Simulation of what a galaxy and its surrounding satellites look like in dark matter. // EAGLE, Durham University
The American Astronomical Society (AAS) winter meeting is big enough that no matter your particular area of interest, there's sure to be something to grab your attention. If cosmology is your favorite topic, then you were in luck. There were talks ranging from inflation and parallel universes to the most recent Planck results (in case you're wondering, the universe is still flat). One highlight was the Royal Astronomical Society Gold Medal Talk, given by Carlos Frenk from the University of Durham on the identity of dark matter.
Astronomers have known for a long time that there's a missing matter problem. Fairly simple observations of galaxy rotation curves tell us this much. The most plausible cosmological models also predict large amounts of dark matter. The standard cosmological model is called ΛCDM but, as Frenk pointed out, "It's an implausible theory. It has a lambda, but we don't know what that is. It has CDM [cold dark matter], but we don't know what that is either." Still, Frenk doesn't advise giving up just yet. The lambda refers to dark energy, and Frenk leaves that question for another day. But CDM was put through the wringer. While ΛCDM provides a good fit — a stunning fit, in fact — to the observations we're able to make with instruments like Planck, it turns out it doesn't quite refute the potential for warm dark matter (WDM) either. The two predict similar enough results that we're not able to distinguish them from astronomical evidence, or so Frenk argued.
Hot dark matter (HDM) was also once a possibility, but cosmologists determined decades ago that simulations relying on HDM were completely unable to reproduce the universe we obviously live in. This is a bit of a shame because we at least knew what HDM was: neutrinos. We're not quite sure yet what CDM or WDM are exactly. But this is an astronomy conference, so we'll leave that bit to the particle physicists. What we can test with astronomy are the effects that CDM or WDM has on the largest scales of the universe.
Frenk identified four major challenges for ΛCDM, which mostly arise from discrepancies between what simulations of our universe predict based on ΛCDM and what observers actually measure. The details lie in the shapes of dwarf galaxies and in the distribution of satellite dwarf galaxies around galaxies like our Milky Way. The question, of course, is: Who is correct? Do observers always draw the correct conclusions about their data? Do simulations accurately recreate the universe we live in? Frenk implied a mixture of both. It appears that many differences can be explained by galaxy formation simulations not including enough details about how stellar feedback and gas dynamics operate to accurately portray the universe we observe. Conversely, other differences might be explained by observers making claims about galaxy shapes that Frenk argued could just as easily be described in a way that supports ΛCDM.
So ΛCDM conquered its challenges. It is the reigning theory for a reason. Unfortunately, Frenk argued that WDM passed just as successfully. So who wins? We'll have to wait a bit longer. Gravitational lensing could provide some answers about the existence of dark matter galaxies, and the Gaia mission will offer unprecedented detail on galaxy evolution by showing the motion of individual stars. Either would provide crucial observational evidence that could decide between CDM and WDM.
Or, Frenk warned, physicists could find the particle itself with experiments like CERN's Large Hadron Collider and answer all our questions at once. Of course we're all on the same side, looking for the same truth. But if someone has to get there first, don't you want it to be the astronomers?