6 research outputs found
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 a scalar field during inflation
The leading quantum correction to the power spectrum of a
gravitationally-coupled light scalar field is calculated, assuming that it is
generated during a phase of single-field, slow-roll inflation.Comment: 33 pages, uses feynmp.sty and ioplatex journal style. v2: matches
version published in JCAP. v3: corrects sign error in Eq. (58). Corrects
final coefficient of the logarithm in Eq. (105). Small corrections to
discussion of divergences in 1-point function. Minor improvements to
discussion of UV behaviour in Sec. 4.
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
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