arXiv subjects

N. Pires

Publications and source records attributed to N. Pires.

11 recordsLinked to original sources

arXiv2017

Coupling dark-baryonic matter density profile for vacuum decay scenarios

The cosmological consequences of an interacting model in which vacuum decay law is deducted from the effect that vacuum decay has on the dark matter evolution are investigated. Here, the baryonic matter is also considered as a fluid gravitationally coupled with dark matter. It is made a careful analysis to constrain this model with the observational data of growth rate of cosmic structures. The theoretical growth rate is followed since the primordial recombination and the main physical processes on the baryonic component are considered. As a complementary constraint, this model is compared with the observed CMB-BAO ratio as well with the gas mass fraction of cluster of galaxies. We found the best fit values for dark matter $Ω_{d0} = 0.269 ^{+0.023}_{-0.023}$ and for the decay parameter $ε= 0.02 ^{+0.04}_{-0.05}$.

astro-ph.CO↗

arXiv2015

Equivalence between generalized phenomenological schemes for the interaction of cosmological fluids: applications to arbitrary linear barotropic fluids and vacuum decay

Interactions between cosmic fluids may appear in many cosmological scenarios that go far beyond the usually studied energy exchange in the dark sector. In the absence of known microscopic interaction mechanisms, phenomenological ansatzes are usually proposed in order to describe such models. In this paper, we derive a generalization of one of the most frequently used of such ansatzes:the one based on a initial proposal of Shapiro, Solà, España-Bonet and Ruiz-Lapuente who described a time-dependent cosmological "constant" whose variation arises from quantum effects near the Planck scale [I. L. Shapiro, J. Solà, C. España-Bonet, and P. Ruiz-Lapuente, (2003). This physically motivated model was based on a single free parameter $ν$, and was subsequently studied by Wang and Meng (2005), under the pure phenomenological reasoning that the vacuum decay would slightly modify the exponent describing how the energy density of dark matter decreases with the scale factor. This modification is described by a single parameter $\varepsilon$ (= 3 $ν$ of the former paper). The generalization derived in the present article requires two free parameters ($\varepsilon_{1}$, $\varepsilon_{2}$). We show that this extension is dynamically equivalent to the ansatz proposed by Barrow and Clifton (2006) which deals with the transfer of energy between any two fluids using a two-parameter scheme ($α_{1}$, $α_{2}$) and that has the advantage of exhibiting explicitly the form of the interaction factor appearing in the continuity equation of each fluid.

gr-qc↗

arXiv2010

Cosmographic constraints on a class of Palatini f(R) gravity

Modified gravity, known as $f(R)$ gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. For this kind of gravity the expansion of the Universe may accelerate while containing only baryonic and cold dark matter. The aim of the present investigation is to place cosmographic constraints on the class of theories of the form $f(R)=R - α/R^n$ within the Palatini approach. Although extensively discussed in recent literature and confronted with several observational data sets, cosmological tests are indeed inconclusive about the true signal of $n$ in this class of theories. This is particularly important to define which kind of corrections (infra-red or high-energy) to general relativity this class of theory indeed represent. We shed some light on this question by examining the evolution of the deceleration parameter $q(z)$ for these theories. We find that for a large range of $α$, models based on $f(R) = R - α/R^{n}$ gravity in the Palatini approach can only have positive values for $n$, placing thus a broad restriction on this class of gravity.

astro-ph.CO↗

arXiv2009

Current lookback time-redshift bounds on dark energy

We investigate observational constraints on dark energy models from lookback time (LT) estimates of 32 old passive galaxies distributed over the redshift interval $0.11 \leq z \leq 1.84$. To build up our LT sample we combine the age measurements for these 32 objects with estimates of the total age of the Universe, as obtained from current CMB data. We show that LT data may provide bounds on the cosmological parameters with accuracy competitive with type Ia Supernova methods. In order to break possible degeneracies between models parameters, we also discuss the bounds when our lookback time versus redshift sample is combined with with the recent measurement of the baryonic acoustic oscillation peak and the derived age of the Universe from current CMB measurements.

