MC
5501

## Candidate

Amirhossein Dehghanizadeh | Applied Mathematics, University of Waterloo

## Title

The Art of Avoiding Singularities & Listening to the Dark Universe through Dark Sirens

## Abstract

This work has mainly two parts. The first part relates to early universe cosmology. The second part relates to the late time cosmology from the viewpoint of observational gravitational-waves (GW) cosmology.

In the first part, motivated by addressing the singularity problem, we review a recent model of the early universe, called Cuscuton bounce. This model utilizes a theory of modified gravity by the same name, i.e. Cuscuton, which was originally proposed as a dark-energy candidate, to produce a bouncing cosmology. It has been shown that within Cuscuton model, we can have a regular bounce without violation of the null energy condition which is a common problem in most bouncing-cosmology models. In addition, the perturbations do not show any instabilities and with the help of a spectator field can generate a scale-invariant scalar power spectrum.

We will then set out to investigate if this model has strong coupling problem or any distinguishing and detectable signatures for non-Gaussianities. We expand the action to third-order and obtain all the interaction terms that can generate non-Gaussianities or potentially lead to a strong coupling problem (breakdown of the perturbation theory). While we do not expect the breakdown of the theory, any distinct and detectable sign of non-Gaussianities would provide an exciting opportunity to test the model with upcoming cosmological observations over the next decade.

In the second part, we first propose a new statistical approach for measuring the Hubble constant using the cross-correlation of galaxies and Gravitational Wave (GW) sources. We perform some numerical tests incorporating Bayestar which is a package developed for simulating GW observations. In addition, we develop a thorough framework for the theoretical prediction of the GW bias parameter for different choices of galaxy properties. This framework allows us to test the impact of different GW formation channels on the GW bias parameters. We include different astrophysical processes that can affect the final distribution of GW sources to calculate their power spectrum. Our numerical packages can be used for both Spectroscopic redshift and Photometric redshift galaxy catalogues. We plan to publish our code for calculating GW bias parameter publicly.