arXiv subjects

Rafael C. Nunes

Publications and source records attributed to Rafael C. Nunes.

121 records · Page 7Linked to original sources

arXiv2017

New observational constraints on $f(R)$ gravity from cosmic chronometers

We use the recently released cosmic chronometer data and the latest measured value of the local Hubble parameter, combined with the latest joint light curves of Supernovae Type Ia, and Baryon Acoustic Oscillation distance measurements, in order to impose constraints on the viable and most used $f(R)$ gravity models. We consider four $f(R)$ models, namely the Hu-Sawicki, the Starobinsky, the Tsujikawa, and the exponential one, and we parametrize them introducing a distortion parameter $b$ that quantifies the deviation from $Λ$CDM cosmology. Our analysis reveals that a small but non-zero deviation from $Λ$CDM cosmology is slightly favored, with the corresponding fittings exhibiting very efficient $AIC$ and $BIC$ Information Criteria values. Clearly, $f(R)$ gravity is consistent with observations, and it can serve as a candidate for modified gravity.

astro-ph.CO↗

arXiv2016

Probing the interaction between dark matter and dark energy in the presence of massive neutrinos

We consider the possibility of an interaction in the dark sector in the presence of massive neutrinos, and study the observational constraints on three different scenarios of massive neutrinos using the most recent CMB anisotropy data in combination with type Ia supernovae, baryon acoustic oscillations, and Hubble parameter measurements. When a sterile neutrino is introduced in the interacting dark sector scenario in addition to the standard model prediction of neutrinos, we find that the coupling parameter, characterizing the interaction between dark matter and dark energy, is non-zero at 2$σ$ confidence level. The interaction model with sterile neutrino is also found to be a promising one to alleviate the current tension on Hubble constant. We do not find the evidence for a coupling in the dark sector when the possibility of a sterile neutrino is discarded.

astro-ph.CO↗

arXiv2016

Entropic gravity from noncommutative black holes

In this paper we will investigate the effects of a noncommutative (NC) space-time on the dynamics of the universe. We will generalize the black hole entropy formula for a NC black hole. Then, using the entropic gravity formalism, we will show that the noncommutativity changes the strength of the gravitational field. By applying this result to a homogeneous and isotropic universe containing nonrelativistic matter and a cosmological constant, we will show that the model modified by the noncommutativity of the space-time is a better fit to the obtained data than the standard one.

gr-qc↗

arXiv2016

New observational constraints on f(T) gravity from cosmic chronometers

We use the local value of the Hubble constant recently measured with 2.4% precision, as well as the latest compilation of cosmic chronometers data, together with standard probes such as Supernovae Type Ia and Baryon Acoustic Oscillation distance measurements, in order to impose constraints on the viable and most used f(T) gravity models, where T is the torsion scalar in teleparallel gravity. In particular, we consider three f(T) models with two parameters, out of which one is independent, and we quantify their deviation from $Λ$CDM cosmology through a sole parameter. Our analysis reveals that for one of the models a small but non-zero deviation from $Λ$CDM cosmology is slightly favored, while for the other models the best fit is very close to $Λ$CDM scenario. Clearly, f(T) gravity is consistent with observations, and it can serve as a candidate for modified gravity.

gr-qc↗

arXiv2016

A note on the relations between thermodynamics, energy definitions and Friedmann equations

In what follows, we investigate the relation between the Friedmann and thermodynamic pressure equations, through solving the Friedmann and thermodynamic pressure equations simultaneously. Our investigation shows that a perfect fluid, as a suitable solution for the Friedmann equations leading to the standard modelling of the universe expansion history, cannot simultaneously satisfy the thermodynamic pressure equation and those of Friedmann. Moreover, we consider various energy definitions, such as the Komar mass, and solve the Friedmann and thermodynamic pressure equations simultaneously to get some models for dark energy. The cosmological consequences of obtained solutions are also addressed. Our results indicate that some of obtained solutions may unify the dominated fluid in both the primary inflationary and current accelerating eras into one model. In addition, by taking into account a cosmic fluid of a known equation of state, and combining it with the Friedmann and thermodynamic pressure equations, we obtain the corresponding energy of these cosmic fluids and face their limitations. Finally, we point out the cosmological features of this cosmic fluid and also study its observational constraints.

gr-qc↗

arXiv2016

Gravitationally induced particle production and its impact on structure formation

In this paper we investigate the influence of a continuous particles creation processes on the linear and nonlinear matter clustering, and its consequences on the weak lensing effect induced by structure formation. We study the line of sight behavior of the contribution to the bispectrum signal at a given angular multipole $l$, showing that the scale where the nonlinear growth overcomes the linear effect depends strongly of particles creation rate.

gr-qc↗

arXiv2016

New constraints on interacting dark energy from cosmic chronometers

We use the latest compilation of observational Hubble parameter measurements estimated with the differential evolution of cosmic chronometers, in combination with the local value of the Hubble constant recently measured with 2.4% precision, to constrain the cosmological scenario where dark energy interacts directly with the dark matter sector. To diminish the degeneracy between the parameters we additionally consider standard probes, such as Supernovae Type Ia from joint light curves (JLA) sample, Baryon Acoustic Oscillation distance measurements (BAO), and cosmic microwave background data from Planck 2015 estimations. Our analysis shows that the direct interaction between dark energy and dark matter is mildly flavored, while the dark energy equation-of-state parameter is w < - 1 at 3$σ$ confidence level.

