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We assume the cosmological dark sector to consist of pressureless matter and holographic dark energy with a cutoff length proportional to the Ricci scale. The requirement of separate energy-momentum conservation of the components is shown to establish a relation between the matter fraction and the (necessarily time-dependent) equation-of-state parameter of the dark energy. Focusing on intrinsically adiabatic pressure perturbations of the dark-energy component, the matter perturbations are found as linear combinations of the total energy-density perturbations of the cosmic medium and the relative (nonadiabatic) perturbations of the components. The resulting background dynamics is consistent with observations from supernovae of type Ia, baryonic acoustic oscillations and the differential age of old objects. The perturbation dynamics, on the other hand, is plagued by instabilities which excludes any phantom-type equation of state. The only stable configuration is singled out by a fixed relation between the present matter fraction $\Omega_{m0}$ and the present value $\omega_{0}$ of the equation-of-state parameter of the dark energy. However, this instability-avoiding configuration is only marginally consistent with the observationally preferred background values of the mentioned parameters., Comment: 29 pages, 8 figures, new material and new results added, conclusions enlarged and sharpened, references added, to appear in PRD

We propose and constrain with the latest observational data three parameterizations of the deceleration parameter, valid from the matter era to the far future. They are well behaved and do not diverge at any redshift. On the other hand, they are model independent in the sense that in constructing them the only assumption made was that the Universe is homogeneous and isotropic at large scales., Comment: 3 pages, 1 figure. Key words: Cosmology; Deceleration parameter. Submitted to the Proceedings of "13th Marcel Grossmann Meeting". A much extended version can be found in Physical Review D 86, 083509 (2012)

We explore whether the generalized second law of thermodynamics is fulfilled in the transition from a generic initial Einstein static phase to the inflationary phase, with constant Hubble rate, and from the end of the latter to the conventional thermal radiation dominated era of expansion. As it turns out, the said law is satisfied provided the radiation component does not contribute largely to the total energy of the static phase., Comment: 3 pages, 1 figure. Key words: Early universe; Thermodynamics. Submitted to the Proceedings of "13th Marcel Grossmann Meeting". An extended version was published in Physics Letters B 707 (2012) 8-10

We describe Lema\^{i}tre-Tolman-Bondi cosmological models where an anisotropic pressures is considered. By using recent astronomical observations coming from supernova of Ia types we constraint the values of the parameters that characterize our models., Comment: 13 pages, 2 figures, Accepted for publication in MPLA

We propose, and constrain with the latest observational data, three model-independent parametrizations of the cosmic deceleration parameter $q(z)$. They are well behaved and stay finite at all redshifts. We construct them by fixing the value of $q$ at high redshift, $q(z \gg 1) = 1/2$ (as demanded by cosmic structure formation), and at the far future, $q(z = -1) = -1$, and smoothly interpolating $q(z)$ between them. The fixed point at $z = -1$ is not arbitrarily chosen; it readily follows from the second law of thermodynamics. This fairly reduces the ample latitude in parameterizing $q(z)$., Comment: 22 pages, 10 figures; this version matches published paper

Warm inflationary universe models in the context of logamediate expansion are studied. General conditions required for these models to be realizable and discussed. This study is done in the weak and strong dissipative regimes. The parameters of our models are constrained from the observational data., Comment: 16 pages, 2 figures. Accepted for publication in IJMPD. arXiv admin note: text overlap with arXiv:1108.1371

This paper studies whether the generalized second law of thermodynamics is fulfilled in the transition from a generic initial Einstein static phase to the inflationary phase, with constant Hubble rate, and from the end of the latter to the conventional era of thermal radiation dominated expansion. As it turns out, the said law is satisfied provided the radiation component does not largely contribute to the total energy of the static phase., Comment: 8 pages, 1 figure; to be published in Phys. Letters B

Warm-intermediate inflationary universe models in the context of braneworld cosmologies, are studied. This study is done in the weak and strong dissipative regimes. We find that, the scalar potentials and dissipation coefficients in terms of the scalar field, evolves as type-power-law and powers of logarithms, respectively. General conditions required for these models to be realizable are derived and discussed. We also study the scalar and tensor perturbations for each regime. We use recent astronomical observations to constraint the parameters appearing in the braneworld models., Comment: 19 pages, 2 figures. Published in Eur. Phys. J. C. arXiv admin note: substantial overlap with arXiv:0903.4214 and arXiv:1003.5645

