arXiv subjects

J. S. Alcaniz

Publications and source records attributed to J. S. Alcaniz.

At least 127 records · Page 7Linked to original sources

arXiv2006

$β$-exponential inflation

An inflationary scenario driven by a slow rolling homogeneous scalar field whose potential $V(Φ)$ is given by a generalized exponential function is discussed. Within the {\sl slow-roll} approximation we investigate some of the main predictions of the model and compare them with current data from Cosmic Microwave Background and Large-Scale Structure observations. In particular, we show that this single scalar field model admits a wider range of solutions than do conventional exponential scenarios and predicts acceptable values of the scalar spectral index and of the tensor-to-scalar ratio for the remaining number of {\sl e-folds} lying in the interval $N = 54 \pm 7$ and energy scales of the order of Planck scale. The running of the spectral index is briefly discussed to show that both negative and positive values are predicted by the model here proposed.

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arXiv2006

"Expansion" around the vacuum: how far can we go from Lambda?

The cosmological constant ($Λ$), i.e., the energy density stored in the true vacuum state of all existing fields in the Universe, is the simplest and the most natural possibility to describe the current cosmic acceleration. However, despite its observational successes, such a possibility exacerbates the well known $Λ$ problem, requiring a natural explanation for its small, but nonzero, value. In this paper we discuss how different our Universe may be from the $Λ$CDM model by studying observational aspects of a kind of "expansion" around the vacuum given by the equation of (EOS) $p_{d}=-ρ_{d} - A ρ_{d}^α$. In different parameter regimes such a parametrization is capable of describing both quintessence-like and phantom-like dark energy, transient acceleration, and various (non)singular possibilities for the final destiny of the Universe, including singularities at finite values of the scale factor, the so-called "Big Rip", as well as sudden future singularities. By using some of the most recent cosmological observations we show that if the functional form of the dark energy EOS has additional parameters very little can be said about their values from the current observational results, which postpones, until the arrival of more precise observational data, a definitive answer to the question posed above.

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arXiv2006

Energy Conditions and Supernovae Observations

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker approach to cosmological modelling where the equation of state of the cosmological fluid is unknown, the energy conditions provide model-independent bounds on the behavior of the distance modulus of cosmic sources as a function of the redshift. We use both the gold and the legacy samples of current type Ia supenovae to carry out a model-independent analysis of the energy conditions violation in the context of standard cosmology.

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arXiv2006

Dark Energy and Some Alternatives: a Brief Overview

The high-quality cosmological data, which became available in the last decade, have thrusted upon us a rather preposterous composition for the universe which poses one of the greatest challenges theoretical physics has ever faced: the so-called dark energy. By focusing our attention on specific examples of dark energy scenarios, we discuss three different candidates for this dark component, namely, a decaying vacuum energy or time-varying cosmological constant [$Λ(t)$], a rolling homogeneous quintessence field ($Φ$), and modifications in gravity due to extra spatial dimensions. As discussed, all these candidates [along with the vacuum energy or cosmological constant ($Λ$)] seem somewhat to be able to explain the current observational results, which hampers any definitive conclusion on the actual nature of the dark energy.

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arXiv2006

A simplified approach for Chaplygin-type cosmologies

A new class of accelerating cosmological models driven by a one-parameter version of the general Chaplygin-type equation of state is proposed. The simplified version is naturally obtained from causality considerations with basis on the adiabatic sound speed $v_S$ plus the observed accelerating stage of the universe. We show that very stringent constraints on the unique free parameter $α$ describing the simplified Chaplygin model can be obtained from a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations (BAO). In our analysis we have considered separately the SNe type Ia gold sample measured by Riess et al. (2004) and the Supernova Legacy Survey (SNLS) from Astier et al. (2006). At 95.4% (c.l.), we find for BAO + \emph{gold} sample, $0.91 \leq α\leq 1.0$ and $Ω_{\rm{M}}= 0.28^{+0.043}_{-0.048}$ while BAO + SNLS analysis provides $0.94 \leq α\leq 1.0$ and $Ω_{\rm M}=0.27^{+0.048}_{-0.045}$.

