The known unknowns of dark matter annihilation

One of cosmology’s most persistent mysteries is that of the nature of dark matter – the invisible matter that holds galaxies together. The leading theory is that dark matter is made of an as-yet undiscovered fundamental particle, with the strange property that it is its own antiparticle. That means that pairs of dark matter particles can annihilate, leaving behind radiation or high-energy particles that we can potentially detect. Searches for this radiation in nearby galaxies and in the center of our own galaxy make up a major part of the effort to finally identify dark matter.

While gamma ray telescopes and cosmic ray detectors search for the products of dark matter annihilation in the local Universe, CAASTRO Affiliate Dr Katherine Mack (University of Melbourne) considered how dark matter annihilation could affect the formation of the first stars and galaxies in the Universe in her recent publication. She asked the question, “When is dark matter annihilation at its peak?”

In the early Universe, dark matter begins to come together via gravity to form “halos” – quasi-spherical clumps in which galaxies and clusters form. The annihilation rate depends on the square of the halo density so a high-density halo has a much higher rate of annihilation than a low-density halo. Over time, more halos form and become denser while, at the same time, the Universe expands and gets less dense. The question then becomes, “Is the annihilation rate higher at early times when the Universe is denser, or at late times when halos are more common and denser?”

The answer, according to Dr Mack’s research, depends on the unknown properties of the dark matter halos. She shows that we need to understand the formation of dark matter halos better before we can accurately predict their effects on early stars and galaxies. Using halo parameters drawn from the literature, she found that uncertainties up to factors of 10,000 exist for the annihilation rate during the time of the first star formation. These uncertainties include the density distribution within the halos and the size of the smallest halos that form. They also make it hard to say when the “smooth” component of the annihilation, from dark matter distributed diffusely in the Universe, is overtaken by the “clumpy” component from individual halos.

by K. Mack

Publication details:

Katherine J. Mack in MNRAS (2014) “Known Unknown of Dark Matter Annihilation over Cosmic Time”