The dilaton-gravity sector of a linear in the scalar curvature, scale invariant Two Measures Field Theory (TMT), is explored in detail in the context of closed FRW cosmology and shown to allow stable emerging universe solutions. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. We study the transition from the emerging phase to inflation, and then to a zero cosmological constant phase. We also study the spectrum of density perturbations and the constraints that impose on the parameters of the theory., Comment: 10 pages, 2 figures. Accepted for publication in Phys. Lett. B

The two dominant components of the cosmic budget today, pressureles matter and dark energy, may or may not be interacting with each other. Currently, both possibilities appear compatible with observational data. We propose several criteria based on the history of the Hubble factor that can help discern whether they are interacting and whether dark energy is phantom or quintessence in nature., Comment: 22 pages, 7 figures. Accepted for publication in IJMPD

Different holographic dark-energy models are studied from a unifying point of view. We compare models for which the Hubble scale, the future event horizon or a quantity proportional to the Ricci scale are taken as the infrared cutoff length. We demonstrate that the mere definition of the holographic dark-energy density generally implies an interaction with the dark-matter component. We discuss the relation between the equation-of-state parameter and the energy density ratio of both components for each of the choices, as well as the possibility of non-interacting and scaling solutions. Parameter estimations for all three cutoff options are performed with the help of a Bayesian statistical analysis, using data from supernovae type Ia and the history of the Hubble parameter. The $\Lambda$CDM model is the clear winner of the analysis. According to the Bayesian Information Criterion ($BIC$), all holographic models should be considered as ruled out, since the difference $\Delta BIC$ to the corresponding $\Lambda$CDM value is $> 10$. According to the Akaike Information Criterion ($AIC$), however, we find $\Delta AIC$ $< 2$ for models with Hubble-scale and Ricci-scale cutoffs, indicating, that they may still be competitive. As we show for the example of the Ricci-scale case, also the use of certain priors, reducing the number of free parameters to that of the $\Lambda$CDM model, may result in a competitive holographic model., Comment: 37 pages, 11 figures, 3 tables, statistical analysis improved, accepted for publication in Phys.Rev.D

We study the evolution of spatial curvature for thawing class of dark energy models. We examine the evolution of the equation of state parameter, $w_\phi$, as a function of the scale factor $a$, for the case in which the scalar field $\phi$ evolve in nearly flat scalar potential. We show that all such models provide the corresponding approximate analytical expressions for $w_\phi(\Omega_\phi,\Omega_k)$ and $w_\phi(a)$. We present observational constraints on these models., Comment: 14 pages, 6 figures. Accepted for publication in Phys. Lett. B

This paper studies the stability of warm inflationary solutions when the viscous pressure is taken into account. The latter is a very natural and physically motivated ingredient of warm inflation and is seen to widen the stability range of warm inflation. The spectral index parameters, $n_{s}$, $n_{T}$, and their ratio are derived. The corresponding WMAP7 data are used to fix some parameters of the model. Two specific examples are discussed in detail: (i) a potential given by $V(\phi, T) = v_{1}(\phi)\, + \, v_{2}(T)$, and (ii) a potential of the form $V(\phi, T) = \alpha \, v_{1}(\phi) \, v_{2}(T)$. In both cases, the viscosity has little impact on the said ratio., Comment: 16 pages, 2 figures (replaced); to be published in JCAP

We consider a scale invariant model which includes a $R^{2}$ term in action and show that a stable "emerging universe" scenario is possible. The model belongs to the general class of theories, where an integration measure independent of the metric is introduced. To implement scale invariance (S.I.), a dilaton field is introduced. The integration of the equations of motion associated with the new measure gives rise to the spontaneous symmetry breaking (S.S.B) of S.I. After S.S.B. of S.I. in the model with the $R^{2}$ term (and first order formalism applied), it is found that a non trivial potential for the dilaton is generated. The dynamics of the scalar field becomes non linear and these non linearities are instrumental in the stability of some of the emerging universe solutions, which exists for a parameter range of the theory., Comment: 21 pages, 4 figures. Accepted for publication in JCAP

