407 research outputs found
Primordial Black Holes in an Accelerating Universe
General expressions are given for the generation of Primordial Black Holes
(PBH) in a universe with a presently accelerated expansion due to a(n
effective) cosmological constant. We give expressions both for a powerlaw
scalefree primordial spectrum and for spectra which are not of that type.
Specializing to the case of a pure cosmological constant and assuming
flatness, we show that a comological constant with
will decrease the mass variance at the PBH formation time by about 15% compared
with a critical density universe.Comment: 9 pages, uses LaTeX, version accepted in Phys. Lett. B, results
unchange
Why do cosmological perturbations look classical to us?
According to the inflationary scenario of cosmology, all structure in the
Universe can be traced back to primordial fluctuations during an accelerated
(inflationary) phase of the very early Universe. A conceptual problem arises
due to the fact that the primordial fluctuations are quantum, while the
standard scenario of structure formation deals with classical fluctuations. In
this essay we present a concise summary of the physics describing the
quantum-to-classical transition. We first discuss the observational
indistinguishability between classical and quantum correlation functions in the
closed system approach (pragmatic view). We then present the open system
approach with environment-induced decoherence. We finally discuss the question
of the fluctuations' entropy for which, in principle, the concrete mechanism
leading to decoherence possesses observational relevance.Comment: 12 pages, Revtex, invited contribution to a special issue of Advanced
Science Letters, final versio
Can Lightcone Fluctuations be Probed with Cosmological Backgrounds?
Finding signatures of quantum gravity in cosmological observations is now
actively pursued both from the theoretical and the experimental side. Recent
work has concentrated on finding signatures of light-cone fluctuations in the
CMB. Because in inflationary scenarios a Gravitational Wave Background (GWB) is
always emitted much before the CMB, we can ask, in the hypothesis where this
GWB could be observed, what is the imprint of light cone fluctuations on this
GWB. We show that due to the flat nature of the GWB spectrum, the effect of
lightcone fluctuations are negligible.Comment: 10 pages, references adde
The End of Cosmic Growth
The growth of large scale structure is a battle between gravitational
attraction and cosmic acceleration. We investigate the future behavior of
cosmic growth under both general relativity (GR) and modified gravity during
prolonged acceleration, deriving analytic asymptotic behaviors and showing that
gravity generally loses and growth ends. We also note the `why now' problem is
equally striking when viewed in terms of the shut down of growth. For many
models inside GR the gravitational growth index also shows today as a
unique time between constant behavior in the past and a higher asymptotic value
in the future. Interestingly, while models depart in this respect
dramatically from GR today and in the recent past, their growth indices are
identical in the asymptotic future and past.Comment: 5 pages, 6 figures; v2 minor edits, matches accepted PR
The growth of matter perturbations in f(R) models
We consider the linear growth of matter perturbations on low redshifts in
some dark energy (DE) models. We discuss the definition of dark energy
(DE) in these models and show the differences with scalar-tensor DE models. For
the model recently proposed by Starobinsky we show that the growth
parameter takes the value for
and for , allowing
for a clear distinction from CDM. Though a scale-dependence appears in
the growth of perturbations on higher redshifts, we find no dispersion for
on low redshifts up to , is also
quasi-linear in this interval. At redshift , the dispersion is still
small with . As for some scalar-tensor models, we
find here too a large value for ,
for and for
. These values are largely outside the range found for DE
models in General Relativity (GR). This clear signature provides a powerful
constraint on these models.Comment: 14 pages, 7 figures, improved presentation, references added, results
unchanged, final version to be published in JCA
Dispersion in the growth of matter perturbations
We consider the linear growth of matter perturbations on low redshifts in
modified gravity Dark Energy (DE) models where G_eff(z,k) is explicitly
scale-dependent. Dispersion in the growth today will only appear for scales of
the order the critical scale ~ \lambda_{c,0}, the range of the fifth-force
today. We generalize the constraint equation satisfied by the parameters
\gamma_0(k) and \gamma'_0(k) \equiv \frac{d\gamma(z,k)}{dz}(z=0) to models with
G_{eff,0}(k) \ne G. Measurement of \gamma_0(k) and \gamma'_0(k) on several
scales can provide information about \lambda_{c,0}. In the absence of
dispersion when \lambda_{c,0} is large compared to the probed scales,
measurement of \gamma_0 and \gamma'_0 provides a consistency check independent
of \lambda_{c,0}. This applies in particular to results obtained earlier for a
viable f(R) model.Comment: 8 pages, 5 figure
Parameter extraction by Planck for a CDM model with broken scale invariance and cosmological constant
We consider a class of spatially flat cold dark matter (CDM) models, with a
cosmological constant and a broken-scale-invariant (BSI) steplike primordial
spectrum of adiabatic perturbations, previously found to be in very good
agreement with observations. Performing a Fisher matrix analysis, we show that
in case of a large gravitational waves (GW) contribution some free parameters
(defining the step) of our BSI model can be extracted with remarkable accuracy
by the Planck satellite, thanks to the polarisation anisotropy measurements.
Further, cosmological parameters can still be found with very good precision,
despite a larger number of free parameters than in the simplest inflationary
models.Comment: Final version to appear in MNRAS. Minor changes. 5 pages, 1 LaTeX
figure, uses mn.st
Scalar Field Cosmologies With Inverted Potentials
Regular bouncing solutions in the framework of a scalar-tensor gravity model
were found in a recent work. We reconsider the problem in the Einstein frame
(EF) in the present work. Singularities arising at the limit of physical
viability of the model in the Jordan frame (JF) are either of the Big Bang or
of the Big Crunch type in the EF. As a result we obtain integrable scalar field
cosmological models in general relativity (GR) with inverted double-well
potentials unbounded from below which possess solutions regular in the future,
tending to a de Sitter space, and starting with a Big Bang. The existence of
the two fixed points for the field dynamics at late times found earlier in the
JF becomes transparent in the EF.Comment: 18 pages, 4 figure
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