arXiv subjects

J. S. Alcaniz

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

At least 91 records · Page 5Linked to original sources

arXiv2011

$Λ(t)$CDM Model as a Unified Origin of Holographic and Agegraphic Dark Energy Models

Motivated by the fact that any nonzero $Λ$ can introduce a length scale or a time scale into Einstein's theory, $r_Λ=ct_Λ=\sqrt{3/|Λ|}$. Conversely, any cosmological length scale or time scale can introduce a $Λ(t)$, $Λ(t)=3/r^2_Λ(t)=3/(c^2t^2_Λ(t))$. In this letter, we investigate the time varying $Λ(t)$ corresponding to the length scales, including the Hubble horizon, the particle horizon and the future event horizon, and the time scales, including the age of the universe and the conformal time. It is found out that, in this scenario, the $Λ(t)$CDM model can be taken as the unified origin of the holographic and agegraphic dark energy models with interaction between the matter and the dark energy, where the interacting term is determined by $Q=-\dotρ_Λ$. We place observational constraints on the $Λ(t)$CDM models originating from different cosmological length scales and time scales with the recently compiled "Union2 compilation" which consists of 557 Type Ia supernovae (SNIa) covering a redshift range $0.015\leq z \leq 1.4$. In conclusion, an accelerating expansion universe can be derived in the cases taking the Hubble horizon, the future event horizon, the age of the universe and the conformal time as the length scale or the time scale.

astro-ph.CO↗

arXiv2011

Probing the time dependence of dark energy

A new method to investigate a possible time-dependence of the dark energy equation of state $w$ is proposed. We apply this methodology to two of the most recent data sets of type Ia supernova (Union2 and SDSS) and the baryon acoustic oscillation peak at $z = 0.35$. For some combinations of these data, we show that there is a clear departure from the standard $Λ$CDM model at intermediary redshifts, although a non-evolving dark energy component ($dw/dz = 0$) cannot be ruled out by these data. The approach developed here may be useful to probe a possible evolving dark energy component when applied to upcoming observational data.

astro-ph.CO↗

arXiv2011

Cosmography and cosmic acceleration

We investigate the prospects for determining the accelerating history of the Universe from upcoming measurements of the expansion rate $H(z)$. In our analyses, we use Monte Carlo simulations based on $w$CDM models to generate samples with different characteristics and calculate the evolution of the deceleration parameter $q(z)$. We show that a cosmographic (and, therefore, model-independent) evidence for cosmic acceleration ($q(z<z_t) < 0$, where $z_t$ is the transition redshift) will only be possible with an accuracy in $H(z)$ data greater than the expected in current planned surveys. A brief discussion about the prospects for reconstructing the dark energy equation of state from the parameters $H(z)$ and $q(z)$ is also included.

astro-ph.CO↗

arXiv2011

The Deformable Universe

The concept of smooth deformations of a Riemannian manifolds, recently evidenced by the solution of the Poincaré conjecture, is applied to Einstein's gravitational theory and in particular to the standard FLRW cosmology. We present a brief review of the deformation of Riemannian geometry, showing how such deformations can be derived from the Einstein-Hilbert dynamical principle. We show that such deformations of space-times of general relativity produce observable effects that can be measured by four-dimensional observers. In the case of the FLRW cosmology, one such observable effect is shown to be consistent with the accelerated expansion of the universe.

gr-qc↗

arXiv2010

Cosmological Consequences of Exponential Gravity in Palatini Formalism

We investigate cosmological consequences of a class of exponential $f(R)$ gravity in the Palatini formalism. By using the current largest type Ia Supernova sample along with determinations of the cosmic expansion at intermediary and high-$z$ we impose tight constraints on the model parameters. Differently from other $f(R)$ models, we find solutions of transient acceleration, in which the large-scale modification of gravity will drive the Universe to a new decelerated era in the future. We also show that a viable cosmological history with the usual matter-dominated era followed by an accelerating phase is predicted for some intervals of model parameters.

astro-ph.CO↗

arXiv2010

Current constraints on the epoch of cosmic acceleration

The cosmographic expansion history of the universe is investigated by using the 557 type Ia supernovae from the Union2 Compilation set along with the current estimates involving the product of the CMB acoustic scale $\ell_{A}$ and the BAO peak at two different redshifts. Using a well-behaved parameterization for the deceleration parameter, $q(z) = q_0 + q_1z/(1 + z)$, we estimate the accelerating redshift $z_{acc}=-q_0/(q_0 + q_1)$ (at which the universe switches from deceleration to acceleration) and investigate the influence of a non-vanishing spatial curvature on these estimates. We also use the asymptotic value of $q(z)$ at high-$z$ to place more restrictive bounds on the model parameters $q_0$ and $q_1$, which results in a more precise determination of the epoch of cosmic acceleration.

