostly positive despite some offstage diva behavior. One thing it drove home to me was how hard it is to communicate across a gulf of different training and experience. Not just from scientists to the public, but between scientists.
At present, 100% of the evidence for dark matter is astronomical in nature. Perhaps 99% of the ideas for what the dark matter is originate from particle physics. (I reserve 1% for those who hold out hope for MACHOs or very cold molecular gas.) It is hard to explain these topics to the public – even the interested, well educated public – when there persists a huge gulf between these two sciences.
To be sure, there is a lot of common ground in fundamental physics, which is a basic requirement for both fields. Yet there is a huge gulf in field-specific knowledge. Astronomers typically have no more training in particle physics than an undergraduate course in quantum mechanics, and nobody in the field holds it against them if they did poorly in it. Particle physicists usually have no training in astronomy at all.
The emergence of the dark matter paradigm has led to an awkward partnership in which each field takes from the other what it needs without much question. Astronomers need dark matter – preferably cold, or at most lukewarm. Whether it is WIMPs or axions or sterile neutrinos or any of the host of other ideas that particle physics is all too eager to provide is largely irrelevant. Particle physics is simply a black box that provides a source for the material with the right dynamical properties.
In the same manner, particle physicists hear there’s a need for dark matter and run with it. I have lost count of the number of physics seminars that I’ve attended that start with a single slide on the astronomical evidence – usually flat rotation curves plus clusters of galaxies and/or the cosmic microwave background – and launch from there into whatever form of particle dark matter the speaker wishes to advocate. On all of those occasions, I have been the only person in the room who has written refereed papers on all three subjects. Yet I frequently encounter a casual arrogance that, as an astronomer, I can’t possibly have a relevant opinion about the nature of dark matter. The widespread attitude is “Thanks for the data. We’ll take it form here.”
Flat rotation curves are only the beginning of the story. There is so much more. The amplitude of flat rotation correlates with observed mass (Tully-Fisher). The shapes of rotation curves varies systematically, even predictably, with the distribution of stars and gas. Galactic disks are composed of stars both ancient and recently formed, both of which are dynamically cold. There is a correlation between age and velocity dispersion, with stars heating as they age. But their orbits are still close to circular, placing constraints on halo triaxiality and merger histories that do not sit well with theory. Oh, the surprise theorists express when I point out that a pet idea they just came up with was excluded decades ago by data they’d never heard of.
They were off and running with “we need dark matter.” Everything else gets dismissed as “gastrophysics:” needless complexity irrelevant to fundamental physics. Sometimes that is true – there is nothing fundamental about how supernova return energy to the interstellar medium. Sometimes it is false – there certainly is something fundamental about the coupling between mass and light in spiral galaxies.
I suspect many professional particle physicists who read the last sentence will not know what I’m talking about. That is exactly the problem. Trying to solve the mass discrepancy problem in ignorance of basic facts is like playing solitaire without a full deck of cards.