arXiv subjects

J. A. S. Lima

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

At least 55 records · Page 3Linked to original sources

arXiv2015

Nonsingular Decaying Vacuum Cosmology and Entropy Production

The thermodynamic behavior of a decaying vacuum cosmology describing the entire cosmological history evolving between two extreme (early and late time) de Sitter eras is investigated. The thermal evolution from the early de Sitter to the radiation phase is discussed in detail. The temperature evolution law and the increasing entropy function are analytically determined. The entropy of the effectively massless particles is initially zero but evolves continuously to the present day maximum value within the current Hubble radius, $S_0 \sim 10^{88}$ in natural units. By using the Gibbons-Hawking temperature relation for the de Sitter spacetime, it is found that the ratio between the primeval and the late time vacuum energy densities is $ρ_{vI}/ρ_{v0} \sim 10^{123}$, as required by some naive estimates from quantum field theory.

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arXiv2015

Decaying Vacuum Inflationary Cosmologies: A Complete Scenario Including Curvature Effects

We propose a large class of nonsingular cosmologies of arbitrary spatial curvature whose cosmic history is determined by a primeval dynamical $Λ(t)$-term. For all values of the curvature, the models evolve between two extreme de Sitter phases driven by the relic time-varying vacuum energy density. The transition from inflation to the radiation phase is universal and points to a natural solution of the graceful exit problem regardless of the values of the curvature parameter. The flat case recovers the scenario recently discussed in the literature (Perico et al., Phys. Rev. D88, 063531, 2013). The early de Sitter phase is characterized by an arbitrary energy scale $H_I$ associated to the primeval vacuum energy density. If $H_I$ is fixed to be nearly the Planck scale, the ratio between the relic and the present observed vacuum energy density is $ρ_{vI}/ρ_{v0} \simeq 10^{123}$.

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arXiv2014

Gravitationally Induced Particle Production: Thermodynamics and Kinetic Theory

A relativistic kinetic description for the irreversible thermodynamic process of gravitationally induced particle production is proposed in the context of an expanding Friedmann-Robertson-Walker (FRW) geometry. We show that the covariant thermodynamic treatment referred to as "adiabatic" particle production provoked by the cosmic time-varying gravitational field has a consistent kinetic counterpart. The variation of the distribution function is associated to a non-collisional kinetic term of quantum-gravitational origin which is proportional to the ratio $Γ/H$, where $Γ$ is the gravitational particle production rate and H is the Hubble parameter. For $Γ<< H$ the process is negligible and as should be expected it also vanishes (regardless of the value of $Γ$) in the absence of gravitation. The resulting non-equilibrium distribution function has the same functional form of equilibrium with the evolution laws corrected by the particle production process. The macroscopic temperature evolution law is also kinetically derived for massive and massless particles. The present approach points to the possibility of an exact (semi-classical) quantum-gravitational kinetic treatment by incorporating back-reaction effects in the cosmic background.

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arXiv2014

Is the transition redshift a new cosmological number?

Observations from Supernovae Type Ia (SNe Ia) provided strong evidence for an expanding accelerating Universe at intermediate redshifts. This means that the Universe underwent a transition from deceleration to acceleration phases at a transition redshift $z_t$ of the order unity whose value in principle depends on the cosmology as well as on the assumed gravitational theory. Since cosmological accelerating models endowed with a transition redshift are extremely degenerated, in principle, it is interesting to know whether the value of $z_t$ itself can be observationally used as a new cosmic discriminator. After a brief discussion of the potential dynamic role played by the transition redshift, it is argued that future observations combining SNe Ia, the line-of-sight (or "radial") baryon acoustic oscillations, the differential age of galaxies, as well as the redshift drift of the spectral lines may tightly constrain $z_t$, thereby helping to narrow the parameter space for the most realistic models describing the accelerating Universe.

astro-ph.CO↗

arXiv2014

Cosmic acceleration without dark energy: Background tests and thermodynamic analysis

A new cosmic scenario with gravitationally induced particle creation is proposed. In this model the Universe evolves from an early to a late time de Sitter era, with the recent accelerating phase driven only by the negative creation pressure associated with the cold dark matter component. The model can be interpreted as an attempt to reduce the so-called cosmic sector (dark matter plus dark energy) and relate the two cosmic accelerating phases (early and late time de Sitter expansions). A detailed thermodynamic analysis including possible quantum corrections is also carried out. For a very wide range of the free parameters, it is found that the model presents the expected behavior of an ordinary macroscopic system in the sense that it approaches thermodynamic equilibrium in the long run (i.e., as it nears the second de Sitter phase). Moreover, an upper bound is found for the Gibbons-Hawking temperature of the primordial de Sitter phase. Finally, when confronted with the recent observational data, the current `quasi'-de Sitter era, as predicted by the model, is seen to pass very comfortably the cosmic background tests.

