17 research outputs found
String Gas Cosmology: Progress and Problems
String Gas Cosmology is a model of the evolution of the very early universe
based on fundamental principles and key new degrees of freedom of string theory
which are different from those of point particle field theories. In String Gas
Cosmology the universe starts in a quasi-static Hagedorn phase during which
space is filled with a gas of highly excited string states. Thermal
fluctuations of this string gas lead to an almost scale-invariant spectrum of
curvature fluctuations. Thus, String Gas Cosmology is an alternative to
cosmological inflation as a theory for the origin of structure in the universe.
This short review focuses on the building blocks of the model, the predictions
for late time cosmology, and the main problems which the model faces.Comment: 17 pages, 4 figures, invited short review for the Special Issue of
CQG on String Cosmology, typo correcte
Unconventional Cosmology
I review two cosmological paradigms which are alternative to the current
inflationary scenario. The first alternative is the "matter bounce", a
non-singular bouncing cosmology with a matter-dominated phase of contraction.
The second is an "emergent" scenario, which can be implemented in the context
of "string gas cosmology". I will compare these scenarios with the inflationary
one and demonstrate that all three lead to an approximately scale-invariant
spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer
School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept.
12 - 17 2012, to be publ. in the proceedings; these lecture notes form an
updated version of arXiv:1003.1745 and arXiv:1103.227
A rule of thumb for cosmological backreaction
In the context of second order perturbation theory, cosmological backreaction
is seen to rescale both time and the scale factor. The issue of the homogeneous
limit of long-wavelength perturbations is addressed and backreaction is
quantified in terms of a gauge-invariant metric function that is the true
physical degree of freedom in the homogeneous limit. The time integral of this
metric function controls whether backreaction hastens or delays the expansion
of the universe. As an example, late-time acceleration of the universe is shown
to be inconsistent with a perturbative approach. Any tendency to accelerate the
expansion requires negative non-adiabatic pressure fluctuations.Comment: 5 pages, references added, comment clarified in Introductio
One-loop corrections to the curvature perturbation from inflation
An estimate of the one-loop correction to the power spectrum of the
primordial curvature perturbation is given, assuming it is generated during a
phase of single-field, slow-roll inflation. The loop correction splits into two
parts, which can be calculated separately: a purely quantum-mechanical
contribution which is generated from the interference among quantized field
modes around the time when they cross the horizon, and a classical contribution
which comes from integrating the effect of field modes which have already
passed far beyond the horizon. The loop correction contains logarithms which
may invalidate the use of naive perturbation theory for cosmic microwave
background (CMB) predictions when the scale associated with the CMB is
exponentially different from the scale at which the fundamental theory which
governs inflation is formulated.Comment: 28 pages, uses feynmp.sty and ioplatex journal style. v2: supersedes
version published in JCAP. Some corrections and refinements to the discussion
and conclusions. v3: Corrects misidentification of quantum correction with an
IR effect. Improvements to the discussio
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Dark Energy from structure: a status report
The effective evolution of an inhomogeneous universe model in any theory of
gravitation may be described in terms of spatially averaged variables. In
Einstein's theory, restricting attention to scalar variables, this evolution
can be modeled by solutions of a set of Friedmann equations for an effective
volume scale factor, with matter and backreaction source terms. The latter can
be represented by an effective scalar field (`morphon field') modeling Dark
Energy.
The present work provides an overview over the Dark Energy debate in
connection with the impact of inhomogeneities, and formulates strategies for a
comprehensive quantitative evaluation of backreaction effects both in
theoretical and observational cosmology. We recall the basic steps of a
description of backreaction effects in relativistic cosmology that lead to
refurnishing the standard cosmological equations, but also lay down a number of
challenges and unresolved issues in connection with their observational
interpretation.
The present status of this subject is intermediate: we have a good
qualitative understanding of backreaction effects pointing to a global
instability of the standard model of cosmology; exact solutions and
perturbative results modeling this instability lie in the right sector to
explain Dark Energy from inhomogeneities. It is fair to say that, even if
backreaction effects turn out to be less important than anticipated by some
researchers, the concordance high-precision cosmology, the architecture of
current N-body simulations, as well as standard perturbative approaches may all
fall short in correctly describing the Late Universe.Comment: Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 59
pages, 2 figures; matches published versio
The Effect of Large-Scale Inhomogeneities on the Luminosity Distance
We study the form of the luminosity distance as a function of redshift in the
presence of large scale inhomogeneities, with sizes of order 10 Mpc or larger.
We approximate the Universe through the Swiss-cheese model, with each spherical
region described by the Tolman-Bondi metric. We study the propagation of light
beams in this background, assuming that the locations of the source and the
observer are random. We derive the optical equations for the evolution of the
beam area and shear. Through their integration we determine the configurations
that can lead to an increase of the luminosity distance relative to the
homogeneous cosmology. We find that this can be achieved if the Universe is
composed of spherical void-like regions, with matter concentrated near their
surface. For inhomogeneities consistent with the observed large scale
structure, the relative increase of the luminosity distance is of the order of
a few percent at redshifts near 1, and falls short of explaining the
substantial increase required by the supernova data. On the other hand, the
effect we describe is important for the correct determination of the energy
content of the Universe from observations.Comment: 27 pages, 5 figures Revised version. References added. Conclusions
clarifie