Knocking on Heaven's Door - Lisa Randall [181]
[ FIGURE 78 ] Dark matter searches take a three-pronged approach. Underground detectors look for dark matter directly hitting target nuclei. The LHC might create dark matter that leaves evidence in its experimental apparatuses. And satellites or telescopes might find evidence of dark matter annihilating and producing visible matter out in space.
TRANSPARENT MATTER
We know the density of dark matter, that it is cold (which is to say, it moves slowly relative to the speed of light), and that it interacts at most extremely weakly—certainly with no significant interaction with light. And that’s about it. Dark matter is transparent. We don’t know its mass, if it has any non-gravitational interactions, or how it was created in the early universe. We know its average density, but there could be one proton mass per cubic centimeter in our galaxy or there could be one thousand trillion times the proton mass stored in a compact object that is distributed throughout the universe every kilometer cubed. Either gives the same average dark matter density, and either could have seeded the formation of structure.
So although we know it’s out there, we don’t yet know the nature of dark matter. It could be small black holes or objects from other dimensions. Most likely, it is simply a new elementary particle that doesn’t have the usual Standard Model interactions—perhaps a stable neutral remnant of a soon-to-be-discovered physical theory that will appear at the weak mass scale. Even if that’s the case, we would want to know what the properties of the dark matter particle are—its mass and its interactions and if it is part of some such larger sector of new particles.
One reason the elementary particle interpretation is currently favored is the point alluded to above—the abundance of dark matter, the fraction of energy it carries—supports this hypothesis. The surprising fact is that a stable particle whose mass is roughly the weak energy scale that the LHC will explore (again via E = mc2) has a relic density today—the fraction of energy stored in the particles in the universe—in the right ballpark to be dark matter.
The logic goes as follows. As the universe evolved, the temperature decreased. Heavier particles that were abundant when the universe was hotter are much more dispersed in the later cooler universe since the energy at low temperature is insufficient to create them. Once the temperature dropped sufficiently, heavy particles efficiently annihilated with heavy antiparticles so that both of them disappeared, but the reverse process where they were created no longer occurred at any significant rate. Therefore, due to annihilation, the number density of heavy particles decreased very rapidly as the universe cooled down.
Of course, in order to annihilate, particles and antiparticles have to first find each other.69 But as their number decreased and they became more diffuse, this became less likely. As a consequence, particles annihilated less efficiently later in the universe’s evolution since it takes two of them in the same place to tango.
The result is that substantially more stable, weak-mass particles could remain today than a naive application of thermodynamics would suggest—at some point both particles and antiparticles became so dilute that they just couldn’t find and eliminate each other. How many particles are left today depends on the mass and the interactions of the putative dark matter