37 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.
Forecasting constraints on the baryon mass fraction in the IGM from fast radio bursts and type Ia supernovae
Fast Radio Bursts (FRBs) are transient events with a high energy and short
duration in the radio frequency. By identifying the origin of the pulse, it is
possible to measure the redshift of the host galaxy, which can be used to
constrain cosmological and astrophysical parameters and test aspects of
fundamental physics when combined with the observed dispersion measure ().
However, some factors limit the application of FRBs in cosmology: (i) the
current poor modelling of the fluctuations in the due to spatial variation
in the cosmic electrons density; (ii) the fact that the fraction of baryon mass
in the intergalactic medium () is degenerated with some cosmological
parameters; (iii) the limited current knowledge about host galaxy contribution
(). In this work, we investigate the impact of different redshift
distribution models of FRBs to constrain the baryon fraction in the IGM and
host galaxy contribution. We use a cosmological model-independent method
developed in previous work \cite{Lemos2023} to perform the analysis and combine
simulated FRB data from Monte Carlo simulation and supernovae data. Since the
physical mechanism responsible for the burst is still unknown, we assume four
distribution models for the FRBs, namely gamma-ray bursts (GRB), star formation
rate (SFR), uniform and equidistant (ED). Also, we consider samples with , 30, 100 and 500 points and three different values of the fluctuations of
electron density in the , pc/cm. Our analysis
shows that the GRB, SFR and Uniform distribution models present consistent
results within for the free parameters and
and highlights the crucial role of fluctuations in obtaining more precise
measurements.Comment: 9 pages, 2 figure
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