2.2 Dark Matter
What is Dark Matter?
- So far, we’ve been able to say what it isn’t: black holes, white dwarfs, faint stars, dust, or anything made of atoms.
- We’ve backed ourselves into a corner and we have to assume that Dark Matter is a new type of matter.
- We believe that Dark Matter is comprised of particles left over from the “quark soup” phase of cosmic evolution.
What are some potential candidates for Dark Matter?
- Interestingly, the new forms of matter weren’t invented by cosmologists, but rather by particle physicists trying to unify the particles and forces of nature.
- One of the hallmarks of string theory is supersymmetry, which implies that every particle we have seen has a supersymmetric partner particle with a different spin.
- One of these classes of particles is called the neutralino. Most of these supersymmetric particles decay very rapidly, but the neutralino lives a very long time. Remarkably, the number of neutralinos left from the quark soup phase would be about the right number to make up Dark Matter (under mild assumptions). If it exists, the neutralino would weigh approximately 100-300 times that of the proton.
- Another Dark Matter candidate is called the axion. If it exists, the axion would weigh a millionth of a millionth of what an electron weighs—there would have to be lots of them to account for Dark Matter. Again, there would be about the right number of these left over from the quark soup.
- The neutrino was once a candidate for Dark Matter before we knew how much they weighed, but now we know that there wouldn’t be enough of them for their mass to account for Dark Matter.
How can we find Dark Matter?
- Weigh it—This is how Dark Matter was first discovered by astronomers such as Zwicky and Rubin. We can “weigh” cosmic bodies by examining the gravitational forces and using gravitational lensing.
- Make it—Particle accelerators are in a sense “Dark Matter factories.” Candidate particles for Dark Matter have the potential to be created in accelerators.
- Catch it—Dark Matter particle candidates like the neutralino are extremely “shy” and difficult to detect. These detectors must be very large and shielded from the radiation of the Earth.
- Infer it—Occasionally, Dark Matter particles bump into each other and annihilate in our halo, producing positrons, neutrinos, and gamma rays, which are all easier to detect than the Dark Matter particles themselves.