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arXiv2014

A viable Starobinsky-like inflationary scenario in the light of Planck and BICEP2 results

The recent CMB data from Planck and BICEP2 observations have opened a new window for inflationary cosmology. In this Essay we compare three Starobinsky-like inflationary scenarios: (i) the original Starobinsky proposal; (ii) a family of dynamically broken SUGRA models; and (iii) a class of "decaying" vacuum $Λ(H)$ cosmologies. We then focus on the $Λ(H)$ variant, which spans the complete cosmic history of the universe from an early inflationary stage, followed by the "graceful exit" into the standard radiation regime, the matter epoch and, finally, the late-time accelerated expansion. Computing the effective potential we find that the "running" $Λ(H)$ models also provide a prediction for the tensor-to-scalar ratio of the CMB spectrum, $r \simeq 0.16$, which is compatible to within $1σ$ with the value $r=0.20^{+0.07}_{-0.05}$ recently measured by the BICEP2 collaboration.

astro-ph.CO↗

arXiv2014

A unified solution to the small scale problems of the $Λ$CDM model

We study, by means of the model proposed in Del Popolo (2009), the effect of baryon physics on the small scale problems of the CDM model. We show that, using this model, the cusp/core problem, the missing satellite problem (MSP), the Too Big to Fail (TBTF) problem, and the angular momentum catastrophe can be reconciled with observations. Concerning the cusp/core problem, the interaction among dark matter (DM) and baryonic clumps of 1% the mass of the halo, through dynamical friction (DF), is able to flatten the inner cusp of the density profiles. We moreover assume that haloes form primarily through quiescent accretion, in agreement with the spherical collapse model (SCM)-secondary infall model (SIM) prescriptions. The results of this paper follow from the two assumptions above. Concerning the MSP and TBTF problem, applying to the Via Lactea II (VL2) subhaloes a series of corrections similar to those of Brooks et al. (2013), namely applying a Zolotov et al. (2012)-like correction obtained with our model, and further correcting for the UV heating and tidal stripping, we obtain that the number of massive, luminous satellites is in agreement with the number observed in the MW. The model also produces an angular momentum distribution in agreement with observations, that is with the distribution of the angular spin parameter and angular momentum of the dwarfs studied by van den Bosch, Burkert, & Swaters (2001). In conclusion, the small scale problems of the CDM model can all be solved by introducing baryon physics.

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arXiv2014

Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach

Light is affected by local inhomogeneities in its propagation, which may alter distances and so cosmological parameter estimation. In the era of precision cosmology, the presence of inhomogeneities may induce systematic errors if not properly accounted. In this vein, a new interpretation of the conventional Dyer-Roeder (DR) approach by allowing light received from distant sources to travel in regions denser than average is proposed. It is argued that the existence of a distribution of small and moderate cosmic voids (or "black regions") implies that its matter content was redistributed to the homogeneous and clustered matter components with the former becoming denser than the cosmic average in the absence of voids. Phenomenologically, this means that the DR smoothness parameter (denoted here by $α_E$) can be greater than unity, and, therefore, all previous analyses constraining it should be rediscussed with a free upper limit. Accordingly, by performing a statistical analysis involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a flat $Λ$CDM model we obtain for the extended parameter, $α_E=1.26^{+0.68}_{-0.54}$ ($1σ$). The effects of $α_E$ are also analyzed for generic $Λ$CDM models and flat XCDM cosmologies. For both models, we find that a value of $α_E$ greater than unity is able to harmonize SNe Ia and cosmic microwave background observations thereby alleviating the well-known tension between low and high redshift data. Finally, a simple toy model based on the existence of cosmic voids is proposed in order to justify why $α_E$ can be greater than unity as required by supernovae data.

astro-ph.CO↗

arXiv2013

On the equivalence of Lambda(t) and gravitationally induced particle production cosmologies

The correspondence between cosmological models powered by a decaying vacuum energy density and gravitationally induced particle production is investigated. Although being physically different in the physics behind them we show that both classes of cosmologies under certain conditions can exhibit the same dynamic and thermodynamic behavior. Our method is applied to obtain three specific models that may be described either as Lambda(t)CDM or gravitationally induced particle creation cosmologies. In the point of view of particle production models, the later class of cosmologies can be interpreted as a kind of one-component unification of the dark sector. By using current type Ia supernovae data, recent estimates of the cosmic microwave background shift parameter and baryon acoustic oscillations measurements we also perform a statistical analysis to test the observational viability within the two equivalent classes of models and we obtain the best-fit of the free parameters. By adopting the Akaike information criterion we also determine the rank of the models considered here. Finally, the particle production cosmologies (and the associated decaying Lambda(t)-models) are modeled in the framework of field theory by a phenomenological scalar field model.