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arXiv2006

Scalar-field-dominated cosmology with a transient accelerating phase

A new cosmological scenario driven by a slow rolling homogeneous scalar field whose exponential potential $V(Φ)$ has a quadratic dependence on the field $Φ$ in addition to the standard linear term is discussed. The derived equation of state for the field predicts a transient accelerating phase, in which the Universe was decelerated in the past, began to accelerate at redshift $z \sim 1$, is currently accelerated, but, finally, will return to a decelerating phase in the future. This overall dynamic behavior is profoundly different from the standard $Λ$CDM evolution, and may alliviate some conflicts in reconciling the idea of a dark energy-dominated universe with observables in String/M-theory. The theoretical predictions for the present transient scalar field plus dark matter dominated stage are confronted with cosmological observations in order to test the viability of the scenario.

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arXiv2006

Supernova constraints on decaying vacuum cosmology

There is mounting observational evidence that the expansion of our Universe is undergoing a late-time acceleration. Among many proposals to describe this phenomenon, the cosmological constant seems to be the simplest and the most natural explanation. However, despite its observational successes, such a possibility exacerbates the well known cosmological constant problem, requiring a natural explanation for its small, but nonzero, value. In this paper we consider a cosmological scenario driven by a varying cosmological term, in which the vacuum energy density decays linearly with the Hubble parameter. We show that this model is indistinguishable from the standard one in that the early radiation phase is followed by a long dust-dominated era, and only recently the varying cosmological term becomes dominant, accelerating the cosmic expansion. In order to test the viability of this scenario we have used the most recent type Ia supernova data, i.e., the High-Z SN Search (HZS) Team and the Supernova Legacy Survey (SNLS) Collaboration data. In particular, for the SNLS sample we have found the present matter density and Hubble parameters in the intervals [0.27, 0.37] and [0.68, 0.72], respectively (at 95% c.l.), which is in good agreement with the currently accepted estimates for these parameters.

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arXiv2006

Lookback time as a test for brane cosmology

The observed late-time acceleration of the Universe may be the result of unknown physical processes involving either modifications of gravitation theory or the existence of new fields in high energy physics. In the former case, such modifications are usually related to the possible existence of extra dimensions (which is also required by unification theories), giving rise to the so-called brane cosmology. In this paper we investigate the viability of this idea by considering a particular class of brane scenarios in which a large scale modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. To this end, differently from other recent analyses, we combine \emph{orthogonal} age and distance measurements at intermediary and high redshifts. We use observations of the lookback time to galaxy clusters, indirect estimates of the age of the Universe from the most recent Large-Scale Structure (LSS) and Cosmic Microwave Background (CMB) data, along with the recent detection of the baryon acoustic oscillations at $z = 0.35$. In agreement with other recent analyses we show that, although compatible with these age and distance measurements, a spatially closed scenario is largely favoured by the current observational data. By restricting our analysis to a spatially flat universe, we also find that the standard $Λ$CDM model is favoured over the particular braneworld scenario here investigated.

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arXiv2006

Supernovae observations and cosmic topology

Two fundamental questions regarding our description of the Universe concern the geometry and topology of its 3-dimensional space. While geometry is a local characteristic that gives the intrinsic curvature, topology is a global feature that characterizes the shape and size of the 3-space. The geometry constrains, but does not dictate the the spatial topology. We show that, besides determining the spatial geometry, the knowledge of the spatial topology allows to place tight constraints on the density parameters associated with dark matter ($Ω_m$) and dark energy ($Ω_Λ$). By using the Poincaré dodecahedral space as the observable spatial topology, we reanalyze the current type Ia supenovae (SNe Ia) constraints on the density parametric space $Ω_{m} - Ω_Λ$. From this SNe Ia plus cosmic topology analysis, we found best fit values for the density parameters, which are in agreement with a number of independent cosmological observations.

