716 research outputs found
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
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
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
Signature of the interaction between dark energy and dark matter in observations
We investigate the effect of an interaction between dark energy and dark
matter upon the dynamics of galaxy clusters. This effect is computed through
the Layser-Irvine equation, which describes how an astrophysical system reaches
virial equilibrium and was modified to include the dark interactions. Using
observational data from almost 100 purportedly relaxed galaxy clusters we put
constraints on the strength of the couplings in the dark sector. We compare our
results with those from other observations and find that a positive (in the
sense of energy flow from dark energy to dark matter) non vanishing interaction
is consistent with the data within several standard deviations.Comment: 13 pages, 3 figures; matches PRD published versio
One Loop Back Reaction On Chaotic Inflation
We extend, for the case of a general scalar potential, the inflaton-graviton
Feynman rules recently developed by Iliopoulos {\it et al.} As an application
we compute the leading term, for late co-moving times, of the one loop back
reaction on the expansion rate for . This is
expressed as the logarithmic time derivative of the scale factor in the
coordinate system for which the expectation value of the metric has the form:
. This quantity should be a gauge
independent observable. Our result for it agrees exactly with that inferred
from the effect previously computed by Mukhanov {\it et al.} using canonical
quantization. It is significant that the two calculations were made with
completely different schemes for fixing the gauge, and that our computation was
done using the standard formalism of covariant quantization. This should settle
some of the issues recently raised by Unruh.Comment: 41 pages, LaTeX 2 epsilo
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