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arXiv2013

A 3% Determination of $H_0$ at Intermediate Redshifts

Recent determinations of the Hubble constant, H_0, at extremely low and very high redshifts based on the cosmic distance ladder (grounded with trigonometric parallaxes) and a cosmological model (applied to Planck 2013 data) respectively, are revealing an intriguing discrepancy (nearly 9% or 2.4sigma) that is challenging astronomers and theoretical cosmologists. In order to shed some light on this problem, here we discuss a new determination of H_0 at intermediate redshifts (z ~ 1), using the following four cosmic probes: (i) measurements of the angular diameter distances (ADD) for galaxy clusters based on the combination of Sunyaev-Zeldovich effect and X-ray data (0.14 < z < 0.89$), (ii) the inferred ages of old high redshift galaxies (OHRG) (0.62 < z < 1.70), (iii) measurements of the Hubble parameter H(z) (0.1 < z < 1.8), and (iv) the baryon acoustic oscillation (BAO) signature (z=0.35). In our analysis, assuming a flat LCDM cosmology and considering statistical plus systematic errors we obtain H_0 = 74.1^{+2.2}_{-2.2} km/s.Mpc (1sigma) which is a 3% determination of the Hubble constant at intermediate redshifts. We stress that each individual test adopted here has error bars larger than the ones appearing in the calibration of the extragalactic distance ladder. However, the remarkable complementarity among the four tests works efficiently in reducing greatly the possible degeneracy on the space parameter (Omega_m,h) ultimately providing a value of H_0 that is in excellent agreement with the determination using recessional velocities and distances to nearby objects.

astro-ph.CO↗

arXiv2013

Complete Cosmic History with a dynamical Lambda(H) term

In the present mainstream cosmology, matter and spacetime emerged from a singularity and evolved through four distinct periods: early inflation, radiation, dark matter and late-time inflation (driven by dark energy). During the radiation and dark matter dominated stages, the universe is decelerating while the early and late-time inflations are accelerating stages. A possible connection between the accelerating periods remains unknown, and, even more intriguing, the best dark energy candidate powering the present accelerating stage (Lambda-vacuum) is plagued with the cosmological constant and coincidence puzzles. Here we propose an alternative solution for such problems based on a large class of time-dependent vacuum energy density models in the form of power series of the Hubble rate, Lambda=Lambda(H). The proposed class of Lambda(H)-decaying vacuum model provides: i) a new mechanism for inflation (different from the usual inflaton models), (ii) a natural mechanism for a graceful exit, which is universal for the whole class of models; iii) the currently accelerated expansion of the universe, iv) a mild dynamical dark energy at present; and v) a final de Sitter stage. Remarkably, the late-time cosmic expansion history of our class of models is very close to the concordance LambdaCDM model, but above all it furnishes the necessary smooth link between the initial and final de Sitter stages through the radiation- and matter-dominated epochs.

astro-ph.CO↗

arXiv2013

From inflation to dark energy through a dynamical Lambda: an attempt at alleviating fundamental cosmic puzzles

After decades of successful hot big-bang paradigm, Cosmology still lacks a framework in which the early inflationary phase of the universe smoothly matches the radiation epoch and evolves to the present `quasi' de Sitter spacetime. No less intriguing is that the current value of the effective vacuum energy density is vastly smaller than the value that triggered inflation. In this Essay we propose a new class of cosmologies capable of overcoming, or highly alleviating, some of these acute cosmic puzzles. Powered by a decaying vacuum energy density, the spacetime emerges from a pure nonsingular de Sitter vacuum stage, "gracefully" exits from inflation to a radiation phase followed by dark matter and vacuum regimes, and, finally, evolves to a late time de Sitter phase.

astro-ph.CO↗

arXiv2013

Shear and rotation in Chaplygin cosmology

We study the effect of shear and rotation on results previously obtained dealing with the application of the spherical collapse model (SCM) to generalized Chaplygin gas (gCg) dominated universes. The system is composed of baryons and gCg and the collapse is studied for different values of the parameter $α$ of the gCg. We show that the joint effect of shear and rotation is that of slowing down the collapse with respect to the simple SCM. This result is of utmost importance for the so-called unified dark matter models, since the described slow down in the growth of density perturbation can solve one of the main problems of the quoted models, namely the instability described in previous papers [e.g., H. B. Sandvik {\it et al.}, Phys. Rev. D {\bf 69}, 123524 (2004)] at the linear perturbation level.