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arXiv2006

Nonextensivity in Geological Faults?

Geological fault systems, as the San Andreas fault (SAF) in USA, constitute typical examples of self-organizing systems in nature. In this paper, we have considered some geophysical properties of the SAF system to test the viability of the nonextensive models for earthquakes developed in [Phys. Rev. E {\bf 73}, 026102, 2006]. To this end, we have used 6188 earthquakes events ranging in the magnitude interval $2 < m < 8$ that were taken from the Network Earthquake International Center catalogs (NEIC, 2004-2006) and the Bulletin of the International Seismological Centre (ISC, 1964-2003). For values of the Tsallis nonextensive parameter $q \simeq 1.68$, it is shown that the energy distribution function deduced in above reference provides an excellent fit to the NEIC and ISC SAF data.

cond-mat.stat-mech↗

arXiv2006

A note on cosmological parameters and the topology of the universe

Given the wealth of increasingly accurate cosmological observations, especially the recent results from the WMAP, and the development of methods and strategies in the search for cosmic topology, it is reasonable to expect that we should be able to detect the spatial topology of the Universe in the near future. Motivated by this, we examine to what extent a possible detection of a nontrivial topology of positively curved universe may be used to place constraints on the matter content of the Universe. We show through concrete examples that the knowledge of the spatial topology allows to place constraints on the density parameters associated to dark matter ($Ω_m$) and dark energy ($Ω_Λ$)

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arXiv2006

Cosmological Parameters and Cosmic Topology

Geometry constrains but does not dictate the topology of the 3--dimensional space. In a locally spatially homogeneous and isotropic universe, however, the topology of its spatial section dictates its geometry. We show that, besides determining the geometry, the knowledge of the spatial topology through the circles--in--the--sky offers an effective way of setting constraints on the density parameters associated with dark matter ($Ω_m$) and dark energy ($Ω_Λ$). By assuming the Poincaré dodecahedral space as the circles--in--the--sky detectable topology of the spatial sections of the Universe, we re-analyze the constraints on the density parametric plane $Ω_{m}$--$ Ω_Λ $ from the current type Ia supenovae (SNe Ia) plus X-ray gas mass fraction data, and show that a circles--in--the sky detection of the dodecahedral space topology give rise to strong and complementary constraints on the region of the density parameter plane currently allowed by these observational data sets.

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arXiv2006

Constraints on the DGP Model from Recent Supernova Observations and Baryon Acoustic Oscillations

Although there is mounting observational evidence that the expansion of our universe is undergoing a late-time acceleration, the mechanism for this acceleration is yet unknown. In the so-called Dvali-Gabadadze-Porrati (DGP) model this phenomena is attributed to gravitational leakage into extra dimensions. In this work, we mainly focus our attention to the constraints on the model from the gold sample of type Ia supernovae (SNeIa), the first year data from the Supernova Legacy Survey (SNLS) and the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS). At 99.73% confidence level, the combination of the three databases provides Omega_m=0.270^{+0.018}_{-0.017} and Omega_{r_c}=0.216^{+0.012}_{-0.013} (hence a spatially closed universe with Omega_k=-0.350^{+0.080}_{-0.083}), which seems to be in contradiction with the most recent WMAP results indicating a flat universe. Based on this result, we also estimated the transition redshift (at which the universe switches from deceleration to acceleration) to be 0.70 < z_{q=0} < 1.01, at 2 sigma confidence level.

