Thursday, February 2, 2017

Thermal Relic Warm Dark Matter Can't Have A Mass Of Less Than 2 keV

The mass parameter space of one of the leading dark matter models remains in the same range where estimates from other means of estimated have put it. 
We study the substructure content of the strong gravitational lens RXJ1131-1231 through a forward modelling approach that relies on generating an extensive suite of realistic simulations. The statistics of the substructure population of halos depends on the properties of dark matter. We use a merger tree prescription that allows us to stochastically generate substructure populations whose properties depend on the dark matter particle mass. These synthetic halos are then used as lenses to produce realistic mock images that have the same features, e.g. luminous arcs, quasar positions, instrumental noise and PSF, as the data. By analysing the data and the simulations in the same way, we are able to constrain models of dark matter statistically using Approximate Bayesian Computing (ABC) techniques. This method relies on constructing summary statistics and distance measures that are sensitive to the signal being targeted. We find that using the HST data for \RXJ we are able to rule out a warm dark matter thermal relict mass below 2 keV at the 2 sigma confidence level.
Simon Birrer, Adam Amara, and Alexandre Refregier, "Lensing substructure quantification in RXJ1131-1231: A 2 keV lower bound on dark matter thermal relict mass" (January 31, 2017).

The bounds of an axion dark matter candidate's mass have also recently been significantly constrained. As previously mentioned, primordial black holes (or, more generally MACHOS) are also strongly disfavored by the evidence. Also, self-interacting dark mater has been pretty much ruled out, and the parameter space for a dark photon mass in SIDM theories has already been significantly restrained. And, WIMPS have been ruled out in the mass range from about 1 GeV to 10 TeV.

There are also strict limits on the rates at which dark matter particles of 10 GeV mass (or more) can annihilate.
Examination of the cosmic rays produced by a dwarf galaxy with an apparent high proportion of dark matter places strict limits on the dark matter annihilation cross-section and mean dark matter lifetime for dark matter candidates with 10 GeV or more of mass. 
The age of the universe is about 4.35*10^17 seconds (13.8 billion years). The minimum mean lifetime of dark matter with various assumptions given the observations made in this study is from 10^25 to 10^27 seconds. Thus, 99.999999% or more of the dark matter, if it exists and has 10 GeV or heavier particles, that ever in existence during the lifetime of the universe must still exist.
Planck data likewise confirm that there are strict limits on the rate at which dark matter can annihilate. High velocity galactic cluster impacts like the Bullet Cluster and El Gordo also disfavor Cold Dark Matter models. And, experimentally observations have likewise rruled out proton decay at a high probability.

While they were never much of a dark matter candidate, per se, the data increasingly rule out a light sterile neutrino that oscillates with ordinary active neutrinos. 

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