38 research outputs found
Accelerating universe from gravitational leakage into extra dimensions: confrontation with SNeIa
There is mounting observational evidence that the expansion of our universe
is undergoing an acceleration. A dark energy component has usually been invoked
as the most feasible mechanism for the acceleration. However, it is desirable
to explore alternative possibilities motivated by particle physics before
adopting such an untested entity. In this work, we focus our attention on an
acceleration mechanism: one arising from gravitational leakage into extra
dimensions. We confront this scenario with high- type Ia supernovae compiled
by Tonry et al. (2003) and recent measurements of the X-ray gas mass fractions
in clusters of galaxies published by Allen et al. (2002,2003). A combination of
the two databases gives at a 99% confidence level that
, , and
, indicating a closed universe. We then
constrain the model using the test of the turnaround redshift, , at
which the universe switches from deceleration to acceleration. We show that, in
order to explain that acceleration happened earlier than within
the framework of gravitational leakage into extra dimensions, a low matter
density, , or a closed universe is necessary.Comment: 16 pages, 4 figures, accepted for publication in Ap
Revisiting the statistical isotropy of GRB sky distribution
The assumption of homogeneity and isotropy on large scales is one of the main
hypotheses of the standard cosmology. In this paper, we test the hypothesis of
isotropy from the two-point angular correlation function of 2626 gamma-ray
bursts (GRB) of the FERMI GRB catalogue. We show that the uncertainties in the
GRB positions induce spurious anisotropic signals in their sky distribution.
However, when such uncertainties are taken into account no significant evidence
against the large-scale statistical isotropy is found. This result remains
valid even for the sky distribution of short-lived GRB, contrarily to previous
reports.Comment: 9 pages, 10 figures, 2 tables, match accepted versio
Screening mechanisms in hybrid metric-Palatini gravity
We investigate the efficiency of screening mechanisms in the hybrid
metric-Palatini gravity. The value of the field is computed around spherical
bodies embedded in a background of constant density. We find a thin shell
condition for the field depending on the background field value. In order to
quantify how the thin shell effect is relevant, we analyze how it behaves in
the neighborhood of different astrophysical objects (planets, moons or stars).
We find that the condition is very well satisfied except only for some peculiar
objects. Furthermore we establish bounds on the model using data from solar
system experiments such as the spectral deviation measured by the Cassini
mission and the stability of the Earth-Moon system, which gives the best
constraint to date on theories. These bounds contribute to fix the range
of viable hybrid gravity models.Comment: 7 pages, 2 figures. Accepted for publication in Phys. Rev.
Is there evidence for a hotter Universe?
The measurement of present-day temperature of the Cosmic Microwave Background
(CMB), K (1), made by the Far-InfraRed
Absolute Spectrophotometer (FIRAS), is one of the most precise measurements
ever made in Cosmology. On the other hand, estimates of the Hubble Constant,
, obtained from measurements of the CMB temperature fluctuations assuming
the standard CDM model exhibit a large () tension when
compared with low-redshift, model-independent observations. Recently, some
authors argued that a slightly change in could alleviate or solve the
-tension problem. Here, we investigate evidence for a hotter or colder
universe by performing an independent analysis from currently available
temperature-redshift measurements. Our analysis (parametric and
non-parametric) shows a good agreement with the FIRAS measurement and a
discrepancy of from the values required to solve the
tension. This result reinforces the idea that a solution of the
-tension problem in fact requires either a better understanding of the
systematic errors on the measurements or new physics.Comment: 4 pages, 2 figures, 1 table. Accepted for publication in European
Physical Journal
Cosmological model-independent constraints on the baryon fraction in the IGM from fast radio bursts and supernovae data
Fast Radio Bursts (FRBs) are millisecond-duration radio transients with an
observed dispersion measure () greater than the expected Milky Way
contribution, which suggests that such events are of extragalactic origin.
Although some models have been proposed to explain the physics of the pulse,
the mechanism behind the FRBs emission is still unknown. From FRBs data with
known host galaxies, the redshift is directly measured and can be combined with
estimates of the to constrain the cosmological parameters, such as the
baryon number density and the Hubble constant. However, the poor knowledge of
the fraction of baryonic mass in the intergalactic medium () and its
degeneracy with the cosmological parameters impose limits on the cosmological
application of FRBs. In this work we present a cosmological model-independent
method to determine the evolution of combining the latest FRBs
observations with localized host galaxy and current supernovae data. We
consider constant and time-dependent parameterizations and show,
through a Bayesian model selection analysis, that a conclusive answer about the
time-evolution of depend strongly on the fluctuations due to the
spatial variation in cosmic electron density (). In particular, our
analysis show that the evidence varies from strong (in favor of a growing
evolution of with redshift) to inconclusive, as larger values of
are considered.Comment: 8 pages, 3 figure