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arXiv2006

Constraining nonextensive statistics with plasma oscillation data

We discuss experimental constraints on the free parameter of the nonextensive kinetic theory from measurements of the thermal dispersion relation in a collisionless plasma. For electrostatic plane-wave propagation, we show through a statistical analysis that a good agreement between theory and experiment is possible if the allowed values of the $q$-parameter are restricted by $q=0.77 \pm 0.03$ at 95% confidence level (or equivalently, $2-q = 1.23$, in the largely adopted convention for the entropy index $q$). Such a result rules out (by a large statistical margin) the standard Bohm-Gross dispersion relation which is derived assuming that the stationary Maxwellian distribution ($q=1$) is the unperturbed solution.

cond-mat.stat-mech↗

arXiv2006

Nonextensive models for earthquakes

We have revisited the fragment-asperity interaction model recently introduced by Sotolongo-Costa and Posadas (Physical Review Letters 92, 048501, 2004) by considering a different definition for mean values in the context of Tsallis nonextensive statistics and introducing a new scale between the earthquake energy and the size of fragment $ε\propto r^3$. The energy distribution function (EDF) deduced in our approach is considerably different from the one obtained in the above reference. We have also tested the viability of this new EDF with data from two different catalogs (in three different areas), namely, NEIC and Bulletin Seismic of the Revista Brasileira de Geof\'ısica. Although both approaches provide very similar values for the nonextensive parameter $q$, other physical quantities, e.g., the energy density differs considerably, by several orders of magnitude.

cond-mat.stat-mech↗

arXiv2005

Observational constraints on the time-dependence of dark energy

One of the most important questions nowadays in physics concerns the nature of the so-called dark energy. It is also a consensus among cosmologists that such a question will not be answered on the basis only of observational data. However, it is possible to diminish the range of possibilities for this dark component by comparing different dark energy scenarios and finding which models can be ruled out by current observations. In this paper, by asssuming three distinct parametrizations for the low-redshift evolution of the dark energy equation of state (EOS), we consider the possibility of discriminating between evolving dark energy and $Λ$CDM models from a joint analysis involving the most recent radio sources gravitational lensing sample, namely, the Cosmic All Sky Survey (CLASS) statistical data and the recently published \emph{gold} SNe Ia sample. It is shown that this particular combination of observational data restricts considerably the dark energy parameter space, which enables possible distinctions between time-dependent and constant EOS's.

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arXiv2005

Interpreting Cosmological Vacuum Decay

The cosmological vacuum decay scenario recently proposed by Wang and Meng \cite{wm} is rediscussed. From thermodynamic arguments it is found that the $ε$ parameter quantifying the vacuum decay rate must be positive in the presence of particle creation. If there is no particle creation, the proper mass of Cold Dark Matter (CDM) particles is necessarily a time dependent quantity, scaling as $m(t) = m_o a(t)^ε$. By considering the presence of baryons in the cosmological scenario, it is also shown that their dynamic effect is to alter the transition redshift $z_*$ (the redshift at which the Universe switches from decelerating to accelerating expansion), predicting values of $z_*$ compatible with current estimates based on type Ia supernova. In order to constrain the $Ω_m - ε$ plane, a joint statistical analysis involving the current supernovae observations, gas mass fraction measurements in galaxy clusters and CMB data is performed. At 95% c.l. it is found that the vacuum decay rate parameter lies on the interval $ε= 0.11 \pm 0.12$). The possibility of a vacuum decay into photons is also analyzed. In this case, the energy density of the radiation fluid scales as $ρ_r = ρ_{ro}a^{-4 + ε}$, and its temperature evolution law obeys $T(t) = T_oa(t)^{ε/4 - 1}$.

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arXiv2005

Energy Conditions and Segre Classification of Phantom Fields

Recent discoveries in the field of observational cosmology have provided increasing evidence that the Universe is undergoing a late time acceleration, which has also stimulated speculations on the nature of the dark component responsible for such a phenomenon. Among several candidates discussed in the current literature, phantom fields, an exotic scalar field with a negative kinetic term and that violates most of the classical energy conditions, appear as a real possibility according to recent observational analysis. In this paper we examine the invariant characterization for the energy-momentum tensor of phantom fields through the Segre algebraic classification in the framework of general relativity. We also discuss some constraints which are imposed on the values of $V(ϕ)$ from the classical energy conditions.

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