22 research outputs found
Dynamical Relaxation of the Cosmological Constant and Matter Creation in the Universe
In this Letter we discuss the issues of the graceful exit from inflation and
of matter creation in the context of a recent scenario \cite{RHBrev} in which
the back-reaction of long wavelength cosmological perturbations induces a
negative contribution to the cosmological constant and leads to a dynamical
relaxation of the bare cosmological constant. The initially large cosmological
constant gives rise to primordial inflation, during which cosmological
perturbations are stretched beyond the Hubble radius. The cumulative effect of
the long wavelength fluctuations back-reacts on the background geometry in a
form which corresponds to the addition of a negative effective cosmological
constant to the energy-momentum tensor. In the absence of an effective scalar
field driving inflation, whose decay can reheat the Universe, the challenge is
to find a mechanism which produces matter at the end of the relaxation process.
In this Letter, we point out that the decay of a condensate representing the
order parameter for a ``flat'' direction in the field theory moduli space can
naturally provide a matter generation mechanism. The order parameter is
displaced from its vacuum value by thermal or quantum fluctuations, it is
frozen until the Hubble constant drops to a sufficiently low value, and then
begins to oscillate about its ground state. During the period of oscillation it
can decay into Standard Model particles similar to how the inflaton decays in
scalar-field-driven models of inflation.Comment: 6 page
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
Loop Corrections to Cosmological Perturbations in Multi-field Inflationary Models
We investigate one-loop quantum corrections to the power spectrum of
adiabatic perturbation from entropy modes/adiabatic mode cross-interactions in
multiple DBI inflationary models. We find that due to the non-canonical kinetic
term in DBI models, the loop corrections are enhanced by slow-varying parameter
and small sound speed . Thus, in general the loop-corrections
in multi-DBI models can be large. Moreover, we find that the loop-corrections
from adiabatic/entropy cross-interaction vertices are IR finite.Comment: 21 pages, 7 figures; v2, typos corrected, ref added; v3 typos
corrected, version for publishing in jca
The effect of extra dimensions on gravity wave bursts from cosmic string cusps
We explore the kinematical effect of having extra dimensions on the gravity
wave emission from cosmic strings. Additional dimensions both round off cusps,
and reduce the probability of their formation. We recompute the gravity wave
burst, taking into account these two factors, and find a potentially
significant damping on the gravity waves of the strings.Comment: 33 pages, 8 figures, published versio
Non-linear Realizations of Conformal Symmetry and Effective Field Theory for the Pseudo-Conformal Universe
The pseudo-conformal scenario is an alternative to inflation in which the
early universe is described by an approximate conformal field theory on flat,
Minkowski space. Some fields acquire a time-dependent expectation value, which
breaks the flat space so(4,2) conformal algebra to its so(4,1) de Sitter
subalgebra. As a result, weight-0 fields acquire a scale invariant spectrum of
perturbations. The scenario is very general, and its essential features are
determined by the symmetry breaking pattern, irrespective of the details of the
underlying microphysics. In this paper, we apply the well-known coset technique
to derive the most general effective lagrangian describing the Goldstone field
and matter fields, consistent with the assumed symmetries. The resulting action
captures the low energy dynamics of any pseudo-conformal realization, including
the U(1)-invariant quartic model and the Galilean Genesis scenario. We also
derive this lagrangian using an alternative method of curvature invariants,
consisting of writing down geometric scalars in terms of the conformal mode.
Using this general effective action, we compute the two-point function for the
Goldstone and a fiducial weight-0 field, as well as some sample three-point
functions involving these fields.Comment: 49 pages. v2: minor corrections, added references. v3: minor edits,
version appearing in JCA
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
On globally static and stationary cosmologies with or without a cosmological constant and the Dark Energy problem
In the framework of spatially averaged inhomogeneous cosmologies in classical
General Relativity, effective Einstein equations govern the regional and the
global dynamics of averaged scalar variables of cosmological models. A
particular solution may be characterized by a cosmic equation of state. In this
paper it is pointed out that a globally static averaged dust model is
conceivable without employing a compensating cosmological constant. Much in the
spirit of Einstein's original model we discuss consequences for the global, but
also for the regional properties of this cosmology. We then consider the wider
class of globally stationary cosmologies that are conceivable in the presented
framework. All these models are based on exact solutions of the averaged
Einstein equations and provide examples of cosmologies in an out-of-equilibrium
state, which we characterize by an information-theoretical measure. It is shown
that such cosmologies preserve high-magnitude kinematical fluctuations and so
tend to maintain their global properties. The same is true for a
driven cosmos in such a state despite of exponential expansion. We
outline relations to inflationary scenarios, and put the Dark Energy problem
into perspective. Here, it is argued, on the grounds of the discussed
cosmologies, that a classical explanation of Dark Energy through backreaction
effects is theoretically conceivable, if the matter-dominated Universe emerged
from a non-perturbative state in the vicinity of the stationary solution. We
also discuss a number of caveats that furnish strong counter arguments in the
framework of structure formation in a perturbed Friedmannian model.Comment: 33 pages, matches published version in Class. Quant. Gra
Reconstructing Single Field Inflationary Actions From CMBR Data
This paper describes a general program for deriving the action of single
field inflation models with nonstandard kinetic energy terms using CMBR power
spectrum data. This method assumes that an action depends on a set of
undetermined functions, each of which is a function of either the inflaton wave
function or its time derivative. The scalar, tensor and non-gaussianity of the
curvature perturbation spectrum are used to derive a set of reconstruction
equations whose solution set can specify up to three of the undetermined
functions. The method is then used to find the undetermined functions in
various types of actions assuming power law type scalar and tensor spectra. In
actions that contain only two unknown functions, the third reconstruction
equation implies a consistency relation between the non-gaussianty, sound speed
and slow roll parameters. In particular we focus on reconstructing a
generalized DBI action with an unknown potential and warp factor. We find that
for realistic scalar and tensor spectra, the reconstructed warp factor and
potential are very similar to the theoretically derived result. Furthermore,
physical consistency of the reconstructed warp factor and potential imposes
strict constraints on the scalar and tensor spectral indices.Comment: 33 pages, 3 figures: v3 - References adde