arXiv subjects

J. A. S. Lima

Publications and source records attributed to J. A. S. Lima.

At least 73 records · Page 4Linked to original sources

arXiv2012

Black Hole Formation with an Interacting Vacuum Energy Density

We discuss the gravitational collapse of a spherically symmetric massive core of a star in which the fluid component is interacting with a growing vacuum energy density. The influence of the variable vacuum in the collapsing core is quantified by a phenomenological β-parameter as predicted by dimensional arguments and the renormalization group approach. For all reasonable values of this free parameter, we find that the vacuum energy density increases the collapsing time but it cannot prevent the formation of a singular point. However, the nature of the singularity depends on the values of β. In the radiation case, a trapped surface is formed for β<1/2 whereas for β>1/2, a naked singularity is developed. In general, the critical value is β=1-2/3(1+ω), where the ω-parameter describes the equation of state of the fluid component.

gr-qc↗

arXiv2012

Influence of Small-Scale Inhomogeneities on the Cosmological Consistency Tests

The current cosmological dark sector (dark matter plus dark energy) is challenging our comprehension about the physical processes taking place in the Universe. Recently, some authors tried to falsify the basic underlying assumptions of such dark matter-dark energy paradigm. In this Letter, we show that oversimplifications of the measurement process may produce false positives to any consistency test based on the globally homogeneous and isotropic LCDM model and its expansion history based on distance measurements. In particular, when local inhomogeneity effects due to clumped matter or voids are taken into account, an apparent violation of the basic assumptions ("Copernican Principle") seems to be present. Conversely, the amplitude of the deviations also probes the degree of reliability underlying the phenomenological Dyer-Roeder procedure by confronting its predictions with the accuracy of the weak lensing approach. Finally, a new method is devised to reconstruct the effects of the inhomogeneities in a LCDM model, and some suggestions of how to distinguish between clumpiness (or void) effects from different cosmologies are discussed.

astro-ph.CO↗

arXiv2012

Decaying Vacuum Cosmology and its Scalar Field Description

We discuss the cosmological consequences of an interacting model in the dark sector in which the $Λ$ component evolves as a truncated power series of the Hubble parameter. In order to constrain the free parameters of the model we carry out a joint statistical analysis involving observational data from current type Ia supernovae, recent estimates of the cosmic microwave background shift parameter and baryon acoustic oscillations measurements. Finally, we adopt a theoretical method to derive the coupled scalar field version for this non-equilibrium decaying vacuum accelerating cosmology.

astro-ph.CO↗

arXiv2012

Constraining the dark energy and smoothness parameter with type Ia Supernovae and Gamma-Ray Bursts

The existence of inhomogeneities in the observed Universe modifies the distance-redshift relations thereby affecting the results of cosmological tests in comparison to the ones derived assuming spatially uniform models. By modeling the inhomogeneities through a Zeldovich-Kantowski-Dyer-Roeder (ZKDR) approach which is phenomenologically characterized by a smoothness parameter $α$, we rediscuss the constraints on the cosmic parameters based on Supernovae type Ia and Gamma-Ray Bursts (GRBs) data. The present analysis is restricted to a flat $Λ$CDM model with the reasonable assumption that $Λ$ does not clump. A $χ^{2}$-analysis using 557 SNe Ia data from the Union2 Compilation Data (Amanullah {\it et al.} 2010) constrains the pair of parameters ($Ω_m, α$) to $Ω_m=0.27_{-0.03}^{+0.08}$($2σ$) and $α\geq 0.25$. A similar analysis based only on 59 Hymnium GRBs (Wei 2010) constrains the matter density parameter to be $Ω_m= 0.35^{+0.62}_{-0.24}$ ($2σ$) while all values for the smoothness parameter are allowed. By performing a joint analysis, it is found that $Ω_m = 0.27^{+0.06}_{-0.03}$ and $α\geq 0.52$. As a general result, although considering that current GRB data alone cannot constrain the smoothness $α$ parameter our analysis provides an interesting cosmological probe for dark energy even in the presence of inhomogeneities.

