Observables In Cosmology: Three Astronomical Perspectives

TL;DR

This thesis explores three cosmological observables: conformal invariance of galaxy counts, polarization-based galaxy shape rotation for cosmic shear, and higher-order gravity theories affecting gravitational wave properties.

Contribution

It introduces conformal invariance of galaxy counts, proposes a polarization-based method to measure cosmic shear, and develops a higher-order gravity theory impacting gravitational wave characteristics.

Findings

Galaxy number counts are invariant under conformal transformations.

Polarization measurements can reveal cosmic shear effects.

Higher-order gravity theories modify gravitational wave speeds and frequencies.

Abstract

In this thesis, I present three projects I carried out during m PhD. In the first project, I introduce Conformal Transformations and the Galaxy Number County. I explicitly show that the Galaxy Number Counts is invariant under Conformal Transformations, which makes it a good physical observable. In the second project, I study how weak lensing, and in particular cosmic shear, affects the shape of the galaxy images. I show that, if the light polarisation is also measured, the rotation of the main axes of the elliptical galaxy shape becomes a cosmological observable. I show how this can be used to estimate cosmic shear and its correlation functions. In the third project, I define a higher order (Riemann-squared and -cubed) Lagrangian Effective Theory of Gravity. I compute the linear correction to the speed and the quasinormal frequencies of the gravitational waves in this theory around a Schwarzschild-like background.