Final PhD Oral Examination (Thesis Title: “Cosmological Tests of Gravity”)

Speaker: 
ALI NARIMANI
Event Date and Time: 
Mon, 2016-07-25 09:00 - 11:00
Location: 
Room 200, Graduate Student Centre
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

Abstract:
General Relativity (GR) has long been acclaimed for its elegance and
simplicity, and has successfully passed many stringent observational tests since it
was introduced a century ago. However, there are two regimes in which the theory
has yet to be fully challenged. One of them is in the neighbourhood of very strong
gravitational fields, and the other is the behaviour of gravity on cosmological scales.
While strong field gravity has challenged theorists because of the desire to find
consistency between GR and quantum mechanics, cosmology has motivated
extensions to GR via the empirical discoveries of dark matter and dark energy.

In this thesis, we study a diverse range of modifications to GR and confront
them with observational data. We discuss how a generic theory of modified gravity
can be parameterized for studies within cosmology, and we introduce a general
parameterization that is simpler than those that have been previously considered.
This parameterization is then applied to investigate a specific theory, known as
``gravitational aether''. The gravitational aether theory was created to solve one of the
theoretical inconsistencies that exists between GR and quantum mechanics, namely
the fact that vacuum fluctuations appear not to gravitate. Cosmology is unique in
testing this theory, and we find that the gravitational aether solution is excluded when
all of the available cosmological data are combined. Nevertheless, a generalization of
this theory provides a consistent way to describe the strength of coupling between
pressure and gravity, and we present the most accurate measurements of this
coupling parameter.

In addition, we discuss the constraints that can be placed on modified gravity
models using the latest data from cosmic microwave background (CMB) anisotropies,
combined with several other probes of large-scale structure. Currently the most
accurate CMB anisotropy measurements come from the Planck 2015 CMB power
spectra, which we use, along with other cosmological data sets, to perform an
extensive study of modified theories of gravity. We find that GR remains the simplest
model that can explain all of the data. We end with a discussion of the prospects for
future experiments that can improve our understanding of gravity.

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