astro-ph.CO↗

arXiv2013

Black Hole Formation with an Interacting Vacuum Energy Density: Curvature Effects

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 filling a FLRW type geometry with an arbitrary curvature parameter is investigated. The complete set of exact solutions for all values of the free parameters are obtained and the influence of the curvature term on the collapsing time, black hole mass and other physical quantities are also discussed in detail. We show that for the same initial conditions the total black hole mass depends only on the effective matter density parameter (including the vacuum component). It is also shown that the analytical condition to form a black hole i.e. the apparent horizon is not altered by the contribution of the curvature terms, however, the remaining physical quantities are quantitatively modified.

gr-qc↗

arXiv2013

Expansion History with Decaying Vacuum: A Complete Cosmological Scenario

We propose a novel cosmological scenario with the space-time emerging from a pure initial de Sitter stage and subsequently evolving into the radiation, matter and dark energy dominated epochs, thereby avoiding the initial singularity and providing a complete description of the expansion history and a natural solution to the horizon problem. The model is based on a dynamical vacuum energy density which evolves as a power series of the Hubble rate. The transit from the inflation into the standard radiation epoch is universal, giving a clue for a successful description of the graceful exit. Since the resulting late time cosmic history is very close to the concordance LCDM model, the new unified framework embodies a more complete past cosmic evolution than the standard cosmology.

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arXiv2012

Spherical collapse model with shear and angular momentum in dark energy cosmologies

We study, for the first time, how shear and angular momentum modify typical parameters of the spherical collapse model, in dark energy dominated universes. In particular, we study the linear density threshold for collapse $δ_\mathrm{c}$ and the virial overdensity $Δ_\mathrm{V}$, for several dark-energy models and its influence on the cumulative mass function. The equations of the spherical collapse are those obtained in Pace et al. (2010), who used the fully nonlinear differential equation for the evolution of the density contrast derived from Newtonian hydrodynamics, and assumed that dark energy is present only at the background level. With the introduction of the shear and rotation terms, the parameters of the spherical collapse model are now mass-dependant. The results of the paper show, as expected, that the new terms considered in the spherical collapse model oppose the collapse of perturbations on galactic scale giving rise to higher values of the linear overdensity parameter with respect to the non-rotating case. We find a similar effect also for the virial overdensity parameter. For what concerns the mass function, we find that its high mass tail is suppressed, while the low mass tail is slightly affected except in some cases, e.g. the Chaplygin gas case.

astro-ph.CO↗

arXiv2012

New Cosmic Accelerating Scenario without Dark Energy

We propose an alternative, nonsingular, cosmic scenario based on gravitationally induced particle production. The model is an attempt to evade the coincidence and cosmological constant problems of the standard model ($Λ$CDM) and also to connect the early and late time accelerating stages of the Universe. Our space-time emerges 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. Next, the radiation becomes sub-dominant with the Universe entering in the cold dark matter dominated era. Finally, the negative pressure associated with the creation of cold dark matter (CCDM model) particles accelerates the expansion and drives the Universe to a final de Sitter stage. The late time cosmic expansion history of the CCDM model is exactly like in the standard $Λ$CDM model, however, there is no dark energy. This complete scenario is fully determined by two extreme energy densities, or equivalently, the associated de Sitter Hubble scales connected by $ρ_I/ρ_f=(H_I/H_f)^{2} \sim 10^{122}$, a result that has no correlation with the cosmological constant problem. We also study the linear growth of matter perturbations at the final accelerating stage. It is found that the CCDM growth index can be written as a function of the $Λ$ growth index, $γ_Λ \simeq 6/11$. In this framework, we also compare the observed growth rate of clustering with that predicted by the current CCDM model. Performing a $χ^{2}$ statistical test we show that the CCDM model provides growth rates that match sufficiently well with the observed growth rate of structure.

astro-ph.CO↗

arXiv2012

Extended Spherical Collapse and the Accelerating Universe

The influence of the shear stress and angular momentum on the nonlinear spherical collapse model is discussed in the framework of the Einstein-de Sitter (EdS) and $Λ$CDM models. By assuming that the vacuum component is not clustering within the homogeneous nonspherical overdensities, we show how the local rotation and shear affects the linear density threshold for collapse of the non-relativistic component ($δ_\mathrm{c}$) and its virial overdensity ($Δ_\mathrm{V}$). It is also found that the net effect of shear and rotation in galactic scale is responsible for higher values of the linear overdensity parameter as compared with the standard spherical collapse model (no shear and rotation).

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