#### Date of Award

2016

#### Degree Type

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Graduate Group

Physics & Astronomy

#### First Advisor

Mark Trodden

#### Abstract

In this thesis we describe theoretical approaches to the problem of cosmic acceleration in the early and late universe. The first approach we consider relies upon the modification of Einstein gravity by the inclusion of mass terms as well as couplings to higher-derivative scalar fields possessing generalized internal shift symmetries - the Galileons. The second half of the thesis is concerned with the quantum-mechanical consistency of a theory of the early universe known as the pseudo-conformal mechanism which, in contrast to inflation, relies not on the effects of gravity but on conformal field theory (CFT) dynamics.

It is possible to couple Dirac-Born-Infeld (DBI) scalars possessing generalized Galilean internal shift symmetries (Galileons) to nonlinear massive gravity in four dimensions, in such a manner that the interactions maintain the Galilean symmetry. Such a construction is of interest because it is not possible to couple such fields to massless General Relativity in the same way. Using tetrad techniques we show that this massive gravity-Galileon theory possesses a primary constraint necessary to ensure propagation with the correct number of degrees of freedom.

We study the background cosmology of this theory around cosmologically relevant spacetimes and find that, as in pure massive gravity, spatially flat solutions do not exist. Spatially open solutions do exist - consisting of a branch of self-accelerating solutions that are identical to those of pure massive gravity, and a new second branch of solutions which do not appear without the inclusion of Galileons. We study the propagating degrees of freedom of the massive gravity-Galileon theory around the self-accelerating solutions and identify the conditions necessary for the theory to remain free of ghost-like instabilities. We show that on the self-accelerating branch the kinetic terms for the vector and scalar modes of the massive graviton vanish, as in the case of pure massive gravity.

We conclude our exploration of massive gravity by considering the possibility of variable-mass massive gravity, where the fixed graviton mass is replaced by the expectation value of a rolling scalar field. We ask whether self-inflation can be driven by the self-accelerated branch of this theory, and we find that, while such solutions can exist for a short period, they cannot be sustained for a cosmologically useful time. Furthermore, we demonstrate that there generally exist future curvature singularities of the ``big brake" form in cosmological solutions to these theories.

In the second half of the thesis we construct the gravitational dual of the pseudo-conformal universe, a proposed alternative to inflation in which a CFT in nearly flat space develops a time dependent vacuum expectation value. Constructing this dual amounts to finding five-dimensional domain-wall spacetimes with anti-de Sitter asymptotics, for which the wall has the symmetries of four-dimensional de Sitter space. This holographically realizes the characteristic symmetry breaking pattern O(2,4) to O(1,4) of the pseudo-conformal universe. We present an explicit example with a massless scalar field, using holographic renormalization to obtain general expressions for the renormalized scalar and stress-tensor one-point functions. We discuss the relationship between these solutions and those of four-dimensional holographic CFTs with boundaries, which break O(2,4) to O(2,3).

Finally, we undertake a systematic study of one and two point functions of CFTs on spaces of maximal symmetry with and without boundaries and investigate their spectral representations. Integral transforms are found, relating the spectral decomposition to renormalized position space correlators. Several applications are presented, including the holographic boundary CFTs as well as spacelike boundary CFTs, which provide realizations of the pseudo-conformal universe.

#### Recommended Citation

Stokes, James, "Theoretical Studies of Cosmic Acceleration" (2016). *Publicly Accessible Penn Dissertations*. 2040.

https://repository.upenn.edu/edissertations/2040