A warm inflationary universe model in loop quantum cosmology is studied. In general we discuss the condition of inflation in this framework. By using a chaotic potential, $V(\phi)\propto \phi^2$, we develop a model where the dissipation coefficient $\Gamma=\Gamma_0=$ constant. We use recent astronomical observations for constraining the parameters appearing in our model., Comment: 15 pages, 1 figure, Accepted for publication in Phys. Rev. D

In this article we study an intermediate inflationary universe models using the Gauss-Bonnet brane. General conditions required for these models to be realizable are derived and discussed. We use recent astronomical observations to constraint the parameters appearing in the model., Comment: 16 pages, 2 figures, accepted for publication in European Physical Journal C

In a logamediate inflationary universe model we introduce the curvaton field in order to bring this inflationary model to an end. In this approach we determine the reheating temperature. We also outline some interesting constraints on the parameters that describe our models. Thus, we give the parameter space in this scenario., Comment: Accepted by PRD

In this article we study closed inflationary universe models using the Gauss-Bonnet Brane. We determine and characterize the existence of a universe with $\Omega > 1$, with an appropriate period of inflation. We have found that this model is less restrictive in comparison with the standard approach where a scalar field is considered. We use recent astronomical observations to constrain the parameters appearing in the model., Comment: 21 pages, 4 figures. Accepted by Annals of Physics

In this work we study the stability of the Jordan-Brans-Dicke (JBD) static universe. This is motivated by the possibility that the universe might have started out in an asymptotically JBD static state, in the context of the so called emergent universe scenario. We extent our previous results on stability of JBD static universe by considering spatially homogeneous Bianchi type IX anisotropic perturbation modes and by including more general perfect fluids. Contrary to general relativity, we have found that the JBD static universe, dominated by a standard perfect fluid, could be stable against isotropic and anisotropic perturbations. The implications of these results for the initial state of the universe and its pre-inflationary evolution are discussed., Comment: 24 pages, 2 figures, typos corrected, some improvements in section 4 and comments added in conclusions. References added. Accepted for publication in JCAP

The tachyonic inflationary universe model in the context of intermediate inflation is studied. General conditions for this model to be realizable are discussed. In the slow-roll approximation, we describe in great details the characteristics of this model., Comment: 13 pages, 1 figure. Accepted for publication in PRD

Warm inflationary universe models in the context of intermediate expansion, between power law and exponential, are studied. General conditions required for these models to be realizable are derived and discussed. This study is done in the weak and strong dissipative regimes. The inflaton potentials considered in this study are negative-power-law and powers of logarithms, respectively. The parameters of our models are constrained from the WMAP three and five year data., Comment: 15 pages, 2 figures. Accepted for publication in JCAP

A cosmological model dominated at the beginning by a dark radiation followed by a period of inflation is presented. This model is based on a Randall-Sundrum II type brane-world. Current observational data are used to fix the parameters associated to the dark radiation., Comment: 14 pages, 1 Figure, Accepted for publication in Phys. Lett. B

Brane inflationary universe model in the context of intermediate inflation is studied. General conditions for this model to be realizable are discussed. In the high-energy limit we describe in great details the characteristic of this model., Comment: 12 pages and 2 figures

It is argued that cosmological models that feature a flow of energy from dark energy to dark matter may solve the coincidence problem of late acceleration (i.e., "why the energy densities of both components are of the same order precisely today?"). However, much refined and abundant observational data of the redshift evolution of the Hubble factor are needed to ascertain whether they can do the job., Comment: 25 pages, 11 figures; accepted for publication in JCAP

Warm inflationary universe model in a tachyon field theory is studied in the weak dissipative regime. We develop our model for an exponential potential and the dissipation parameter $\Gamma=\Gamma_0$=constant. We describe scalar and tensor perturbations for this scenario., Comment: 9 pages, accepted by European Physical Journal C

Tachyon-Brane inflationary universe model in the context of a Chaplygin gas equation of state is studied. General conditions for this model to be realizable are discussed. In the high-energy limit and by using an exponential potential we describe in great details the characteristic of this model. Recent observational data from the Wilkinson Microwave Anisotropy Probe experiment are employed to restrict the parameters of the model., Comment: 15 pages, 3 figures, Accepted for publication in General Relativity and Gravitation

