Anybody else tired of the science media jumping on every piece of evidence for Cold Dark Matter, and turning it into possible evidence for string theory, supersymmetry, and the multiverse. I wish that the scientific journalists at Scientific American and New Scientist thought critically about the physics news that they are reporting. How can a particle be heavier than a proton, but have no electric charge or strong nuclear 'charge'?
Cold Dark Matter is an oxymoron because the rest mass of a particle is related to its capability to interact with other particles and/or fields (especially the Higgs field.) Heavier particles have more interactions with other particles, whereas lighter particles have less interactions. Mass is proportional to the number and strength of the particles interactions with other particles. Saying the words "Cold dark matter" is like saying the words "Skinny fat people." It just doesn't make sense because a particle can't be both heavy in mass but light in interactions. (Note that this is also why I think that supersymmetry and any supersymmetric string theories are silly...if your theory invents new particles that are really heavy, but hardly interact with anything...such as gravitinos or neutralinos...then please throw your theory away and start from scratch. You are missing the whole point...the mass of a particle is proportional to its capability to interact and/or decay. Note that the same goes for a theories that predict 'sterile' neutrinos of GeV or TeV mass.)
But let's step back for a second, and ask the question: what are the implications of GeV dark matter?
In order to have GeV dark matter, you need to explain the following:
(1) Why there's no evidence for the GeV dark matter particles in any of the particle collider experiments? Why haven't we seen any of these particles when we collide together matter/anti-matter pairs with TeV of energy?
(2) Why doesn't the GeV dark matter just clump together at the center of galaxies? The reason that we invented the concept of dark matter was to explain the higher than expected velocity of stars on the outer-edge of galaxies (and of higher than expected velocity of entire galaxies rotating about each other.)
GeV cold dark matter would just clump together because there's nothing (except Fermi-Dirace statistics and perhaps the weak force) to keep the particles from clumping together into an extremely tight ball. The fact that the recent "evidence" for GeV dark matter is coming from GeV gamma ray emission only in the center of the galaxy is a tell-tale sign that it's not coming from dark matter, but rather that it's coming from objects with extreme temperatures.
(3) According to astrophysical observation, there's no spike in the density of dark matter in the center of galaxies. Dark matter is actually quite diffuse in galaxies, and even extends out past where there's no more stars. (see image below from the new movie Dark Universe.) So, why would there be spike in the GeV emission at the center of galaxies? (It's not due to dark matter collisions, or else it would be diffuse throughout the galaxy.)
So, let's get a few things clear about the terms: cold, warm, and hot dark matter. These terms seem to be thrown around a lot without much formal definition.
Hot dark matter is matter that moving close to the speed of light, i.e. that it's kinetic energy is comparative in size to or greater than its rest mass energy. If a group of particles has a temperature of a few Kelvin (like the temperature of the Cosmic Microwave Background), then this means that we could consider the particles to be "hot" if the rest mass of the particles is less than a few Kelvin (i.e. ~ 0.0001 eV.) If we take their temperature to be the temperature at which the Cosmic Microwave Background was formed, then we could consider the particles to be "hot" if the rest mass of the particles is less than the approximately 3,000 Kelvin (i.e. 0.3 eV.)
If neutrinos have masses on the order of 0.1 eV, then these neutrinos would be considered to be 'hot' early in the universe and 'warm' later in the universe.
Neutrinos just barely fit the definition of hot (depending on their actual mass and depending on their temperature.) A keV-rest-mass sterile neutrino (i.e. a neutrino that is right handed most of the time) would be considered 'warm' early on in the universe and be considered 'cold' now, because its rest mass-energy is much greater than its temperature. A GeV dark matter particle would be "cold" both now and back when the Cosmic Microwave Background radiation formed.
So, now let's look at what the astrophysical simulations are suggesting to us as far as the rest mass of dark matter. Below are two graphs that try to estimate the rest mass of dark matter particles. The first is from Horiuchi et al. and the second is from Schneider et al. In both graphs, it's clear that dark matter particles greater than 20 keV and less than 1 keV are highly ruled out. GeV particles are definitely ruled out.
So, my suggestion to the science media is to stop making any connection between dark matter and light emission in the center of the galaxy in the GeV scale. The two things are completely unrelated.
So, to conclude this post, I'll like to leave you all with a few lines from an article written in 2011 on warm dark matter, which can be found here.
"But astronomers have been saying for a long time that if dark matter were warm, this fit would be much better. Physicists have preferred cold dark matter because of the possibilities of detection; indeed, that’s where most of the efforts, including CDMS, XENON, Edelweiss, and the search at the LHC are sensitive."
Warm dark matter works! Warm dark matter . So if warm dark matter turns out to be the answer to our astronomy conundrums, what does this mean for the physics? It means that we’re not going to find dark matter where we’re looking right now. Perhaps it’s a sterile neutrino; a fourth family of neutrino that doesn’t couple to the other ones the way we’re accustomed to? Perhaps it’s a new type of particle that we haven’t considered before? Or perhaps it something like an axion, only they aren’t born either cold (like standard theory predicts) hot (like a thermal relic would be), but warm, due to a new type of interaction or coupling?