36 research outputs found
On the growth of structure in theories with a dynamical preferred frame
We study the cosmological stability of a class of theories with a dynamical
preferred frame. For a range of actions, we find cosmological solutions which
are compatible with observations of the recent history of the Universe: a
matter dominated era followed by accelerated expansion. We then study the
evolution of linear perturbations on these backgrounds and find conditions on
the parameters of the theory which allow for the growth of structure sourced by
the new degrees of freedom
The Vector-Tensor nature of Bekenstein's relativistic theory of Modified Gravity
Bekenstein's theory of relativistic gravity is conventionally written as a
bi-metric theory. The two metrics are related by a disformal transformation
defined by a dynamical vector field and a scalar field. In this comment we show
that the theory can be re-written as Vector-Tensor theory akin to
Einstein-Aether theories with non-canonical kinetic terms. We discuss some of
the implications of this equivalence.Comment: Updated version: Notation cleaned up and some typos corrected-TG
Vector field models of modified gravity and the dark sector
We present a comprehensive investigation of cosmological constraints on the
class of vector field formulations of modified gravity called Generalized
Einstein-Aether models. Using linear perturbation theory we generate cosmic
microwave background and large-scale structure spectra for general parameters
of the theory, and then constrain them in various ways. We investigate two
parameter regimes: a dark-matter candidate where the vector field sources
structure formation, and a dark-energy candidate where it causes late-time
acceleration. We find that the dark matter candidate does not fit the data, and
identify five physical problems that can restrict this and other theories of
dark matter. The dark energy candidate does fit the data, and we constrain its
fundamental parameters; most notably we find that the theory's kinetic index
parameter can differ significantly from its CDM
value.Comment: 16 pages, 11 figure