In this article we study closed inflationary universe models by means of a tachyonic field. We described a general treatment for created a universe with $\Omega>1$ in patch cosmology, which is able to represent General Relativity, Gauss-Bonnet or Randall-Sundrum patches. We use recent data from astronomical observations to constrain the parameters appearing in our model., Comment: 15 pages, 2 figures. Accepted by Modern Physics Letters A

The coincidence problem of late cosmic acceleration constitutes a serious riddle with regard to our understanding of the evolution of the Universe. Here we argue that this problem may someday be solved -or better understood- by expressing the Hubble expansion rate as a function of the ratio of densities (dark matter/dark energy) and observationally determining the said rate in terms of the redshift., Comment: 14 pages, 5 figures. Key words: Cosmology, dark energy, coincidence problem. Slightly extended version of the paper published in Phys. Rev. D as rapid communication

Warm inflationary universe models in the context of a Chaplygin gas equation are studied. General conditions required for these models to be realizable are derived and discussed. By using a chaotic potential we develop models for a dissipation coefficient of the form $\Gamma\propto \phi^n$, with $n=0$ or $n\neq 0$., Comment: 17 pages, 4 figures, Accepted for publication in Physics Letters B

Brane inflationary universe model in the context of a Chaplygin gas equation of state is studied. General conditions for this model to be realizable are discussed. In the high-energy limit and by using a chaotic potential we describe in great details the characteristic of this model. The parameters of the model are restricted by using recent astronomical observations., Comment: 13 pages, 2 figures, Accepted for publication in Physics Letters B

Tachyonic inflationary universe model in the context of a Chaplygin gas equation of state is studied. General conditions for this model to be realizable are discussed. By using an effective exponential potential we describe in great details the characteristic of the inflationary universe model. The parameters of the model are restricted by using recent astronomical observations., Comment: 13 pages, 1 figure, Accepted by Physics Letters B

In this article, we study a type of one-field approach for open inflationary universe scenario in the context of braneworld models with a Gauss-Bonnet correction term. For a one-bubble universe model, we determine and characterize the existence of the Coleman-De Lucia instanton together with the period of inflation after tunneling has occurred. Our results are compared those analogous obtained when the usual Einstein Theory of Gravitation is used., Comment: Revtex, 4 Figures. Accepted by Modern Physics Letters A

In this paper we study emergent universe model in the context of a self interacting Jordan-Brans-Dicke theory. The model presents a stable past eternal static solution which eventually enters a phase where the stability of this solution is broken leading to an inflationary period. We also establish constraints for the different parameters appearing in our model., Comment: 18 pages, 4 figures. Accepted for publication in JCAP

The curvaton in an intermediate inflationary universe model is studied. This study has allowed us to find some interesting constraints on different parameters that appear in the model., Comment: 16 pages, 2 figures, to appear in Physical Review D

In this paper, warm inflationary models on a brane scenario are studied. Here we consider slow-roll inflation and high-dissipation regime in a high-energy scenario. General conditions required for these models to be realizable are derived. We describe scalar and tensor perturbations for these scenarios. Specifically we study power-law potentials considering a dissipation parameter to be a constant on the one hand and $\phi$ dependent on the other hand. We use recent astronomical observations to restrict the parameters appearing in our model., Comment: 20 pages and 2 figures

Warm inflationary universe models on a warped Dvali-Gabadadze-Porrati brane are studied. General conditions required for these models to be realizable are derived and discussed. By using an effective exponential potential we develop models for constant and variable dissipation coefficient ratio $r=\frac{\Gamma}{3 H}$. We use recent astronomical observations for constraining the parameters appearing in our models., Comment: 15 pages and 1 figure. Accepted for publication in Phys. Lett. B

In this article we study closed inflationary universe models by using a tachyonic field theory. We determine and characterize the existence of an universe with $\Omega > 1$, and which describes a period of inflation. We find that considered models are less restrictive compared to the standard ones with a scalar field. We use recent astronomical observations to constraint the parameters appearing in the model. Obtained results are compared to those found in the standard scalar field inflationary universes., Comment: 8 pages, 2 figures. Minor changes, typos corrected. Accepted for publication in Eur. Phys. J. C

