1,604 research outputs found
Low red-shift effects of local structure on the Hubble parameter in presence of a cosmological constant
In order to estimate the effects of local structure on the Hubble parameter
we calculate the low-redshift expansion for and for
an observer at the center of a spherically symmetric matter distribution in
presence of a cosmological constant. We then test the accuracy of the formulae
comparing them with fully relativistic non perturbative numerical calculations
for different cases for the density profile. The low red-shift expansion we
obtain gives results more precise than perturbation theory since is based on
the use of an exact solution of Einstein's field equations. For larger density
contrasts the low red-shift formulae accuracy improves respect to the
perturbation theory accuracy because the latter is based on the assumption of a
small density contrast, while the former does not rely on such assumption.
The formulae can be used to take into account the effects on the Hubble
expansion parameter due to the monopole component of the local structure. If
the observations will show deviations from the prediction
compatible with the formulae we have derived, this could be considered an
independent evidence of the existence of a local inhomogeneity, and the
formulae could be used to determine the characteristics of this local
structure.Comment: 14 pages, 4 figure
Reconstructing the metric of the local Universe from number counts observations
Number counts observations available with new surveys such as the Euclid
mission will be an important source of information about the metric of the
Universe. We compute the low red-shift expansion for the energy density and the
density contrast using an exact spherically symmetric solution in presence of a
cosmological constant. At low red-shift the expansion is more precise than
linear perturbation theory prediction. We then use the local expansion to
reconstruct the metric from the monopole of the density contrast. We test the
inversion method using numerical calculations and find a good agreement within
the regime of validity of the red-shift expansion. The method could be applied
to observational data to reconstruct the metric of the local Universe with a
level of precision higher than the one achievable using perturbation theory.Comment: 18 pages,5 figure
Are primordial black holes produced by entropy perturbations in single field inflationary models?
We show that in single field inflationary models the super-horizon evolution
of curvature perturbations on comoving slices , which can cause
the production of primordial black holes (PBH), is not due to entropy
perturbations, but to the background evolution effect on the conversion between
entropy and curvature perturbations. We derive a general relation between the
time derivative of comoving curvature perturbations and entropy perturbations,
in terms of a conversion factor depending on the background evolution. Contrary
to previous results derived in the uniform density gauge assuming the gradient
term can be neglected on super-horizon scales, the relation is valid on any
scale for any minimally coupled single scalar field model, also on sub-horizon
scales where gradient terms are large.
We apply it to the case of quasi-inflection inflation, showing that while
entropy perturbations are decreasing, can grow on super-horizon
scales, due to a large increase of the conversion factor. This happens in the
time interval during which a sufficiently fast decrease of the equation of
state transforms into a growing mode that in slow-roll models would be a
decaying mode. The same mechanism also explains the super-horizon evolution of
in globally adiabatic systems, for which entropy perturbations
vanish on any scale, such as ultra slow-roll inflation and its generalizations
The MESS of cosmological perturbations
We introduce two new effective quantities for the study of comoving curvature
perturbations : the space dependent effective sound speed (SESS) and the
momentum dependent effective sound speed (MESS) . We use the SESS and the MESS
to derive a new set of equations which can be applied to any system described
by an effective stress-energy-momentum tensor (EST), including multi-fields
systems, supergravity and modified gravity theories. We show that this approach
is completely equivalent to the standard one and it has the advantage of
requiring to solve only one differential equation for instead of a
system, without the need of explicitly computing the evolution of entropy
perturbations. The equations are valid for perturbations respect to any
arbitrary flat spatially homogeneous background, including any inflationary and
bounce model.
As an application we derive the equation for for multi-fields
models and show that observed features of the primordial curvature perturbation
spectrum are compatible with the effects of an appropriate local variation of
the MESS in momentum space. The MESS is the natural quantity to parametrize in
a model independent way the effects produced on curvature perturbations by
multi-fields systems, particle production and modified gravity theories and
could be conveniently used in the analysis of LSS observations, such as the
ones from the upcoming EUCLID mission or CMB radiation measurements.Comment: We study the MESS of cosmological perturbations, version accepted in
Physics Letters
El Alcázar de Madīnat al-Zahrā’: permanencia y procesos de cambio
El Alcázar de Madīnat al-Zahrā’: permanencia y procesos de cambi
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