134 research outputs found
Regularized braneworlds of arbitrary codimension
We consider a thick p-brane embedded in an n-dimensional spacetime possessing
radial symmetry in the directions orthogonal to the brane. We first consider a
static brane, and find a general fine tuning relationship between the brane and
bulk parameters required for the brane to be flat. We then consider the
cosmology of a time dependent brane in a static bulk, and find the Friedmann
equation for the brane scale factor a(t). The singularities that would
ordinarily arise when considering arbitrary codimensions are avoided by
regularizing the brane, giving it a finite profile in the transverse
dimensions. However, since we consider the brane to be a strictly local defect,
we find that the transverse dimensions must have infinite volume, and hence
gravity cannot be localized on the brane without resorting to some infra-red
cutoff.Comment: 21 page
Cluster Probes of Dark Energy Clustering
Cluster abundances are oddly insensitive to canonical early dark energy.
Early dark energy with sound speed equal to the speed of light cannot be
distinguished from a quintessence model with the equivalent expansion history
for but negligible early dark energy density, despite the different early
growth rate. However, cold early dark energy, with a sound speed much smaller
than the speed of light, can give a detectable signature. Combining cluster
abundances with cosmic microwave background power spectra can determine the
early dark energy fraction to 0.3 % and distinguish a true sound speed of 0.1
from 1 at 99 % confidence. We project constraints on early dark energy from the
Euclid cluster survey, as well as the Dark Energy Survey, using both current
and projected Planck CMB data, and assess the impact of cluster mass
systematics. We also quantify the importance of dark energy perturbations, and
the role of sound speed during a crossing of
Galileons on Trial
Galileon gravity is a robust theoretical alternative to general relativity
with a cosmological constant for explaining cosmic acceleration, with
interesting properties such as having second order field equations and a shift
symmetry. While either its predictions for the cosmic expansion or growth
histories can approach standard \Lambda CDM, we demonstrate the incompatibility
of both doing so simultaneously. Already current observational constraints can
severely disfavor an entire class of Galileon gravity models that do not couple
directly to matter, ruling them out as an alternative to \Lambda CDM.Comment: v2 matches JCAP versio
Parameterizing scalar-tensor theories for cosmological probes
We study the evolution of density perturbations for a class of models
which closely mimic CDM background cosmology. Using the quasi-static
approximation, and the fact that these models are equivalent to scalar-tensor
gravity, we write the modified Friedmann and cosmological perturbation
equations in terms of the mass of the scalar field. Using the perturbation
equations, we then derive an analytic expression for the growth parameter
in terms of , and use our result to reconstruct the linear matter
power spectrum. We find that the power spectrum at is characterized
by a tilt relative to its General Relativistic form, with increased power on
small scales. We discuss how one has to modify the standard, constant
prescription in order to study structure formation for this class of models.
Since is now scale and time dependent, both the amplitude and transfer
function associated with the linear matter power spectrum will be modified. We
suggest a simple parameterization for the mass of the scalar field, which
allows us to calculate the matter power spectrum for a broad class of
models
Modified Gravity: the CMB, Weak Lensing and General Parameterisations
We examine general physical parameterisations for viable gravitational models
in the framework. This is related to the mass of an additional scalar
field, called the scalaron, that is introduced by the theories. Using a simple
parameterisation for the scalaron mass we show there is an exact
correspondence between the model and popular parameterisations of the modified
Poisson equation and the ratio of the Newtonian potentials
. However, by comparing the aforementioned model against other
viable scalaron theories we highlight that the common form of and
in the literature does not accurately represent behaviour.
We subsequently construct an improved description for the scalaron mass (and
therefore and ) which captures their essential features
and has benefits derived from a more physical origin. We study the scalaron's
observational signatures and show the modification to the background Friedmann
equation and CMB power spectrum to be small. We also investigate its effects in
the linear and non linear matter power spectrum--where the signatures are
evident--thus giving particular importance to weak lensing as a probe of these
models. Using this new form, we demonstrate how the next generation Euclid
survey will constrain these theories and its complementarity to current solar
system tests. In the most optimistic case Euclid, together with a Planck prior,
can constrain a fiducial scalaron mass at
the level. However, the decay rate of the scalaron mass, with
fiducial value , can be constrained to uncertainty
Curing singularities in cosmological evolution of F(R) gravity
We study modified gravity models which are capable of driving the
accelerating epoch of the Universe at the present time whilst not destroying
the standard Big Bang and inflationary cosmology. Recent studies have shown
that a weak curvature singularity with can arise generically in
viable models of present dark energy (DE) signaling an internal
incompleteness of these models. In this work we study how this problem is cured
by adding a quadratic correction with a sufficiently small coefficient to the
function at large curvatures. At the same time, this correction
eliminates two more serious problems of previously constructed viable DE
models: unboundedness of the mass of a scalar particle (scalaron) arising in
gravity and the scalaron overabundance problem. Such carefully
constructed models can also yield both an early time inflationary epoch and a
late time de Sitter phase with vastly different values of . The reheating
epoch in these combined models of primordial and present dark energy is
completely different from that of the old inflationary
model, mainly due to the fact that values of the effective gravitational
constant at low and intermediate curvatures are different for positive and
negative . This changes the number of e-folds during the observable part of
inflation that results in a different value of the primordial power spectrum
index.Comment: Discussion expanded, references added, results unchanged, accepted
for publication in JCAP. A minor typo in Eq. (2.14) has been correcte
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