astro-ph.CO↗

arXiv2016

Probing the cosmological viability of non-gaussian statistics

Based on the relationship between thermodynamics and gravity we propose, with the aid of Verlinde's formalism, an alternative interpretation of the dynamical evolution of the Friedmann-Robertson-Walker Universe. This description takes into account the entropy and temperature intrinsic to the horizon of the universe due to the information holographically stored there through non-gaussian statistical theories proposed by Tsallis and Kaniadakis. The effect of these non-gaussian statistics in the cosmological context is change the strength of the gravitational constant. In this paper, we consider the $w$CDM model modified by the non-gaussian statistics and investigate the compatibility of these non-gaussian modification with the cosmological observations. In order to analyze in which extend the cosmological data constrain these non-extensive statistics, we use type Ia supernovae, baryon acoustic oscillations, Hubble expansion rate function and the linear growth of matter density perturbations data.

gr-qc↗

arXiv2016

Connecting inflation with late cosmic acceleration by particle production

A continuous process of creation of particles is investigated as a possible connection between the inflationary stage with late cosmic acceleration. In this model, the inflationary era occurs due to a continuous and fast process of creation of relativistic particles, and the recent accelerating phase is driven by the non-relativistic matter creation from the gravitational field acting on the quantum vacuum, which finally results in an effective equation of state less than $-1$. Thus, explaining recent results in favor of a phantom dynamics without the need of any modifications in the gravity theory has been proposed. Finally, we confront the model with recent observational data of type Ia Supernova, history of the Hubble parameter, baryon acoustic oscillations, and the cosmic microwave background.

gr-qc↗

arXiv2016

Cosmological consequences of an adiabatic matter creation process

In this paper we investigate the cosmological consequences of a continuous matter creation associated with the production of particles by the gravitational field acting on the quantum vacuum. To illustrate this, three phenomenological models are considered. An equivalent scalar field description is presented for each models. The effects on the cosmic microwave background power spectrum are analyzed for the first time in the context of adiabatic matter creation cosmology. Further, we introduce a model independent treatment, $Om$, which depends only on the Hubble expansion rate and the cosmological redshift to distinguish any cosmological model from $Λ$CDM by providing a null test for the cosmological constant, meaning that, for any two redshifts $z_1$, $z_2$, $Om (z)$ is same, i.e. $Om (z_1)- Om (z_2)= 0$. Also, this diagnostic can differentiate between several cosmological models by indicating their quintessential/ phantom behavior without knowing the accurate value of the matter density, and the present value of the Hubble parameter. For our models, we find that particle production rate is inversely proportional to $Om$. Finally, the validity of the generalized second law of thermodynamics bounded by the apparent horizon has been examined.

gr-qc↗

arXiv2016

Jeans instability criterion from the viewpoint of non-gaussian statistics

In this Letter we have derived the Jeans length in the context of the Kaniadakis statistics. We have compared this result with the Jeans length obtained in the non-extensive Tsallis statistics and discussed the main differences between these two models. We have also obtained the kappa-sound velocity. Finally, we have applied the results obtained here to analyze an astrophysical system.

astro-ph.CO↗

arXiv2015

Phantom behavior via cosmological creation of particles

Recent determinations of the equation of state of dark energy hint that this may well be of the phantom type, i.e., $w_{de} < -1$. If confirmed by future experiments, this would strongly point to the existence of fields that violate the dominant energy condition, which are known to present serious theoretical difficulties. This paper presents an alternative to this possibility, namely, that the measured equation of state, $w_{de}$, is in reality an effective one, the equation of state of the quantum vacuum, $w_Λ = -1$, plus the negative equation of state, $w_{c}$, associated to the production of particles by the gravitational field acting on the vacuum. To illustrate this, three phenomenological models are proposed and constrained with recent observational data.

gr-qc↗

arXiv2014

Dark matter-dark energy interaction for a time-dependent equation of state

In this work we investigate the interaction between dark matter and dark energy for a coupling that obeys the Wang-Meng decaying law, $ρ_{\rm DM}\propto (1+z)^{3-ε}$, and the Barboza-Alcaniz dark energy parametric model, $w=w_0+w'_0z(1+z)/(1+z^2)$. Theoretically, we show that the coupling constant, $ε$, should satisfy the physical constraint $ε\ge0$. We use the most recent data of type Ia supernovae, baryon acoustic oscillations, cosmic microwave background and the Hubble expansion rate function to constrain the free parameters of the model. From a purely observational point of view, we show that is not possible to discard values of the coupling constant in the unphysical region $ε<0$. We show that the uncoupled case, $ε=0$, is in better agreement with the data than any of coupled models in the physical region. We also find that all physically acceptable interaction in dark sector lies in the narrow range $0<ε\le0.034$ ($95\%$ CL).

astro-ph.CO↗