Where to Look for Dark Matter? Part 1


What’s the Universe made of? At first glance, at the night sky, you’d see stars and planets and you might say “It’s made up of stars and planets and hydrogen and helium and other normal stuff.” Congrats. you just named less than 5% of the Universe.

So where’s everything else?

The Planck satellite looked at the Cosmic Microwave Background (CMB) in detail and found that the Universe today was made up of 4.9% normal matter, 26.8% was some mysterious dark matter, and 68.3% was dark energy. When we look around at the universe, we see less than 5% of it. (1) Needless to say, being told that they’re missing a big chunk of the Universe is annoying to astronomers.

There’s a strange link between the smallest realms of physics (subatomic particles) and the largest (cosmology). They put limits on each other. For example, there are three pairs of quarks (and their antiparticle complements): up, down; charm and strange; top and bottom. (2) Cosmologists tapped the particle physicists on the shoulder and said “Our calculations from the Big Bang tell us that a fourth pair of quarks would make the ratio of primordial hydrogen to helium ratio far different than what we can see. You can quit looking. There’s no more quarks.”

The particle physicists turned to the cosmologists and said “Oh, thanks! Well…we’ve got some subatomic particle theories that predict weakly interacting massive particles (WIMPS) that might be your ‘dark matter.’” See, cooperation works!

With the discovery of the Higgs Boson, the basic particles in the Standard Model of particle physics have all been found. There are a lot of reasons particle physicists expect that the Standard Model is not the complete story (and not just because they want jobs!). These theories (string theory, brane theory, etc.) produce even more particles that will be searched for when the Large Hadron Collider starts back up. Some of the theoretical particles (e.g. neutralinos, Lightest Kaluza-Klein particles, etc.) are excellent candidates for WIMPS. Many of them are their own antiparticle that can annihilate–and be detected. The rate equation for this reaction would be the product of the concentration of the reactants:


r is the reaction rate, k is the rate constant and [WIMP} is the concentration of WIMP particles. The rate of the reaction goes up with the square of the density (concentration). (3)  Areas that have collected more dark matter will have higher reaction rates. (4)

As the universe expanded, a point would be reached where the concentration of the WIMPS stopped most annihilations. As gravity magnified the pressure densities from the first instants, the concentrations could possibly rise to the point where annihilations would once again occur–and, with luck, be observable.

So where do we look–and, given that looking farther in space we are looking backward in time, when?

Next: Katherine Mack’s article discusses what we don’t know about dark matter–and how it will affect the search for dark matter.

  1. This article is about what little we know about dark matter. Scientists know even less about dark energy, and I know way less than they do. I’ll leave dark energy for another time, which might well be never. []
  2. They originally named the last two “truth” and “beauty,” but apparently that was a weirdness too far and The Powers That Be changed the to “top” and “bottom.” I liked “truth” and “beauty,” but like Pluto’s status as a dwarf planet, Nature and TPTB don’t care what I like. []
  3. As a biochemist, I’ve only measured reaction rates in solution. I find it helpful to state the obvious here that in this case a change in concentration is a change in density. []
  4. Note: Some particles have been proposed for dark matter candidates that are not self-annihilating. Some of these do come in particle and antiparticle form and might also annihilate. The rate equation would be similar, with concentrations for the particle and antiparticle being multiplied to a rate constant. Discussions about these particle/antiparticle dark matter seem to be less common, so I deleted a discussion I wrote about that situation. Perhaps I should have saved it. It’s not hard to recreate, and I could post it easily enough. []

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