27 research outputs found
Comments on Backreaction and Cosmic Acceleration
In this brief WEB note we comment on recent papers related to our paper "On
Acceleration Without Dark Energy".Comment: 5 pages WEB not
Comments on Long-Wavelength Backreaction and Dark Energy
In this brief note we comment on a recent attempt by Martineau and
Brandenberger (astro-ph/0510523) to explain the acceleration of the Universe
using the back-reaction of long-wavelength perturbations associated with
isocurvature perturbation modes.Comment: 7 page
Gravitational Backreaction of Matter Inhomogeneities
The non-linearity of Einstein's equations makes it possible for small-scale
matter inhomogeneities to affect the Universe at cosmological distances. We
study the size of such effects using a simple heuristic model that captures the
most important backreaction effect due to nonrelativistc matter, as well as
several exact solutions describing inhomogeneous and anisotropic expanding
universes. We find that the effects are or smaller, where
is the Hubble parameter and the typical size scale of inhomogeneities. For
virialized structures this is of order , where is the
characteristic peculiar velocity.Comment: 16 page
Signatures of Primordial Non-Gaussianity in the Large-Scale Structure of the Universe
We discuss how primordial (e.g. inflationary) non-Gaussianity in the
cosmological perturbations is left imprinted in the Large-Scale Structure of
the universe. Our findings show that the information on the primordial
non-Gaussianity set on super-Hubble scales flows into Post-Newtonian terms,
leaving an observable imprint in the Large-Scale Structure. Future
high-precision measurements of the statistics of the dark matter density and
peculiar velocity fields will allow to pin down the primordial non-Gaussianity,
thus representing a tool complementary to studies of the Cosmic Microwave
Background anisotropies.Comment: 8 pages, LaTeX file; Revised to match the final version accepted for
publication on JCAP (some comments and one figure added
Late-time Inhomogeneity and Acceleration Without Dark Energy
The inhomogeneous distribution of matter in the non-linear regime of
galaxies, clusters of galaxies and voids is described by an exact, spherically
symmetric inhomogeneous solution of Einstein's gravitational field equations,
corresponding to an under-dense void. The solution becomes the homogeneous and
isotropic Einstein-de Sitter solution for a red shift , which
describes the matter dominated CMB data with small inhomogeneities
. A spatial volume averaging of physical
quantities is introduced and the averaged time evolution expansion parameter
in the Raychoudhuri equation can give rise in the late-time universe
to a volume averaged deceleration parameter that is negative for a
positive matter density. This allows for a region of accelerated expansion
which does not require a negative pressure dark energy or a cosmological
constant. A negative deceleration parameter can be derived by this volume
averaging procedure from the Lema\^{i}tre-Tolman-Bondi open void solution,
which describes the late-time non-linear regime associated with galaxies and
under-dense voids and solves the ``coincidence'' problem.Comment: LaTex file, 16 pages, no figures. Typo corrections. References added
and updated. Additional material and some conclusions changed. Replacement to
match final published version in Journ. Cosmol. Astropart. Phys. JCAP 200
A cosmic equation of state for the inhomogeneous Universe: can a global far-from-equilibrium state explain Dark Energy?
A system of effective Einstein equations for spatially averaged scalar
variables of inhomogeneous cosmological models can be solved by providing a
`cosmic equation of state'. Recent efforts to explain Dark Energy focus on
`backreaction effects' of inhomogeneities on the effective evolution of
cosmological parameters in our Hubble volume, avoiding a cosmological constant
in the equation of state. In this Letter it is argued that, if kinematical
backreaction effects are indeed of the order of the averaged density (or larger
as needed for an accelerating domain of the Universe), then the state of our
regional Hubble volume would have to be in the vicinity of a
far-from-equilibrium state that balances kinematical backreaction and average
density. This property, if interpreted globally, is shared by a stationary
cosmos with effective equation of state . It
is concluded that a confirmed explanation of Dark Energy by kinematical
backreaction may imply a paradigmatic change of cosmology.Comment: 7 pages, matches published version in Class. Quant. Gra
Can the Acceleration of Our Universe Be Explained by the Effects of Inhomogeneities?
No. It is simply not plausible that cosmic acceleration could arise within
the context of general relativity from a back-reaction effect of
inhomogeneities in our universe, without the presence of a cosmological
constant or ``dark energy.'' We point out that our universe appears to be
described very accurately on all scales by a Newtonianly perturbed FLRW metric.
(This assertion is entirely consistent with the fact that we commonly encounter
.) If the universe is accurately described by a
Newtonianly perturbed FLRW metric, then the back-reaction of inhomogeneities on
the dynamics of the universe is negligible. If not, then it is the burden of an
alternative model to account for the observed properties of our universe. We
emphasize with concrete examples that it is {\it not} adequate to attempt to
justify a model by merely showing that some spatially averaged quantities
behave the same way as in FLRW models with acceleration. A quantity
representing the ``scale factor'' may ``accelerate'' without there being any
physically observable consequences of this acceleration. It also is {\it not}
adequate to calculate the second-order stress energy tensor and show that it
has a form similar to that of a cosmological constant of the appropriate
magnitude. The second-order stress energy tensor is gauge dependent, and if it
were large, contributions of higher perturbative order could not be neglected.
We attempt to clear up the apparent confusion between the second-order stress
energy tensor arising in perturbation theory and the ``effective stress energy
tensor'' arising in the ``shortwave approximation.''Comment: 20 pages, 1 figure, several footnotes and references added, version
accepted for publication in CQG;some clarifying comments adde
On cosmic acceleration without dark energy
We elaborate on the proposal that the observed acceleration of the Universe
is the result of the backreaction of cosmological perturbations, rather than
the effect of a negative-pressure dark-energy fluid or a modification of
general relativity. Through the effective Friedmann equations describing an
inhomogeneous Universe after smoothing, we demonstrate that acceleration in our
local Hubble patch is possible even if fluid elements do not individually
undergo accelerated expansion. This invalidates the no-go theorem that there
can be no acceleration in our local Hubble patch if the Universe only contains
irrotational dust. We then study perturbatively the time behavior of
general-relativistic cosmological perturbations, applying, where possible, the
renormalization group to regularize the dynamics. We show that an instability
occurs in the perturbative expansion involving sub-Hubble modes. Whether this
is an indication that acceleration in our Hubble patch originates from the
backreaction of cosmological perturbations on observable scales requires a
fully non-perturbative approach.Comment: 33 pages, LaTeX file. Revised to match the final version accepted for
publication in NJ
Enhancing the tensor-to-scalar ratio in simple inflation
We show that in theories with a nontrivial kinetic term the contribution of
the gravitational waves to the CMB fluctuations can be substantially larger
than that is naively expected in simple inflationary models. This increase of
the tensor-to-scalar perturbation ratio leads to a larger B-component of the
CMB polarization, thus making the prospects for future detection much more
promising. The other important consequence of the considered model is a higher
energy scale of inflation and hence higher reheating temperature compared to a
simple inflation.Comment: 9 pages, 1 figure and references are added, discussion is slightly
extended, published versio
Can a dust dominated universe have accelerated expansion?
Recently, there has been suggestions that the apparent accelerated expansion
of the universe is due not to a cosmological constant, but rather to
inhomogeneities in the distribution of matter. In this work, we investigate a
specific class of inhomogeneous models that can be solved analytically, namely
the dust-dominated Lemaitre-Tolman-Bondi universe models. We show that they do
not permit accelerated cosmic expansion.Comment: 9 pages, 1 figure. v3: Paper shortened and updated. References added.
v4: Minor LATEX problem fixed. Submitted to JCA