arXiv subjects
Publications and source records attributed to Diego Pavon.
In this paper we show that by considering a universe dominated by two interacting components a superaccelerated expansion can be obtained from a decelerated one by applying a dual transformation that leaves the Einstein's field equations invariant.
We explore the thermodynamics of dark energy taking into account the existence of the observer's event horizon in accelerated universes. Except for the initial stage of Chaplygin gas dominated expansion, the generalized second law of gravitational thermodynamics is fulfilled and the temperature of the phantom fluid results positive. This substantially extends the work of Pollock and Singh [10] on the thermodynamics of super--inflationary expansion.
We analyze the dynamics of models of warm inflation with general dissipative effects. We consider phenomenological terms both for the inflaton decay rate and for viscous effects within matter. We provide a classification of the asymptotic behavior of these models and show that the existence of a late-time scaling regime depends not only on an asymptotic behavior of the scalar field potential, but also on an appropriate asymptotic behavior of the inflaton decay rate. There are scaling solutions whenever the latter evolves to become proportional to the Hubble rate of expansion regardless of the steepness of the scalar field exponential potential. We show from thermodynamic arguments that the scaling regime is associated to a power-law dependence of the matter-radiation temperature on the scale factor, which allows a mild variation of the temperature of the matter/radiation fluid. We also show that the late time contribution of the dissipative terms alleviates the depletion of matter, and increases the duration of inflation.
We determine the range of parameter space of an interacting quintessence (IQ) model that best fits the luminosity distance of type Ia supernovae data and the recent WMAP measurements of Cosmic Microwave Background temperature anisotropies. Models in which quintessence decays into dark matter provide a clean explanation for the coincidence problem. We focus on cosmological models of zero spatial curvature. We show that if the dark energy (DE) decays into cold dark matter (CDM) at a rate that brings the ratio of matter to dark energy constant at late times, the supernovae data are not sufficient to constrain the interaction parameter. On the contrary, WMAP data constrain it to be smaller than $c^2 < 10^{-2}$ at the $3σ$ level. Accurate measurements of the Hubble constant and the dark energy density, independent of the CMB data, would support/disprove this set of models.
We review the notion of holographic dark energy and assess its significance in the light of the well documented cosmic acceleration at the present time. We next propose a model of holographic dark energy in which the infrared cutoff is set by the Hubble scale. The model accounts for the aforesaid acceleration and, by construction, is free of the cosmic coincidence problem.
In this Letter we demonstrate that any interaction of pressureless dark matter with holographic dark energy, whose infrared cutoff is set by the Hubble scale, implies a constant ratio of the energy densities of both components thus solving the coincidence problem. The equation of state parameter is obtained as a function of the interaction strength. For a variable degree of saturation of the holographic bound the energy density ratio becomes time dependent which is compatible with a transition from decelerated to accelerated expansion.
We study the coincidence problem of late cosmic acceleration by assuming that the present ratio between dark matter and dark energy is a slowly varying function of the scale factor. As dark energy component we consider two different candidates, first a quintessence scalar field, and then a tachyon field. In either cases analytical solutions for the scale factor, the field and the potential are derived. Both models show a good fit to the recent magnitude-redshift supernovae data. However, the likelihood contours disfavor the tachyon field model as it seems to prefer a excessively high value for the matter component.
We determine the range of parameter space of Interacting Quintessence Models that best fits the recent WMAP measurements of Cosmic Microwave Background temperature anisotropies. We only consider cosmological models with zero spatial curvature. We show that if the quintessence scalar field decays into cold dark matter at a rate that brings the ratio of matter to dark energy constant at late times,the cosmological parameters required to fit the CMB data are: Ω_x = 0.43 \pm 0.12, baryon fraction Ω_b = 0.08 \pm 0.01, slope of the matter power spectrum at large scals n_s = 0.98 \pm 0.02 and Hubble constant H_0 = 56 \pm 4 km/s/Mpc. The data prefers a dark energy component with a dimensionless decay parameter c^2 =0.005 and non-interacting models are consistent with the data only at the 99% confidence level. Using the Bayesian Information Criteria we show that this exra parameter fits the data better than models with no interaction. The quintessence equation of state parameter is less constrained; i.e., the data set an upper limit w_x \leq -0.86 at the same level of significance. When the WMAP anisotropy data are combined with supernovae data, the density parameter of dark energy increases to Ω_x \simeq 0.68 while c^2 augments to 6.3 \times 10^{-3}. Models with quintessence decaying into dark matter provide a clean explanation for the coincidence problem and are a viable cosmological model, compatible with observations of the CMB, with testable predictions. Accurate measurements of baryon fraction and/or of matter density independent of the CMB data, would support/disprove these models.
