arXiv subjects

R. Silva

Publications and source records attributed to R. Silva.

At least 55 records · Page 3Linked to original sources

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

Non-gaussian statistics and stellar rotational velocities of main sequence field stars

In this letter we study the observed distributions of rotational velocity in a sample of more than 16,000 nearby F and G dwarf stars, magnitude complete and presenting high precision $Vsin i$ measurements. We show that the velocity distributions cannot be fitted by a maxwellian. On the other hand, an analysis based on both Tsallis and Kaniadakis power-law statistics is by far the most appropriate statistics and give a very good fit. It is also shown that single and binary stars have similar rotational distributions. This is the first time, to our knowledge, that these two new statistics are tested for the rotation of such a large sample of stars, pointing solidly to a solution of the puzzling problem on the function governing the distribution of stellar rotational velocity

astro-ph.SR↗

arXiv2009

Power-law statistics and stellar rotational velocities in the Pleiades

In this paper we will show that, the non-gaussian statistics framework based on the Kaniadakis statistics is more appropriate to fit the observed distributions of projected rotational velocity measurements of stars in the Pleiades open cluster. To this end, we compare the results from the $κ$ and $q$-distributions with the Maxwellian.

astro-ph.SR↗

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↗

arXiv2008

Conservative Force Fields in Non-Gaussian Statistics

In this letter, we determine the $κ$-distribution function for a gas in the presence of an external field of force described by a potential U(${\bf r}$). In the case of a dilute gas, we show that the $κ$-power law distribution including the potential energy factor term can rigorously be deduced in the framework of kinetic theory with basis on the Vlasov equation. Such a result is significant as a preliminary to the discussion on the role of long range interactions in the Kaniadakis thermostatistics and the underlying kinetic theory.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2006

The relativistic statistical theory and Kaniadakis entropy: an approach through a molecular chaos hypothesis

We have investigated the proof of the $H$ theorem within a manifestly covariant approach by considering the relativistic statistical theory developed in [G. Kaniadakis, Phy. Rev. E {\bf 66}, 056125, 2002; {\it ibid.} {\bf 72}, 036108, 2005]. As it happens in the nonrelativistic limit, the molecular chaos hypothesis is slightly extended within the Kaniadakis formalism. It is shown that the collisional equilibrium states (null entropy source term) are described by a $κ$ power law generalization of the exponential Juttner distribution, e.g., $f(x,p)\propto (\sqrt{1+ κ^2θ^2}+κθ)^{1/κ}\equiv\exp_κθ$, with $θ=α(x)+β_μp^μ$, where $α(x)$ is a scalar, $β_μ$ is a four-vector, and $p^μ$ is the four-momentum. As a simple example, we calculate the relativistic $κ$ power law for a dilute charged gas under the action of an electromagnetic field $F^{μν}$. All standard results are readly recovered in the particular limit $κ\to 0$.

cond-mat.stat-mech↗

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.

astro-ph↗

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

Relativistic statistical theory and generalized stosszahlansatz

We have investigated the proof of the $H$ theorem within a manifestly covariant approach by considering the relativistic statistical theory developed in [Phy. Rev. E {\bf 66}, 056125, 2002; {\it ibid.} {\bf 72}, 036108 2005]. In our analysis, however, we have not considered the so-called deformed mathematics as did in the above reference. As it happens in the nonrelativistic limit, the molecular chaos hypothesis is slightly extended within the $κ$-formalism, and the second law of thermodynamics implies that the $κ$ parameter lies on the interval [-1,1]. It is shown that the collisional equilibrium states (null entropy source term) are described by a $κ$ power law generalization of the exponential Juttner distribution, e.g., $f(x,p)\propto (\sqrt{1+ κ^2θ^2}+κθ)^{1/κ}\equiv\exp_κθ$, with $θ=α(x)+β_μp^μ$, where $α(x)$ is a scalar, $β_μ$ is a four-vector, and $p^μ$ is the four-momentum. As a simple example, we calculate the relativistic $κ$ power law for a dilute charged gas under the action of an electromagnetic field $F^{μν}$. All standard results are readly recovered in the particular limit $κ\to 0$.

cond-mat.stat-mech↗

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

The $H$-theorem in $κ$-statistics: influence on the molecular chaos hypothesis

We rediscuss recent derivations of kinetic equations based on the Kaniadakis' entropy concept. Our primary objective here is to derive a kinetical version of the second law of thermodynamycs in such a $κ$-framework. To this end, we assume a slight modification of the molecular chaos hypothesis. For the $H_κ$-theorem, it is shown that the collisional equilibrium states (null entropy source term) are described by a $κ$-power law extension of the exponential distribution and, as should be expected, all these results reduce to the standard one in the limit $κ\to 0$.

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

Relativity, nonextensivity, and extended power law distributions

A proof of the relativistic $H$-theorem by including nonextensive effects is given. As it happens in the nonrelativistic limit, the molecular chaos hypothesis advanced by Boltzmann does not remain valid, and the second law of thermodynamics combined with a duality transformation implies that the q-parameter lies on the interval [0,2]. It is also proved that the collisional equilibrium states (null entropy source term) are described by the relativistic $q$-power law extension of the exponential Juttner distribution which reduces, in the nonrelativistic domain, to the Tsallis power law function. As a simple illustration of the basic approach, we derive the relativistic nonextensive equilibrium distribution for a dilute charged gas under the action of an electromagnetic field $F^{μν}$. Such results reduce to the standard ones in the extensive limit, thereby showing that the nonextensive entropic framework can be harmonized with the space-time ideas contained in the special relativity theory.

cond-mat.stat-mech↗

arXiv2004

Non-extensive statistics and the stellar polytrope index

We use physical constrains imposed from the H-Theorem and from the negative nature of the heat capacity of self-gravitating thermodynamically isolated systems to investigate some possible limits on the stellar polytrope index $n$ within the domain of a classical non-extensive kinetic theory.

astro-ph↗

arXiv2003

Reply to ``Comment on Conservative Force Fields in Nonextensive Kinetic Theory"

It is shown that the comment by Costa and Meneses [1] did not point any technical or conceptual flaw in our paper titled ``Conservative Force Fields in Nonextensive Kinetic Theory" (Physica A 316, 289 (2002)). In particular, the application of the nonextensive distribution to the historical problem of the unbounded isothermal atmosphere under constant gravity is theoretically correct. It should be stressed that our solution is somewhat connected with a similar solution to a problem appearing in the astrophysical domain, namely, the so-called singular isothermal sphere. Actually, both problems are solved by a stationary Tsallis q-distribution with a "thermal cut-off" in the allowed values of the energy.

cond-mat.stat-mech↗