The recent detections of gravitational waves (GWs) from binary black holes (BBHs) from LIGO-Virgo and the latest data releases allow to open different major windows on our understanding of the universe, among which GW-based measurements of cosmological parameters, new tests of General Relativity (GR) at cosmological scales, and BBHs population studies. In this talk I will present some results in these directions based on the GWTC-2 catalogue and discuss the interplay among them.
In particular, cosmological tests are based on the fact that the luminosity distance is measured directly from the GW signal. By adding independent information on the redshift we can thus test the distance-redshift relation. Moreover, this is modified in presence of new physics at cosmological scales, providing a possibility to test GR.
I will discuss in particular two ways of obtaining the redshift information for ``dark sirens" (i.e., compact binary coalescences without an electromagnetic counterpart), namely using galaxy catalogues or information from the BBH mass distribution and rate evolution. Besides showing results for the measurement of the Hubble constant and the parameters that govern deviations from GR, I will be focussing on methodological aspects and relevant sources of systematics. I will finally discuss the fact that the two methods are tightly related, so population studies and cosmology have to proceed together. This is particularly relevant for modifications of gravity.
All the python code written to produce the joint population and cosmological analysis, as well as correlation with galaxy catalogues, is (or will shortly be) publicly available.