6,045 research outputs found
The impact on cosmology of a primordial scaling field
A scalar field with an exponential potential has the particular property that
it is attracted into a solution in which its energy scales as the dominant
component (radiation or matter) of the Universe, contributing a fixed fraction
of the total energy density. We briefly discuss the dynamics of such a scalar
field and its impact on Big Bang nucleosynthesis, the growth of large scale
structure and abundance of damped Lyman systems at high redshift.
Given the simplicity of the model, its theoretical motivation, and its success
in matching observations, we argue that it should be taken on par with other
currently viable models of structure formation.Comment: 3 pages, uses prcrc.sty, 2 figures, to be published in proceedings of
Dark Matter '9
Ricci focusing, shearing, and the expansion rate in an almost homogeneous Universe
The Universe is inhomogeneous, and yet it seems to be incredibly
well-characterised by a homogeneous relativistic model. One of the current
challenges is to accurately characterise the properties of such a model. In
this paper we explore how inhomogeneities may affect the overall optical
properties of the Universe by quantifying how they can bias the
redshift-distance relation in a number of toy models that mimic the real
Universe. The models that we explore are statistically homogeneous on large
scales. We find that the effect of inhomogeneities is of order of a few
percent, which can be quite important in precise estimation of cosmological
parameters. We discuss what lessons can be learned to help us tackle a more
realistic inhomogeneous universe.Comment: 22 pages, 8 figure
Eddington's theory of gravity and its progeny
We resurrect Eddington's proposal for the gravitational action in the
presence of a cosmological constant and extend it to include matter fields. We
show that the Newton-Poisson equation is modified in the presence of sources
and that charged black holes show great similarities with those arising in
Born-Infeld electrodynamics coupled to gravity. When we consider homogeneous
and isotropic space-times we find that there is a minimum length (and maximum
density) at early times, clearly pointing to an alternative theory of the Big
Bang. We thus argue that the modern formulation of Eddington's theory,
Born-Infeld gravity, presents us with a novel, non-singular description of the
Universe.Comment: 4 Pages, 2 Figures, Accepted to PRL (typos corrected
A general theory of linear cosmological perturbations: bimetric theories
We implement the method developed in [1] to construct the most general
parametrised action for linear cosmological perturbations of bimetric theories
of gravity. Specifically, we consider perturbations around a homogeneous and
isotropic background, and identify the complete form of the action invariant
under diffeomorphism transformations, as well as the number of free parameters
characterising this cosmological class of theories. We discuss, in detail, the
case without derivative interactions, and compare our results with those found
in massive bigravity.Comment: Published version with extra comments in conclusio
Constraining ultra large-scale cosmology with multiple tracers in optical and radio surveys
Multiple tracers of the cosmic density field, with different bias, number and
luminosity evolution, can be used to measure the large-scale properties of the
Universe. We show how an optimal combination of tracers can be used to detect
general-relativistic effects in the observed density of sources. We forecast
for the detectability of these effects, as well as measurements of primordial
non-Gaussianity and large-scale lensing magnification with current and upcoming
large-scale structure experiments. In particular we quantify the significance
of these detections in the short term with experiments such as the Dark Energy
Survey (DES), and in the long term with the Large Synoptic Survey Telescope
(LSST) and the Square Kilometre Array (SKA). We review the main observational
challenges that must be overcome to carry out these measurements.Comment: 19 pages, 10 figure
The closet non-Gaussianity of anisotropic Gaussian fluctuations
In this paper we explore the connection between anisotropic Gaussian
fluctuations and isotropic non-Gaussian fluctuations. We first set up a large
angle framework for characterizing non-Gaussian fluctuations: large angle
non-Gaussian spectra. We then consider anisotropic Gaussian fluctuations in two
different situations. Firstly we look at anisotropic space-times and propose a
prescription for superimposed Gaussian fluctuations; we argue against
accidental symmetry in the fluctuations and that therefore the fluctuations
should be anisotropic. We show how these fluctuations display previously known
non-Gaussian effects both in the angular power spectrum and in non-Gaussian
spectra. Secondly we consider the anisotropic Grischuk-Zel'dovich effect. We
construct a flat space time with anisotropic, non-trivial topology and show how
Gaussian fluctuations in such a space-time look non-Gaussian. In particular we
show how non-Gaussian spectra may probe superhorizon anisotropy
Archipelagian Cosmology: Dynamics and Observables in a Universe with Discretized Matter Content
We consider a model of the Universe in which the matter content is in the
form of discrete islands, rather than a continuous fluid. In the appropriate
limits the resulting large-scale dynamics approach those of a
Friedmann-Robertson-Walker (FRW) universe. The optical properties of such a
space-time, however, do not. This illustrates the fact that the optical and
`average' dynamical properties of a relativistic universe are not equivalent,
and do not specify each other uniquely. We find the angular diameter distance,
luminosity distance and redshifts that would be measured by observers in these
space-times, using both analytic approximations and numerical simulations.
While different from their counterparts in FRW, the effects found do not look
like promising candidates to explain the observations usually attributed to the
existence of Dark Energy. This incongruity with standard FRW cosmology is not
due to the existence of any unexpectedly large structures or voids in the
Universe, but only to the fact that the matter content of the Universe is not a
continuous fluid.Comment: 49 pages, 15 figures. Corrections made to description of lattice
constructio
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