astro-ph.CO↗

arXiv2010

Cosmographic constraints on a class of Palatini f(R) gravity

Modified gravity, known as $f(R)$ gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. For this kind of gravity the expansion of the Universe may accelerate while containing only baryonic and cold dark matter. The aim of the present investigation is to place cosmographic constraints on the class of theories of the form $f(R)=R - α/R^n$ within the Palatini approach. Although extensively discussed in recent literature and confronted with several observational data sets, cosmological tests are indeed inconclusive about the true signal of $n$ in this class of theories. This is particularly important to define which kind of corrections (infra-red or high-energy) to general relativity this class of theory indeed represent. We shed some light on this question by examining the evolution of the deceleration parameter $q(z)$ for these theories. We find that for a large range of $α$, models based on $f(R) = R - α/R^{n}$ gravity in the Palatini approach can only have positive values for $n$, placing thus a broad restriction on this class of gravity.

astro-ph.CO↗

arXiv2010

Using a phenomenological model to test the coincidence problem of dark energy

By assuming a phenomenological form for the ratio of the DE and DM densities $ρ_X\propto ρ_{m} a^ξ$ (Dalal et al.2001), we discuss the cosmic coincidence problem in light of current observational data. $ξ$ is a key parameter to denote the severity of the coincidence problem. In this scenario, $ξ=3$ and $ξ=0$ correspond to $Λ$CDM and the self-similar solution without coincidence problem, respectively. Hence, any solution with a scaling parameter $0<ξ<3$ makes the coincidence problem less severe. In addition, the standard cosmology without interaction between DE and DM is characterized by $ξ+3ω_X=0$, whereas the inequality $ξ+3ω_X\neq 0$ represents non-standard cosmology. We place observational constraints on the parameters $(Ω_{X,0},ω_{X},ξ)$ of this model, by using the Constitution Set, the CMB shift parameter from the 5-year WMAP and the SDSS baryon acoustic peak. Combining the three samples, we get $Ω_{X,0}=0.72\pm0.02$, $ω_{X}=-0.98\pm0.07$ and $ξ=3.06\pm0.35$ at 68.3% confidence level. The result shows that the $Λ$CDM model still remains a good fit to the recent observational data, as well as, the coincidence problem indeed exists and is quite severe, in the framework of this simple phenomenological model. We further constrain the model with the transition redshift (deceleration / acceleration). It shows that if the transition from deceleration to acceleration happens at the redshit $z > 0.73$, within the framework of this model, we can conclude that the interaction between DE and DM is necessary.

astro-ph.CO↗

arXiv2009

Transient cosmic acceleration

We explore cosmological consequences of two quintessence models in which the current cosmic acceleration is a transient phenomenon. We argue that one of them (in which the EoS parameter switches from freezing to thawing regimes) may reconcile the slight preference of observational data for freezing potentials with the impossibility of defining observables in String/M-theory due to the existence of a cosmological event horizon in asymptotically de Sitter universes.

astro-ph.CO↗

arXiv2009

Constraining dark matter-dark energy interaction with gas mass fraction in galaxy clusters

The recent observational evidence for the current cosmic acceleration have stimulated renewed interest in alternative cosmologies, such as scenarios with interaction in the dark sector (dark matter and dark energy). In general, such models contain an unknown negative-pressure dark component coupled with the pressureless dark matter and/or with the baryons that results in an evolution for the Universe rather different from the one predicted by the standard $Λ$CDM model. In this work we test the observational viability of such scenarios by using the most recent galaxy cluster gas mass fraction versus redshift data (42 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.063 < z < 1.063) (Allen et al. 2008) to place bounds on the parameter $ε$ that characterizes the dark matter/dark energy coupling. The resulting are consistent with, and typically as constraining as, those derived from other cosmological data. Although a time-independent cosmological constant ($Λ$CDM model) is a good fit to these galaxy cluster data, an interacting dark energy component cannot yet be ruled out.

astro-ph.CO↗

arXiv2009

Cosmological consequences of a possible $Λ$-dark matter interaction

We propose a general class of interacting models in which the interaction between the CDM component and $Λ$ is parameterized by an arbitrary function of the cosmic scale factor $ε(a)$. Differently from other dynamical $Λ$ scenarios in which the final stage of cosmic expansion is a de Sitter phase, we find solutions of transient acceleration, in which the $Λ$-dark matter interaction will drive the Universe to a new dark matter-dominated era in the future. We investigate some cosmological consequences of this model and discuss some constraints on its parameters from current SNe Ia, BAO, CMB and $H_0$ data.