astro-ph.CO↗

arXiv2012

Nonextensive Effects on Chandrasekhar's Dynamical Friction

The motion of a point like object of mass M passing through the background potential of massive collisionless particles (m << M) suffers a steady deceleration named dynamic friction. In his classical work, Chandrasekhar assumed a Maxwellian velocity distribution in the halo and neglected the self gravity of the wake induced by the gravitational focusing of the mass M. In this paper, by relaxing the validity of the Maxwellian distribution due to the presence of long range forces, we derive an analytical formula for the dynamic friction in the context of the q-nonextensive kinetic theory. In the extensive limiting case (q = 1), the classical Gaussian Chandrasekhar result is recovered. As an application, the dynamic friction timescale for Globular Clusters spiraling to the galactic center is explicitly obtained. Our results suggest that the problem concerning the large timescale as derived by numerical N-body simulations or semi-analytical models can be understood as a departure from the standard extensive Maxwellian regime as measured by the Tsallis nonextensive q-parameter.

astro-ph.SR↗

arXiv2012

Probing the Cosmic Distance Duality Relation with the Sunyaev-Zeldovich Effect, X-rays Observations and Supernovae Ia

The angular diameter distances toward galaxy clusters can be determined with measurements of the Sunyaev-Zel'dovich effect and X-ray surface brightness combined with the validity of the distance-duality relation, $D_L(z) (1 + z)^{2}/D_{A}(z) = 1$, where $D_L(z)$ and $D_A(z)$ are, respectively, the luminosity and angular diameter distances. This combination enables us to probe galaxy cluster physics or even to test the validity of the distance-duality relation itself. We explore these possibilities based on two different, but complementary approaches. Firstly, in order to constrain the possible galaxy cluster morphologies, the validity of the distance-duality relation (DD relation) is assumed in the $Λ$CDM framework (WMAP7). Secondly, by adopting a cosmological-model-independent test, we directly confront the angular diameters from galaxy clusters with two supernovae Ia (SNe Ia) subsamples (carefully chosen to coincide with the cluster positions). The influence of the different SNe Ia light-curve fitters in the previous analysis are also discussed. We assumed that $η$ is a function of the redshift parametrized by two different relations: $η(z) = 1 + η_{0}z$, and $η(z)=1 + η_{0}z/(1+z)$, where $η_0$ is a constant parameter quantifying the possible departure from the strict validity of the DD relation. The statistical analysis presented here provides new evidence that the true geometry of clusters is elliptical. We find that the two-light curve fitters (SALT2 and MLCS2K2) present a statistically significant conflict, and a joint analysis involving the different approaches suggests that clusters are endowed with an elliptical geometry as previously assumed.

astro-ph.CO↗

arXiv2012

Constraining H0 from Lyman-alpha Forest and Baryon Acoustic Oscillations

A new method is proposed to measure the Hubble constant H0 through the mean transmitted flux observed from high redshift quasars. A semi-analytical model for the cosmological-independent volume density distribution function is adopted which allows one to obtain constraints over the cosmological parameters once a moderate knowlegde of the InterGalactic Medium (IGM) parameters is assumed. By assuming a flat LCDM cosmology, we show that such method alone cannot provide good constraints on the pair of free parameters (h, Omega_m). However, it is possible possible to break the degeneracy on the mass density parameter by applying a joint analysis involving the baryon acoustic oscillations (BAOs). Our analysis based on two different samples of Lyman-alpha forest restricts the parameters on the intervals 0.58 < h < 0.91 and 0.215 < Omega_m < 0.245 (1 sigma). Although the constraints are weaker comparatively to other estimates, we point out that with a bigger sample and a better knowledge of the IGM this method may present competitive results to measure the Hubble constant independently of the cosmic distance ladder.

astro-ph.CO↗

arXiv2011

New constraints on H_0 and Omega_M from SZE/X-RAY data and Baryon Acoustic Oscillations

