Galaxy clusters are powerful probes of cosmology, since their abundance
as a function of redshift and mass depends both on the expansion and
growth of density fluctuations in the
Universe. They are the largest objects in the Universe that have reached
virial equilibrim (or not!), and can be observed in various frequencies,
exposing different aspects of the physical processes happening inside the
cluster constituents, such as galaxies and hot gas.
The main challenge in using clusters to constrain cosmology is that their
masses are not observed directly and need to be inferred from mass proxies,
such as their richness (number of galaxies), X-ray flux or temperature,
SZ flux and their lensing shear. One needs to determine the relationship
between mass and observables, including its redshift evolution, in order to use clusters for cosmology.
With Wayne Hu, I have studied self-calibration techniques, which allow one
to solve simultaneously for parameters of the mass-observable relation, as well as
for the cosmological parameters of interest. I have also investigated the
requirements on redshift precision in order to use self-calibration methods.
With Hiro Oyaizu, Fabian Schmidt and Wayne Hu, I have also looked at the
effect of modified gravity theories on properties of galaxy clusters,
such as their abundance and bias.