astro-ph.CO↗

arXiv2009

Nonextensive Quantum H-Theorem

A proof of the quantum $H$-theorem taking into account nonextensive effects on the quantum entropy $S^Q_q$ is shown. The positiveness of the time variation of $S^Q_q$ combined with a duality transformation implies that the nonextensive parameter $q$ lies in the interval [0,2]. It is also shown that the equilibrium states are described by quantum $q$-power law extensions of the Fermi-Dirac and Bose-Einstein distributions. Such results reduce to the standard ones in the extensive limit, thereby showing that the nonextensive entropic framework can be harmonized with the quantum distributions contained in the quantum statistics theory.

quant-ph↗

arXiv2009

A generalized equation of state for dark energy

A generalized parameterization $w_β(z)$ for the dark energy equation of state (EoS) is proposed and some of its cosmological consequences are investigated. We show that in the limit of the characteristic dimensionless parameter $β\to +1, 0$ and -1 some well-known EoS parameterizations are fully recovered whereas for other values of $β$ the proposed parameterization admits a wider and new range of cosmological solutions. We also discuss possible constraints on the $w_β(z)$ parameters from current observational data.

astro-ph.CO↗

arXiv2009

Nonextensive effects on the phase structure of QHD

We investigate nonextensive effects on phase transition in nuclear matter in the context Walecka many-body field theory. Changes in the general behavior are observed when the results calculated for the nuclear matter at vanishing baryon density are compared to those obtained through the standard Fermi-Dirac distribution. It is observed a dependence between the nonextensive parameter $q$ and the coupling constants $C_S^2$ of the phase transition. A numerical relation for this thermodynamical dependence is also proposed.

nucl-th↗

arXiv2009

Current lookback time-redshift bounds on dark energy

We investigate observational constraints on dark energy models from lookback time (LT) estimates of 32 old passive galaxies distributed over the redshift interval $0.11 \leq z \leq 1.84$. To build up our LT sample we combine the age measurements for these 32 objects with estimates of the total age of the Universe, as obtained from current CMB data. We show that LT data may provide bounds on the cosmological parameters with accuracy competitive with type Ia Supernova methods. In order to break possible degeneracies between models parameters, we also discuss the bounds when our lookback time versus redshift sample is combined with with the recent measurement of the baryonic acoustic oscillation peak and the derived age of the Universe from current CMB measurements.

astro-ph.CO↗

arXiv2009

Geometry of Dark Energy

The acceleration of the universe is described as a dynamical effect of the extrinsic curvature of space-time. By extending previous results, the extrinsic curvature is regarded as an independent spin-2 field, determined by a set of non-linear equations similar to Einstein's equations. In this framework, we investigate some cosmological consequences of this class of scenarios and test its observational viability by performing a statistical analysis with current type Ia Supernova data.

astro-ph.CO↗

arXiv2009

Cosmology with interaction in the dark sector

Unless some unknown symmetry in Nature prevents or suppresses a non-minimal coupling in the dark sector, the dark energy field may interact with the pressureless component of dark matter. In this paper, we investigate some cosmological consequences of a general model of interacting dark matter-dark energy characterized by a dimensionless parameter $ε$. We derive a coupled scalar field version for this general class of scenarios and carry out a joint statistical analysis involving SNe Ia data ({Legacy} and {Constitution} sets), measurements of baryon acoustic oscillation peak at $z = 0.20$ (2dFGRS) and $z = 0.35$ (SDSS), and measurements of the Hubble evolution $H(z)$. For the specific case of vacuum decay ($w = -1$), we find that, although physically forbidden, a transfer of energy from dark matter to dark energy is favored by the data.

astro-ph.CO↗

arXiv2009

Non-gaussian statistics and the relativistic nuclear equation of state

We investigate possible effects of quantum power-law statistical mechanics on the relativistic nuclear equation of state in the context of the Walecka quantum hadrodynamics theory. By considering the Kaniadakis non-Gaussian statistics, characterized by the index $κ$ (Boltzmann-Gibbs entropy is recovered in the limit $κ\to 0$), we show that the scalar and vector meson fields become more intense due to the non-Gaussian statistical effects ($κ\neq 0$). From an analytical treatment, an upper bound on $κ$ ($κ< 1/4$) is found. We also show that as the parameter $κ$ increases the nucleon effective mass diminishes and the equation of state becomes stiffer. A possible connection between phase transitions in nuclear matter and the $κ$-parameter is largely discussed.

nucl-th↗