The Hubble constant, $H_0$, sets the scale of the size and age of the Universe and its determination from independent methods is still worthwhile to be investigated. In this article, by using the Sunyaev-Zel`dovich effect and X-ray surface brightness data from 38 galaxy clusters observed by Bonamente {\it{et al.}} (2006), we obtain a new estimate of $H_0$ in the context of a flat $Λ$CDM model. There is a degeneracy on the mass density parameter ($Ω_{m}$) which is broken by applying a joint analysis involving the baryon acoustic oscillations (BAO) as given by Sloan Digital Sky Survey (SDSS). This happens because the BAO signature does not depend on $H_0$. Our basic finding is that a joint analysis involving these tests yield $H_0= 0.765^{+0.035}_{-0.033}$ km s$^{-1}$ Mpc$^{-1}$ and $Ω_{m}=0.27^{+0.03}_{-0.02}$. Since the hypothesis of spherical geometry assumed by Bonamente {\it {et al.}} is questionable, we have also compared the above results to a recent work where a sample of triaxial galaxy clusters has been considered.

astro-ph↗

arXiv2011

Deformed Distance Duality Relations and Supernovae Dimming

The basic cosmological distances are linked by the Etherington cosmic distance duality relation, $η(z) = D_{L}(z)(1+z)^{-2}/D_{A}(z) \equiv 1$, where $D_{L}$ and $D_{A}$ are, respectively, the luminosity and angular diameter distances. In order to test its validity, some authors have proposed phenomenological expressions for $η(z)$ thereby deforming the original Etherington's relation and comparing the resulting expressions with the available and future cosmological data. The relevance of such studies is unquestionable since any violation of the cosmic distance duality relation could be the signal of new physics or non-negligible astrophysical effects in the usually assumed perfectly transparent Universe. In this letter, we show that under certain conditions such expressions can be derived from a more fundamental approach with the parameters appearing in the $η(z)$ expression defining the cosmic absorption parameter as recently discussed by Chen and Kantowski. Explicit examples involving four different parametrizations of the deformation function are given. Based on such an approach, it is also found that the latest Supernova data can also be explained in the framework of a pure cold dark matter model (Einstein-de Sitter). Two different scenarios with cosmic absorption are discussed. Only if the cosmic opacity is fully negligible, the description of an accelerating Universe powered by dark energy or some alternative gravity theory must be invoked.

astro-ph.CO↗

arXiv2011

Newtonian Perturbations on Models with Matter Creation

Creation of Cold Dark Matter (CCDM) can macroscopically be described by a negative pressure, and, therefore, the mechanism is capable to accelerate the Universe, without the need of an additional dark energy component. In this framework we discuss the evolution of perturbations by considering a Neo-Newtonian approach where, unlike in the standard Newtonian cosmology, the fluid pressure is taken into account even in the homogeneous and isotropic background equations (Lima, Zanchin and Brandenberger, MNRAS {\bf 291}, L1, 1997). The evolution of the density contrast is calculated in the linear approximation and compared to the one predicted by the $Λ$CDM model. The difference between the CCDM and $Λ$CDM predictions at the perturbative level is quantified by using three different statistical methods, namely: a simple $χ^{2}$-analysis in the relevant space parameter, a Bayesian statistical inference, and, finally, a Kolmogorov-Smirnov test. We find that under certain circumstances the CCDM scenario analysed here predicts an overall dynamics (including Hubble flow and matter fluctuation field) which fully recovers that of the traditional cosmic concordance model. Our basic conclusion is that such a reduction of the dark sector provides a viable alternative description to the accelerating $Λ$CDM cosmology.