Scalar and tensorial cosmological perturbations generated in warm inflationary scenarios whose matter-radiation fluid is endowed with a viscous pressure are considered. Recent observational data from the WMAP experiment are employed to restrict the parameters of the model. Although the effect of this pressure on the matter power spectrum is of the order of a few percent, it may be detected in future experiments., Comment: 20 pages, to be published in the Physcal Review D

We study one-field open inflationary models in a universe dominated by tachyon matter. In these scenarios, we determine and characterize the existence of the Coleman-De Lucia (CDL) instanton. Also, we study the Lorentzian regime, that is, the period of inflation after tunnelling has occurred., Comment: 13 pages, 7 figures. Accepted by Physics Letters B

The coincidence problem of late cosmic acceleration is a serious riddle in connection with our understanding of the evolution of the Universe. In this paper we show that an interaction between the dark energy component (either phantom or quintessence) and dark matter can alleviate it. In this scenario the baryon component is independently conserved. This generalizes a previous study [S. del Campo, R. Herrera, and D. Pavon, Phys. Rev. D 71, 123529 (2005)] in which neither baryons nor phantom energy were considered., Comment: 18 pages, 9 figures; to be published in the Physical Review, section D

The curvaton reheating in a non-oscillatory inflationary universe model is studied in a Jordan-Brans-Dicke theory. For different scenarios, the temperature of reheating is computed. The result tells us that the reheating temperature becomes practically independent of the Jordan-Brans-Dicke parameter $w$. This reheating temperature results to be quite different when compared with that obtained from Einstein`s theory of gravity., Comment: Accepted by JCAP, 12 pages, 1 Figure

The curvaton reheating in a tachyonic braneworld inflationary universe model with an exponential potential is studied. We have found that the energy density in the kinetic epoch, has a complicated dependencies of the scale factor. For different scenarios the temperature of reheating is computed, finding an upper limit that lies in the range $10^{14}$--$10^{16}$GeV., Comment: 6 pages

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., Comment: 27 pages, 10 eps figures, to be published in Physical Review D

In this paper we study a type of model for closed inflationary universe models using the Jordan-Brans-Dicke theory. Herein we determine and characterize the existence of $\Omega>$1, together with the period of inflation. We have found that our model, which takes into account a Jordan-Brans-Dicke type of theory, is less restrictive than the one used in Einstein's theory of general relativity. Our results are compared to those found in Einstein's theory of Relativity., Comment: 9 pages, 4 figures, accepted by Class. Quant. Grav

In this article we study closed inflationary universe models proposed by Linde in a brane world cosmological context. In this scenario we determine and characterize the existence of a closed universe, in presence of one self-interacting scalar field with an inflationary stage. Our results are compared to those found in General Relativity., Comment: 9 pages, 3 figures, accepted by International Journal of Modern Physics D. Minor changes and some reference was added

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., Comment: 15 pages, 2 eps figures, uses revtex4, to be published in Physical Review D

In this paper, we study a type of one-field model for open inflationary universe models in the context of the brane world models. In the scenario of a one-bubble universe model, we determine and characterize the existence of the Coleman-De Lucia instanton, together with the period of inflation after tunneling has occurred. Our results are compared to those found in the Einstein theory of Relativistic Models., Comment: 8 pages, 4 Figures, accepted in Physical Review D

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., Comment: 11 pages, no figues, to be published in Gen. Ralativ. Grav

Scalar density cosmological perturbations, spectral indices and reheating in a chaotic inflationary universe model, in which a higher derivative term is added, are investigated. This term is supposed to play an important role in the early evolution of the Universe, specifically at times closer to the Planck era., Comment: 14 pages, accepted for publication in MPLA

In this paper we study a type of one-field model for open inflationary universe models in the context of the Jordan-Brans-Dicke theory. In the scenario of a one-bubble universe model we determine and characterize the existence of the Coleman-De Lucia instanton, together with the period of inflation after tunnelling has occurred. Our results are analogous to those found in the Einstein General Relativity models., Comment: 8 pages, 9 figures. Accepted for publication in the Phys. Rev. D