The generalized second law of gravitational thermodynamics is applied to the present era of accelerated expansion of the Universe. In spite of the fact that the entropy of matter and relic gravitational waves inside the event horizon diminish, the mentioned law is fulfilled provided that the expression for the entropy density of the gravitational waves satisfies a certain condition.
We anlayze the dynamics of warm inflation models with general viscous effects. We characterize the situations yielding asymptotic scaling behavior.
We calculate the current power spectrum of the gravitational waves created at the big bang (and later amplified by the different transitions during the Universe expansion) taking into account the present stage of accelerated expansion. Likewise, we determine the power spectrum in a hypothetical second dust era that would follow the present one if at some future time the dark energy, that supposedly drives the current accelerated expansion, evolved in such a way that it became dynamically equivalent to cold dark matter. The calculated power spectrum as well as the evolution of the density parameter of the waves may serve to discriminate between phases of expansion and may help ascertain the nature of dark energy.
In this paper we show that a universe dominated by two components, namely domain walls and dissipative dark matter such that the former slowly decays into the latter may drive power law cosmological acceleration consistent with the high redshift supernovae data, while the ratio between the energy densities of both components remains fixed at late times thus solving the coincidence problem of present acceleration. Likewise, we estimate the aforesaid ratio at early times.
We find exact solutions leading to power law accelerated expansion for a homogeneous, isotropic and spatially flat universe, dominated by an interacting mixture of cold dark matter and a tachyonic field such that the ratio of the energy densities of both components at late times is constant and no coincidence problem arises.
Non-canonical scaling of the form $ρ_{x} \propto ρ_{m} a^ξ$, where $ρ_{x}$ and $ρ_{m}$ are the energy densities of dark energy and dark matter, respectively, provides a natural way to address the coincidence problem -why the two densities are comparable today. This non-canonical scaling is achieved by a suitable interaction between the dark components. We study the observational constraints imposed on such models from the cosmic microwave background (CMB) anisotropy spectrum. Using the recent WMAP results for the location of the CMB peaks, we determine the admissible parameter space and find that interacting models are well consistent with current observational bounds.
A non--minimally coupled quintessence model is investigated and the conditions for a stationary solution to the coincidence problem are obtained. For a conformally coupled scalar field and dissipative matter, a general solution possessing late acceleration is found. It fits rather well the high redshift supernovae data and gives a good prediction of the age of the Universe. Likewise, the cold dark matter component dominates the cosmological perturbations at late times albeit they decrease with expansion.
We argue that the recently introduced "statefinder parameters" (Sahni et al., JETP Lett. 77, 201 (2003)), that include the third derivative of the cosmic scale factor, are useful tools to characterize interacting quitessence models. We specify the statefinder parameters for two classes of models that solve, or at least alleviate, the coincidence problem.
With the help of Nordtvedt's scalar tensor theory an exact analytical model of a non-minimally coupled scalar field cosmology in which the gravitational coupling $G$ and the Hubble factor $H$ oscillate during the radiation era is presented. A key feature is that the oscillations are confined to the early stages of the radiation dominated era with $G$ approaching its present constant value while $H$ becoming a monotonically decreasing function of time. The Brans Dicke parameter $ω$ is chosen to be a function of Brans Dicke scalar field so that no conflict with observational constraints regarding its present value arises.
In this paper we explore the impact of an era -right after reheating- dominated by mini black holes and radiation on the spectrum of the relic gravitational waves. This era may lower the spectrum several orders of magnitude.