astro-ph.CO↗

arXiv2011

From de Sitter to de Sitter: A New Cosmic Scenario without Dark Energy

In the present lore of cosmology, matter and space-time emerged from a singularity and evolved through four different regimes: inflation, radiation, dark matter and dark energy dominated eras. In the radiation and dark matter dominated stages, the expansion of the Universe decelerates while the inflation and dark energy eras are accelerating regimes. So far there is no clear cut connection between these accelerating periods. More intriguing, the substance driving the present accelerating stage remains a mystery, and the best available candidate (vacuum energy density) is plagued with the coincidence and cosmological constant problems. In this paper we overcome such problems through an alternative cosmic scenario based on gravitationally-induced particle production. The model proposed here is non-singular with the space-time emerging from a pure initial de Sitter stage thereby providing a natural solution to the horizon problem. Subsequently, due to an instability provoked by the production of massless particles, the Universe evolves smoothly to the standard radiation dominated era thereby ending the production of radiation as required by the conformal invariance (Parker's theorem). Next, the radiation becomes subdominant with the Universe entering in the cold dark matter dominated era. Finally, the negative pressure associated with the creation of cold dark matter particles accelerates the expansion and drives the Universe to a final de Sitter stage. The late time cosmic expansion history is exactly like in the standard LCDM model, however, there is no dark energy. The model evolves between two limiting (early and late time) de Sitter regimes. Our scenario is fully determined by two extreme energy densities, or equivalently, the associated de Sitter Hubble scales connected by 122 orders of magnitude.

astro-ph.CO↗

arXiv2011

Dynamics and Constraints of the Massive Gravitons Dark Matter Flat Cosmologies

We discuss the dynamics of the universe within the framework of Massive Graviton Dark Matter scenario (MGCDM) in which gravitons are geometrically treated as massive particles. In this modified gravity theory, the main effect of the gravitons is to alter the density evolution of the cold dark matter component in such a way that the Universe evolves to an accelerating expanding regime, as presently observed. Tight constraints on the main cosmological parameters of the MGCDM model are derived by performing a joint likelihood analysis involving the recent supernovae type Ia data, the Cosmic Microwave Background (CMB) shift parameter and the Baryonic Acoustic Oscillations (BAOs) as traced by the Sloan Digital Sky Survey (SDSS) red luminous galaxies. The linear evolution of small density fluctuations is also analysed in detail. It is found that the growth factor of the MGCDM model is slightly different ($\sim1-4%$) from the one provided by the conventional flat $Λ$CDM cosmology. The growth rate of clustering predicted by MGCDM and $Λ$CDM models are confronted to the observations and the corresponding best fit values of the growth index ($γ$) are also determined. By using the expectations of realistic future X-ray and Sunyaev-Zeldovich cluster surveys we derive the dark-matter halo mass function and the corresponding redshift distribution of cluster-size halos for the MGCDM model. Finally, we also show that the Hubble flow differences between the MGCDM and the $Λ$CDM models provide a halo redshift distribution departing significantly from the ones predicted by other DE models. These results suggest that the MGCDM model can observationally be distinguished from $Λ$CDM and also from a large number of dark energy models recently proposed in the literature.

astro-ph.CO↗

arXiv2011

Accelerating Cosmology Driven by Gravitational Production of Dark Matter Particles: Constraints from SNe Ia and H(z) data

The free parameters of a flat accelerating model without dark energy are constrained by using Supernovae type Ia and observational H(z) data. Instead of the vacuum dominance, the present accelerating stage in this modified Einstein-de Sitter cosmology is a consequence of the gravitationally-induced particle production of cold dark matter. The model present a transition from a decelerating to an accelerating regime at low redshifts, and is also able to harmonize a cold dark matter picture with the latest measurements of the Hubble constant H_0, the Supernovae observations (Constitution sample), and the H(z) data.

astro-ph.CO↗

arXiv2011

Accessing the Acceleration of the Universe with Sunyaev-Zel'dovich and X-ray Data from Galaxy Clusters

By using exclusively the Sunyaev-Zel'dovich effect and X-ray surface brightness data from 25 galaxy clusters in the redshift range 0.023< z < 0.784 we access cosmic acceleration employing a kinematic description. Such result is fully independent on the validity of any metric gravity theory, the possible matter-energy contents filling the Universe, as well as on the SNe Ia Hubble diagram.

