arXiv subjects

J. S. Alcaniz

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

At least 109 records · Page 6Linked to original sources

arXiv2009

Simplified Quartessence Cosmology

We propose a new class of accelerating world models unifying the cosmological dark sector (dark matter and dark energy). All the models are described by a simplified version of the Chaplygin gas Quartessence cosmology. It is found that even for $Ω_k \neq 0$, this Quartessence scenario depends only on a pair of parameters which can severely be constrained by the cosmological tests. As an example we perform a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations. In our analysis we have considered the SNe type Ia Union sample compiled by Kowalski et al. (2008). At 95.4% (c.l.), we find for BAO + Union sample, $α=0.81^{+0.04}_{-0.04}$ and $Ω_{\rm Q4}=1.15^{+0.16}_{-0.17}$. The best fit for this simplified Quartessence scenario is a spatially closed Universe and its reduced $χ^2$ is exactly the same of the flat concordance model (LambdaCDM)

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arXiv2008

New coupled quintessence cosmology

A component of dark energy has been recently proposed to explain the current acceleration of the Universe. Unless some unknown symmetry in Nature prevents or suppresses it, such a field may interact with the pressureless component of dark matter, giving rise to the so-called models of coupled quintessence. In this paper we propose a new cosmological scenario where radiation and baryons are conserved, while the dark energy component is decaying into cold dark matter (CDM). The dilution of CDM particles, attenuated with respect to the usual $a^{-3}$ scaling due to the interacting process, is characterized by a positive parameter $ε$, whereas the dark energy satisfies the equation of state $p_x=ωρ_x$ ($ω< 0$). We carry out a joint statistical analysis involving recent observations from type Ia supernovae, baryon acoustic oscillation peak, and Cosmic Microwave Background shift parameter to check the observational viability of the coupled quintessence scenario here proposed.

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arXiv2008

Cosmological Constraints from Hubble Parameter on f(R) Cosmologies

Modified $f(R)$ gravity in the Palatini approach has been presently applied to Cosmology as a realistic alternative to dark energy. In this concern, a number of authors have searched for observational constraints on several $f(R)$ gravity functional forms using mainly data of type Ia supenovae (SNe Ia), Cosmic Microwave Background ({\rm CMB}) radiation and Large Scale Structure ({\rm LSS}). In this paper, by considering a homogeneous and isotropic flat universe, we use determinations of the Hubble function $H(z)$, which are based on differential age method, to place bounds on the free parameters of the $f(R) = R - β/R^{n}$ functional form. We also combine the $H(z)$ data with constraints from Baryon Acoustic Oscillations ({\rm BAO}) and {\rm CMB} measurements, obtaining ranges of values for $n$ and $β$ in agreement with other independent analyses. We find that, for some intervals of $n$ and $β$, models based on $f(R) = R - β/R^{n}$ gravity in the Palatini approach, unlike the metric formalism, can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

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arXiv2008

Latest supernovae constraints on f(R) cosmologies

A class of modified gravity, known as $f(R)$-gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. In this paper we use the most recent Type-Ia Supernova (SNe Ia) data, the so-called \emph{Union} sample of 307 SNe Ia, to place bounds on a theory of the form $f(R)=R - β/R^n$ within the Palatini approach. Given the complementarity of SNe Ia data with other cosmological observables, a joint analysis with measurements of baryon acoustic oscillation peak and estimates of the {\rm CMB} shift parameter is also performed. We show that, for the allowed intervals of $n$, $Ω_{mo}$, and $β$, models based on $f(R) = R - β/R^{n}$ gravity in the Palatini approach can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

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arXiv2008

Energy conditions constraints on a class of f(R)-gravity

We present and discuss the bounds from the energy conditions on a general f(R) functional form in the framework of metric variational approach. As a concrete application of the energy conditions to locally homogeneous and isotropic f(R)-cosmology, the recent estimated values of the deceleration and jerk parameters are used to examine the bounds from the weak energy condition on the free parameter of the family of f(R)=\sqrt{R^2 - R_0^2} gravity theory.

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arXiv2008

Hybrid dark energy

Extending previous results [Phys. Rev. Lett. 97, 081301 (2006)], we explore the cosmological implications of a new quintessence scenario driven by a slow rolling homogeneous scalar field whose equation of state behaved as freezing over the entire cosmic evolution, is approaching -1 today, but will become thawing in the near future, thereby driving the Universe to an eternal deceleration. We argue that such a mixed behavior, named \emph{hybrid}, may reconcile the slight preference of current observational data for freezing potentials with the impossibility of defining observables in the String/M-theory context due to the existence of a cosmological event horizon in asymptotically de Sitter universes as, e.g., pure freezing scenarios.

