Staying with the Hubble Trouble

Abstract

This thesis investigates the tension between the expansion
rate, also known as the Hubble-Lemaître constant, H0 , measurements by
a number of independent early- and late-time observables.
In the first part, we consider an alternative to the standard theory of
gravity, the generalised Proca (GP) theory, which can potentially alleviate
the Hubble tension. Focusing on the GP Lagrangian at cubic order – the
Cubic Vector Galilen (CVG) model – we derive the simplified equations
for gravity and vector modes and implement them in a modified version
of the ECOSMOG N-body code and augmented it further with ray-tracing
modules taken from Ray-RAMSES. Accordingly, we conduct the first broad
simulation study of a cosmologies based on the CVG theory. They explore
the formation, evolution and clustering of dark matter based on matter,
halo, weak lensing and voids statistics.
In the second part, we attempt to answer whether systematic errors
in strong gravitational time delay measurements could partly explain
the Hubble tension. We quantify the impact of line-of-sight structures
on time-delay measurements and in turn, on the inferred value of H0 ,
and test the reliability of existing procedures for correcting for these
line-of-sight effects. In that pursuit we create realistic lightcones using
multiple lens plane ray-tracing to create a set of simulated strong lensing
systems that are derived from the CosmoDC2 semi-analytical extra-
galactic catalogue.