109 research outputs found
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
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
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
The dispersion of growth of matter perturbations in f(R) gravity
We study the growth of matter density perturbations delta_m for a number of
viable f(R) gravity models that satisfy both cosmological and local gravity
constraints, where the Lagrangian density f is a function of the Ricci scalar
R. If the parameter m=Rf_{,RR}/f_{,R} today is larger than the order of
10^{-6}, linear perturbations relevant to the matter power spectrum evolve with
a growth rate s=d (ln delta_m)/d (ln a) (a is the scale factor) that is larger
than in the LCDM model. We find the window in the free parameter space of our
models for which spatial dispersion of the growth index gamma_0= gamma(z=0) (z
is the redshift) appears in the range of values 0.40< gamma_0<0.55, as well as
the region in parameter space for which there is essentially no dispersion and
gamma_0 converges to values around 0.40<gamma_0<0.43. These latter values are
much lower than in the LCDM model. We show that these unusual dispersed or
converged spectra are present in most of the viable f(R) models with m(z=0)
larger than the order of 10^{-6}. These properties will be essential in the
quest for f(R) modified gravity models using future high-precision observations
and they confirm the possibility to distinguish clearly most of these models
from the LCDM model.Comment: 11 pages, 7 figure
Are f(R) dark energy models cosmologically viable ?
All modified gravity theories are conformally identical to models of
quintessence in which matter is coupled to dark energy with a strong coupling.
This coupling induces a cosmological evolution radically different from
standard cosmology. We find that in all theories that behave as a power
of at large or small (which include most of those proposed so far in
the literature) the scale factor during the matter phase grows as
instead of the standard law . This behaviour is grossly inconsistent
with cosmological observations (e.g. WMAP), thereby ruling out these models
even if they pass the supernovae test and can escape the local gravity
constraints.Comment: 4 pages; v2: revised figure and minor changes to match version
accepted on Phys. Rev. Let
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