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arXiv2008

A parametric model for dark energy

Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in Cosmology but also in high energy physics theories. Following this approach, we explore in this paper some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift $z$ over the entire cosmological evolution, i.e., $z\in [-1,\infty)$. This parameterization allows us to divide the parametric plane $(w_0,w_1)$ in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. A statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution $H(z)$ is performed to check the observational viability of the EoS parameterization here proposed.

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arXiv2008

Observational constraints on late-time Lambda(t) cosmology

The cosmological constant, 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 cosmological constant problem, requiring a natural explanation for its small, but nonzero, value. In this paper we study cosmological consequences of a scenario driven by a varying cosmological term, in which the vacuum energy density decays linearly with the Hubble parameter. We test the viability of this scenario and study a possible way to distinguish it from the current standard cosmological model by using recent observations of type Ia supernova (Supernova Legacy Survey Collaboration), measurements of the baryonic acoustic oscillation from the Sloan Digital Sky Survey and the position of the first peak of the cosmic microwave background angular spectrum from the three-year Wilkinson Microwave Anisotropy Probe.

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arXiv2007

Testing power-law cosmology with galaxy clusters

Power-law cosmologies, in which the cosmological scale factor evolves as a power law in the age, $a \propto t^α$ with $α\ga 1$, regardless of the matter content or cosmological epoch, is comfortably concordant with a host of cosmological observations.} {In this article, we use recent measurements of the X-ray gas mass fractions in clusters of galaxies to constrain the $α$ parameter with curvature $k = \pm1, 0$. We find that the best fit happens for an open scenario with the power index $α= 1.14 \pm 0.05$, though the flat and closed model can not be rule out at very high confidence level.} {Our results are in agreement with other recent analyses and show that the X-ray gas mass fraction measurements in clusters of galaxies provide a complementary test to the power law cosmology.

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arXiv2007

Current constraints on interacting holographic dark energy

Although there is mounting observational evidence that the cosmic expansion is undergoing a late-time acceleration, the physical mechanism behind such a phenomenon is yet unknown. In this paper, we investigate a holographic dark energy (HDE) model with interaction between the components of the dark sector in the light of current cosmological observations. We use both the new \emph{gold} sample of 182 type Ia supernovae (SNe Ia) and the 192 SNe Ia ESSENCE data, the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey and the shift parameter from the three-year Wilkinson Microwave Anisotropy Probe data. In agreement with previous results, we show that these observations suggest a very weak coupling or even a noninteracting HDE. The phantom crossing behavior in the context of these scenarios is also briefly discussed.

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arXiv2007

Cosmological bounds on oscillating dark energy models

We study some cosmological constraints on the two phenomenological models of oscillating dark energy. In these scenarios, the equation of state of dark energy varies periodically and may provide a way to unify the early acceleration (inflation) and the late time acceleration of the universe. These models give also an effective way to tackle the so-called cosmic coincidence problem. We examine observational constraints on this class of models from the latest observational data including the \emph{gold} sample of 182 type Ia supernovae, the CMB shift parameter $R$ and the BAO measurements from the Sloan Digital Sky Survey.

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arXiv2007

Energy conditions in f(R)-gravity

In order to shed some light on the current discussion about f(R)-gravity theories we derive and discuss the bounds imposed by the energy conditions on a general f(R) functional form. The null and strong energy conditions in this framework are derived from the Raychaudhuri's equation along with the requirement that gravity is attractive, whereas the weak and dominant energy conditions are stated from a comparison with the energy conditions that can be obtained in a direct approach via an effective energy-momentum tensor for f(R)-gravity. As a concrete application of the energy conditions to locally homogeneous and isotropic f(R)-cosmology, the recent estimated values of the deceleration and jerk parameters are used to examine the bounds from the weak energy condition on the parameters of two families of f(R)-gravity theories.

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arXiv2007

Coupled quintessence and vacuum decay

We discuss observational consequences of a class of cosmological models characterized by the dilution of pressureless matter attenuated with respect to the usual $a^{-3}$ scaling due to the decay of vacuum energy. We carry out a joint statistical analysis of observational data from the new \emph{gold} sample of 182 SNe Ia, recent estimates of the CMB shift parameter, and BAO measurements from the SDSS to show that such models favor the decay of vacuum only into the dark matter sector, and that the separately conserved baryons cannot be neglected. In order to explore ways to more fundamentally motivated models, we also derive a coupled scalar field version for this general class of vacuum decay scenarios.