astro-ph.CO↗

arXiv2011

Cosmic Distance Duality Relation and the Shape of Galaxy Clusters

Observations in the cosmological domain are heavily dependent on the validity of the cosmic distance-duality (DD) relation, D_L(z) (1 + z)^{2}/D_{A}(z) = 1, an exact result required by the Etherington reciprocity theorem where D_L(z) and D_A(z) are, respectively, the luminosity and angular diameter distances. In the limit of very small redshifts D_A(z) = D_L(z) and this ratio is trivially satisfied. Measurements of Sunyaev-Zeldovich effect (SZE) and X-rays combined with the DD relation have been used to determine D_A(z)from galaxy clusters. This combination offers the possibility of testing the validity of the DD relation, as well as determining which physical processes occur in galaxy clusters via their shapes. We use WMAP (7 years) results by fixing the conventional LCDM model to verify the consistence between the validity of DD relation and different assumptions about galaxy cluster geometries usually adopted in the literature. We assume that $η$ is a function of the redshift parametrized by two different relations: η(z) = 1 + η_{0}z, and η(z)=1 + η_{0}z/(1+z), where η_0 is a constant parameter quantifying the possible departure from the strict validity of the DD relation. In order to determine the probability density function (PDF) of η_{0}, we consider the angular diameter distances from galaxy clusters recently studied by two different groups by assuming elliptical (isothermal) and spherical (non-isothermal) $β$ models. The strict validity of the DD relation will occur only if the maximum value of η_{0} PDF is centered on η_{0}=0. It was found that the elliptical $β$ model is in good agreement with the data, showing no violation of the DD relation (PDF peaked close to η_0=0 at 1-sigma), while the spherical (non-isothermal) one is only marginally compatible at 3-sigma.

astro-ph.CO↗

arXiv2010

Note on "Continuous matter creation and the acceleration of the universe: the growth of density fluctuations"

Recently, de Roany & Pacheco (2010) performed a Newtonian analysis on the evolution of perturbations for a class of relativistic cosmological models with Creation of Cold Dark Matter (CCDM) proposed by the present authors. In this note we demonstrate that the basic equations adopted in their work do not recover the specific (unperturbed) CCDM model. Unlike to what happens in the original CCDM cosmology, their basic conclusions refer to a decelerating cosmological model in which there is no transition from a decelerating to an accelerating regime as required by SNe type Ia and complementary observations.

astro-ph.CO↗

arXiv2010

Confronting Dark Energy Models using Galaxy Cluster Number Counts

The mass function of cluster-size halos and their redshift distribution are computed for 12 distinct accelerating cosmological scenarios and confronted to the predictions of the conventional flat $Λ$CDM model. The comparison with $Λ$CDM is performed by a two-step process. Firstly, we determine the free parameters of all models through a joint analysis involving the latest cosmological data, using SNe type Ia, the CMB shift parameter and BAO. Apart from a brane world inspired cosmology, it is found that the derived Hubble relation of theremaining models reproduce the $Λ$CDM results approximately with the same degree of statistical confidence. Secondly, in order to attempt distinguish the different dark energy models from the expectations of $Λ$CDM, we analyze the predicted cluster-size halo redshift distribution on the basis of two future cluster surveys: (i) an X-ray survey based on the {\tt eROSITA} satellite, and (ii) a Sunayev-Zeldovich survey based on the South Pole Telescope. As a result, we find that the predictions of 8 out of 12 dark energy models can be clearly distinguished from the $Λ$CDM cosmology, while the predictions of 4 models are statistically equivalent to those of the $Λ$CDM model, as far as the expected cluster mass function and redshift distribution are concerned. The present analysis suggest that such a technique appears to be very competitive to independent tests probing the late time evolution of the Universe and the associated dark energy effects.

astro-ph.CO↗

arXiv2010

Are Galaxy Clusters Suggesting an Accelerating Universe?

The present cosmic accelerating stage is discussed through a new kinematic method based on the Sunyaev- Zel'dovich effect (SZE) and X-ray surface brightness data from galaxy clusters. By using the SZE/X-ray data from 38 galaxy clusters in the redshift range $0.14 \leq z \leq 0.89 $ [Bonamente et al., Astrop. J. {\bf 647}, 25 (2006)] it is found that the present Universe is accelerating and that the transition from an earlier decelerating to a late time accelerating regime is relatively recent. The ability of the ongoing Planck satellite mission to obtain tighter constraints on the expansion history through SZE/X-ray angular diameters is also discussed. Our results are fully independent on the validity of any metric gravity theory, the possible matter- energy contents filling the Universe, as well as on the SNe Ia Hubble diagram from which the presenting accelerating stage was inferred.

astro-ph.CO↗