794 research outputs found
Cosmological Density Perturbations From A Quantum Gravitational Model Of Inflation
We derive the implications for anisotropies in the cosmic microwave
background following from a model of inflation in which a bare cosmological
constant is gradually screened by an infrared process in quantum gravity. The
model predicts that the amplitude of scalar perturbations is , that the tensor-to-scalar ratio is , and that the scalar and tensor spectral indices are
and , respectively. By comparing the model's
power spectrum with the COBE 4-year RMS quadrupole, the mass scale of inflation
is determined to be . At this scale
the model produces about e-foldings of inflation, so another prediction
is .Comment: 18 pages, LaTeX 2 epsilon, 1 eps file, uses epsfi
A Scalar Measure Of The Local Expansion Rate
We define a scalar measure of the local expansion rate based on how
astronomers determine the Hubble constant. Our observable is the inverse
conformal d'Alembertian acting on a unit ``standard candle.'' Because this
quantity is an integral over the past lightcone of the observation point it
provides a manifestly causal and covariant technique for averaging over small
fluctuations. For an exactly homogeneous and isotropic spacetime our scalar
gives minus one half times the inverse square of the Hubble parameter. Our
proposal is that it be assigned this meaning generally and that it be employed
to decide the issue of whether or not there is a significant quantum
gravitational back-reaction on inflation. Several techniques are discussed for
promoting the scalar to a full invariant by giving a geometrical description
for the point of observation. We work out an explicit formalism for evaluating
the invariant in perturbation theory. The results for two simple models are
presented in subsequent papers.Comment: 25 pages, LaTeX 2 epsilon, 1 figur
Angular-planar CMB power spectrum
Gaussianity and statistical isotropy of the Universe are modern cosmology's
minimal set of hypotheses. In this work we introduce a new statistical test to
detect observational deviations from this minimal set. By defining the
temperature correlation function over the whole celestial sphere, we are able
to independently quantify both angular and planar dependence (modulations) of
the CMB temperature power spectrum over different slices of this sphere. Given
that planar dependence leads to further modulations of the usual angular power
spectrum , this test can potentially reveal richer structures in the
morphology of the primordial temperature field. We have also constructed an
unbiased estimator for this angular-planar power spectrum which naturally
generalizes the estimator for the usual 's. With the help of a chi-square
analysis, we have used this estimator to search for observational deviations of
statistical isotropy in WMAP's 5 year release data set (ILC5), where we found
only slight anomalies on the angular scales and . Since this
angular-planar statistic is model-independent, it is ideal to employ in
searches of statistical anisotropy (e.g., contaminations from the galactic
plane) and to characterize non-Gaussianities.Comment: Replaced to match the published version. Journal-ref: Phys.Rev. D80
063525 (2009
Energy density and pressure of long wavelength gravitational waves
Inflation leads us to expect a spectrum of gravitational waves (tensor
perturbations) extending to wavelengths much bigger than the present observable
horizon. Although these gravity waves are not directly observable, the energy
density that they contribute grows in importance during the radiation- and
dust-dominated ages of the universe. We show that the back reaction of tensor
perturbations during matter domination is limited from above, since
gravitational waves of wavelength have a share of the total energy
density during matter domination that is at most
equal to the share of the total energy density that they had when the mode
exited the Hubble radius during inflation. This work is to
be contrasted to that of Sahni, who analyzed the energy density of gravity
waves only insofar as their wavelengths are smaller than . Such a
cut-off in the spectral energy of gravity waves leads to the breakdown of
energy conservation, and we show that this anomaly is eliminated simply by
taking into account the energy density and pressure of long wavelength
gravitational waves as well as short wavelength ones.Comment: Updated one reference; 17 pages, no figure
A note on dualities in Einstein's gravity in the presence of a non-minimally coupled scalar field
We show that the action of Einstein's gravity with a scalar field coupled in
a generic way to spacetime curvature is invariant under a particular set of
conformal transformations. These transformations relate dual theories for which
the effective couplings of the theory are scaled uniformly. In the simplest
case, this class of dualities reduce to the S-duality of low-energy effective
action of string theory.Comment: 12 page
One Loop Back Reaction On Power Law Inflation
We consider quantum mechanical corrections to a homogeneous, isotropic and
spatially flat geometry whose scale factor expands classically as a general
power of the co-moving time. The effects of both gravitons and the scalar
inflaton are computed at one loop using the manifestly causal formalism of
Schwinger with the Feynman rules recently developed by Iliopoulos {\it et al.}
We find no significant effect, in marked contrast with the result obtained by
Mukhanov {\it et al.} for chaotic inflation based on a quadratic potential. By
applying the canonical technique of Mukhanov {\it et al.} to the exponential
potentials of power law inflation, we show that the two methods produce the
same results, within the approximations employed, for these backgrounds. We
therefore conclude that the shape of the inflaton potential can have an
enormous impact on the one loop back-reaction.Comment: 28 pages, LaTeX 2 epsilo
Spherical Collapse in Chameleon Models
We study the gravitational collapse of an overdensity of nonrelativistic
matter under the action of gravity and a chameleon scalar field. We show that
the spherical collapse model is modified by the presence of a chameleon field.
In particular, we find that even though the chameleon effects can be
potentially large at small scales, for a large enough initial size of the
inhomogeneity the collapsing region possesses a thin shell that shields the
modification of gravity induced by the chameleon field, recovering the standard
gravity results. We analyse the behaviour of a collapsing shell in a
cosmological setting in the presence of a thin shell and find that, in contrast
to the usual case, the critical density for collapse depends on the initial
comoving size of the inhomogeneity.Comment: matches printed versio
- …