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arXiv2007

Lookback time bounds from energy conditions

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 (FLRW) approach to cosmological modeling, where the energy and matter components of the cosmic fluid are unknown, the energy conditions provide model-independent bounds on the behavior of the lookback time of cosmic sources as a function of the redshift for any value of the spatial curvature. We also confront such bounds with a lookback time sample which is built from the age estimates of 32 galaxies lying in the interval $0.11 \lesssim z \lesssim 1.84$ and by assuming the total expanding age of the Universe to be $13.7 \pm 0.2$ Gyr, as obtained from current cosmic microwave background experiments. In agreement with previous results, we show that all energy conditions seem to have been violated at some point of the recent past of cosmic evolution.

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arXiv2007

Nonextensive effects on the relativistic nuclear equation of state

The Walecka many-body field theory is investigated in the context of quantum nonextensive statistical mechanics, characterized by a dimensionless parameter $q$. We consider nuclear matter described statistically by a power-law distribution which generalizes the standard Fermi-Dirac distribution ($q = 1$). We show that the scalar and vector meson fields become more intense due to the nonextensive effects ($q \neq 1$). From a numerical treatment, we also show that as the nonextensive parameter $q$ increases, the nucleon effective mass diminishes and the equation of state becomes stiffer. Finally, the usual Maxwell construction seems not to be necessary for isotherms with temperatures in the range 14 Mev$<k_BT<$20 MeV.

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arXiv2007

Energy Conditions and Cosmic Acceleration

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors $T_{μν}$ in different frameworks, and to derive general results that hold in a variety of general contexts on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach, 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 for any spatial curvature. We use the most recent type Ia supernovae (SNe Ia) observations, which include the new Hubble Space Telescope SNe Ia events, to carry out a model-independent analysis of the energy conditions violation in the context of the standard cosmology. We show that both the null (NEC), weak (WEC) and dominant (DEC) conditions, which are associated with the existence of the so-called phantom fields, seem to have been violated only recently ($z \lesssim 0.2$), whereas the condition for attractive gravity, i.e., the strong energy condition (SEC) was firstly violated billions of years ago, at $z \gtrsim 1$.

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arXiv2007

CMB and LSS constraints on a single-field model of inflation

A new inflationary scenario whose exponential potential $V(Φ)$ has a quadratic dependence on the field $Φ$ in addition to the standard linear term is confronted with the tree-year observations of the Wilkinson-Microwave Anisotropy Probe and the Sloan Digital Sky Survey data. The number of e-folds ($N$), the ratio of tensor-to-scalar perturbations ($r$), the spectral scalar index of the primordial power spectrum ($n_s$) and its running ($dn_s/d\ln k$) depend on the dimensionless parameter $α$ multiplying the quadratic term in the potential. In the limit $α\to 0$ all the results of the standard exponential potential are fully recovered. For values of $α\neq 0$, we find that the model predictions are in good agreement with the current observations of the Cosmic Microwave Background (CMB) anisotropies and Large-Scale Structure (LSS) in the Universe.

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arXiv2007

Age constraints on the cosmic equation of state

Dark energy is the invisible fuel that seems to drive the current acceleration of the Universe. Its presence, which is inferred from an impressive convergence of high-quality observational results along with some sucessful theoretical predictions, is also supported by the current estimates of the age of the Universe from dating of local and high-$z$ objects. In this work we study observational constraints on the dark energy equation of state ($w$) from lookback time measurements of high-$z$ galaxies, as recently published by the Gemini Deep Deep Survey (GDDS). In order to build up our lookback time sample from these observations we use 8 high-$z$ galaxies in the redshift interval $1.3 \leq z \leq 2.2$ and assume the total expanding age of the Universe to be $t_{0}^{obs} = 13.6^{+0.4}_{-0.3}$ Gyr, as obtained from current cosmic microwave background data. We show that these age measurements are compatible with values of $w$ close to -1, although there is still space for quintessence ($w > -1$) and phantom ($w < -1$) behaviors. In order to break possible degeneracies in the $Ω_{\rm{m}} - w$ plane, we also discuss the bounds on this parametric space when GDDS lookback time measurements are combined with the most recent SNe Ia, CMB and LSS data.

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