Understand
Cosmological data can be used to search for---and characterize---light particles in the standard model, if these populate our Universe.
- In addition to the well-known effect of these light relics in the background cosmology, usually parametrized through a change in the effective number $N_{\rm eff}$ of neutrino species, these particles can become nonrelativistic at later times, affecting the growth of matter fluctuations due to their thermal velocities.
- An extensively studied example is that of massive neutrinos, which are known to produce a suppression in the matter power spectrum due to their free streaming.
- Galaxies, as biased traces of matter fluctuations, can therefore provide us with a wealth of information about both known and unknown degrees of freedom in the standard model.