38 research outputs found

    Accelerating universe from gravitational leakage into extra dimensions: confrontation with SNeIa

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    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-zz 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 Ωm=0.290.02+0.04\Omega_m=0.29^{+0.04}_{-0.02}, Ωrc=0.210.08+0.08\Omega_{rc}=0.21^{+0.08}_{-0.08}, and Ωk=0.360.35+0.31\Omega_k=-0.36^{+0.31}_{-0.35}, indicating a closed universe. We then constrain the model using the test of the turnaround redshift, zq=0z_{q=0}, at which the universe switches from deceleration to acceleration. We show that, in order to explain that acceleration happened earlier than zq=0=0.6z_{q=0} = 0.6 within the framework of gravitational leakage into extra dimensions, a low matter density, Ωm<0.27\Omega_m < 0.27, or a closed universe is necessary.Comment: 16 pages, 4 figures, accepted for publication in Ap

    Revisiting the statistical isotropy of GRB sky distribution

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    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

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    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 f(R)f(R) 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?

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    The measurement of present-day temperature of the Cosmic Microwave Background (CMB), T0=2.72548±0.00057T_0 = 2.72548 \pm 0.00057 K (1σ\sigma), 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, H0H_0, obtained from measurements of the CMB temperature fluctuations assuming the standard Λ\LambdaCDM model exhibit a large (4.1σ4.1\sigma) tension when compared with low-redshift, model-independent observations. Recently, some authors argued that a slightly change in T0T_0 could alleviate or solve the H0H_0-tension problem. Here, we investigate evidence for a hotter or colder universe by performing an independent analysis from currently available temperature-redshift T(z)T(z) measurements. Our analysis (parametric and non-parametric) shows a good agreement with the FIRAS measurement and a discrepancy of 1.9σ\gtrsim 1.9\sigma from the T0T_0 values required to solve the H0H_0 tension. This result reinforces the idea that a solution of the H0H_0-tension problem in fact requires either a better understanding of the systematic errors on the H0H_0 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

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    Fast Radio Bursts (FRBs) are millisecond-duration radio transients with an observed dispersion measure (DMDM) 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 DMDM 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 (fIGMf_{IGM}) 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 fIGMf_{IGM} combining the latest FRBs observations with localized host galaxy and current supernovae data. We consider constant and time-dependent fIGMf_{IGM} parameterizations and show, through a Bayesian model selection analysis, that a conclusive answer about the time-evolution of fIGMf_{IGM} depend strongly on the DMDM fluctuations due to the spatial variation in cosmic electron density (δ\delta). In particular, our analysis show that the evidence varies from strong (in favor of a growing evolution of fIGMf_{IGM} with redshift) to inconclusive, as larger values of δ\delta are considered.Comment: 8 pages, 3 figure
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