So I’ve been hanging out at the Indirect and Direct Detection of Dark Matter conference this week, and been struck by several things.
It is a good meeting, enthusiastic crowd, definite excitement in the air.
It would have read much better if the title had been “Direct and Indirect Detection of Dark Matter”, but that would have been wrong…
This is not my usual turf, though I was on a couple of papers on the adiabatic contraction of collisionless matter during growth of supermassive black holes, which is a way to get very high dark matter densities (QHS95 and SHQ95).
Basically a “spike” in the density of dark matter develops around the black hole, if the growth of the black hole is suitably non-violent in a technical and plausible sort of way. And, actually, even if the black hole growth is violent the spike is still there, just not as sharp, and from a lower base density.
What really matters for the strength of the spike is what the “initial” distribution function of the dark matter is, “before” the black hole, if there is a “before”, and that goes back to cosmology and the formation of early structure. It also is tightly coupled to any dark matter physics.
One of the great hopes in dark matter indirect detection is that if cold dark matter is “simple” in some rigorous sense, and it has some internal physics, then there might be (self)annihilation of the dark matter.
This could show up as a gamma ray signature, somewhere up in the multi-GeV energy range:
simulation of possible γ-ray emission signature of annihilating CDM from
Article in Physics World
This depends on the dark matter having some finite cross-section for interaction, and a finite branching ration via Z0, or some such, to photon pair emission.
The simplest such physics have the γ-ray signal proportional to the density of the dark matter squared, so the spikes in dark matter density around the supermassive black holes can make a big difference in the prospect for detection. Many orders of magnitude stronger signals.
There is a lot of assumptions here, but it makes for fun and easy models to play with, which provide formal constraints on CDM mass, cross-sections and branching ratios, as detection limits get squeezed.
And, there could be actual detection for not-unreasonable simple CDM candidates…
So… what I have learned is:
1) CDM modelers do not generally have good perspective on which astronomical observations are robust to more than one signficant figure, if that.
2) CDM modelers like to take wildly optimistic scenarios for the astrophysical processes to optimize their estimates for likelihood of detection
3) If you do a lot of experiments looking for detections, including post hoc analysis of archival data, then you get a lot of 2.x σ signals.
Any one of which might be interesting individually, but are collectively clearly noise.
4) There is actually reasonably prospect for either indirect or direct detection of cold dark matter in the finite future, possibly within graduate student lifetimes, if nature is kind and CDM is relatively simple with no hidden nastinesses which forbid detection.
I now want to revisit some of the crazier ideas I had about CDM in years past which got blown away as too wild back when…
I still think the True Insight is
hexapodia low surface density dwarf galaxies, but then I have always had a peculiar fondness for these.
PS: Fermi really is unreasonably good at what it does.
I am sooooooooooooooo jealous.