astro-ph.CO↗

arXiv2008

Latest supernovae constraints on f(R) cosmologies

A class of modified gravity, known as $f(R)$-gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. In this paper we use the most recent Type-Ia Supernova (SNe Ia) data, the so-called \emph{Union} sample of 307 SNe Ia, to place bounds on a theory of the form $f(R)=R - β/R^n$ within the Palatini approach. Given the complementarity of SNe Ia data with other cosmological observables, a joint analysis with measurements of baryon acoustic oscillation peak and estimates of the {\rm CMB} shift parameter is also performed. We show that, for the allowed intervals of $n$, $Ω_{mo}$, and $β$, models based on $f(R) = R - β/R^{n}$ gravity in the Palatini approach can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

astro-ph↗

arXiv2007

Lookback time bounds from energy conditions

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach to cosmological modeling, where the energy and matter components of the cosmic fluid are unknown, the energy conditions provide model-independent bounds on the behavior of the lookback time of cosmic sources as a function of the redshift for any value of the spatial curvature. We also confront such bounds with a lookback time sample which is built from the age estimates of 32 galaxies lying in the interval $0.11 \lesssim z \lesssim 1.84$ and by assuming the total expanding age of the Universe to be $13.7 \pm 0.2$ Gyr, as obtained from current cosmic microwave background experiments. In agreement with previous results, we show that all energy conditions seem to have been violated at some point of the recent past of cosmic evolution.

astro-ph↗

arXiv2007

Energy Conditions and Cosmic Acceleration

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors $T_{μν}$ in different frameworks, and to derive general results that hold in a variety of general contexts on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach, where the equation of state of the cosmological fluid is unknown, the energy conditions provide model-independent bounds on the behavior of the distance modulus of cosmic sources as a function of the redshift for any spatial curvature. We use the most recent type Ia supernovae (SNe Ia) observations, which include the new Hubble Space Telescope SNe Ia events, to carry out a model-independent analysis of the energy conditions violation in the context of the standard cosmology. We show that both the null (NEC), weak (WEC) and dominant (DEC) conditions, which are associated with the existence of the so-called phantom fields, seem to have been violated only recently ($z \lesssim 0.2$), whereas the condition for attractive gravity, i.e., the strong energy condition (SEC) was firstly violated billions of years ago, at $z \gtrsim 1$.

astro-ph↗

arXiv2006

Lookback time as a test for brane cosmology

The observed late-time acceleration of the Universe may be the result of unknown physical processes involving either modifications of gravitation theory or the existence of new fields in high energy physics. In the former case, such modifications are usually related to the possible existence of extra dimensions (which is also required by unification theories), giving rise to the so-called brane cosmology. In this paper we investigate the viability of this idea by considering a particular class of brane scenarios in which a large scale modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. To this end, differently from other recent analyses, we combine \emph{orthogonal} age and distance measurements at intermediary and high redshifts. We use observations of the lookback time to galaxy clusters, indirect estimates of the age of the Universe from the most recent Large-Scale Structure (LSS) and Cosmic Microwave Background (CMB) data, along with the recent detection of the baryon acoustic oscillations at $z = 0.35$. In agreement with other recent analyses we show that, although compatible with these age and distance measurements, a spatially closed scenario is largely favoured by the current observational data. By restricting our analysis to a spatially flat universe, we also find that the standard $Λ$CDM model is favoured over the particular braneworld scenario here investigated.

astro-ph↗

arXiv2004

On the width of the last scattering surface

We discuss the physical effects of some accelerated world models on the width of the last scattering surface (LSS) of the cosmic microwave background radiation (CMBR). The models considered in our analysis are X-matter (XCDM) and a Chaplygin type gas. The redshift of the LSS does not depend on the kind of dark energy (if XCDM of Chaplygin). Further, for a Chaplygin gas, the width of the LSS is also only weakly dependent on the kind of scenario (if we have dark energy plus cold dark matter or the unified picture).

astro-ph↗

arXiv2004

Cosmic acceleration in brane cosmology

Cosmic acceleration may be the result of unknown physical processes involving either new fields in high energy physics or modifications of gravitation theory. In the latter case, such modifications are usually related to the existence of extra dimensions (which is also required by unification theories), giving rise to the so-called brane cosmology. In this paper we investigate the phenomenon of the acceleration of the Universe in a particular class of brane scenarios in which a large scale modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. By using the most recent supernova observations we study the transition (deceleration/acceleration) epoch as well as the constraints imposed on the parameters characterizing the model. We show that these models provide a good description for the current supernova data, which may be indicating that the existence of extra dimensions play an important role not only in fundamental physics but also in cosmology.

astro-ph↗

arXiv2002

Deflationary $Λ(t)$ cosmology: observational expressions

We discuss the classical cosmological tests for a large class of FRW type models driven by a decaying vacuum energy density. Analytic expressions for the lookback time, age of the universe, luminosity distance, angular diameter, and galaxy number counts versus redshift are derived and their meaning discussed in detail. It is found that the standard FRW results are significantly altered, showing that such tests may constrain the physical parameters of these models which are also in agreement to the accelerated expansion suggested by the latest SNe type Ia observations.

astro-ph↗