Item

Abstract

We study the implications of a combined analysis of cosmic standard candles
and standard rulers on the viability of cosmological models beyond the
cosmological concordance model. To this end, we employ data in the form of the
joint light-curve analysis supernova compilation, baryon acoustic oscillations,
cosmic microwave background data, and a recently proposed set of Quasars as
objects of known brightness. The advantage of including the latter is that they
extend the local distance measures to redshifts which have previously been out
of reach and we investigate how this allows one to test cosmologies beyond
$\Lambda$CDM. We focus on two particular modifications: One is the theory of a
massive tensor field interacting with the standard metric of gravity, so-called
bigravity, and the other conformal gravity, a theory of gravity that has no
knowledge of fundamental length scales. The former of the two constitutes a
veritable extension of General Relativity, given that it adds to the metric
tensor of gravity a second dynamical tensor field. The resulting dynamics have
been proposed as a self-accelerating cosmology. Conformal gravity on the other
hand is a much more drastic change of the underlying gravitational theory. Its
ignorance towards fundamental length scales offers a completely different
approach to late time acceleration and the so-called cosmological constant
problem. In this sense, both models offer - in one way or another - an
explanation for the cosmological constant problem. We perform a combined
cosmological fit which provides strong constraints on some of these extensions,
while some alternative cosmologies are in fact favoured by the data. We also
briefly comment on the implications of the long-standing $H_0$-tension.