arXiv subjects

R. C. Santos

Publications and source records attributed to R. C. Santos.

25 records · Page 2Linked to original sources

arXiv2008

Simplified Chaplygin Gas: Deriving $H_0$ From Ages of Old High Redshift Objects and Baryon Acoustic Oscillations

The discovery that the expansion of the Universe is accelerating is the most challenging problem of modern cosmology. In the context of general relativity, there are many dark energy candidates to explain the observed acceleration. In this work we focus our attention on two kinds of simplified Chaplygin gas cosmological accelerating models recently proposed in the literature. In the first scenario, the simplified Chaplygin gas works like a Quintessence model while in the second one, it plays the role of a Quartessence (an unification of the dark sector). Firstly, in order to limit the free parameters of both models, we discuss the age of high redshift objects with special emphasis to the old quasar APM 08279+5255 at $z = 3.91$. The basic finding is that this old high redshift object constrain severely the simplified Chaplygin cosmologies. Secondly, through a joint analysis involving the baryon acoustic oscillations (BAO) and a sample of old high redshift galaxies (OHRGs) we also estimate the value of the Hubble parameter, $H_0$. Our approach suggests that the combination of these two independent phenomena provides an interesting method to constrain the Hubble constant.

astro-ph↗

arXiv2008

Clustering, Angular Size and Dark Energy

The influence of dark matter inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component (XCDM model). The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is characterized by a smoothness parameter $α(z)$ and a power index $γ$, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. As a general result, we have found that the $α$ parameter is totally unconstrained by this sample of angular diameter data.

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arXiv2008

Constraining the dark energy and smoothness-parameter with supernovae

The presence of inhomogeneities modifies the cosmic distances through the gravitational lensing effect, and, indirectly, must affect the main cosmological tests. Assuming that the dark energy is a smooth component, the simplest way to account for the influence of clustering is to suppose that the average evolution of the expanding Universe is governed by the total matter-energy density whereas the focusing of light is only affected by a fraction of the total matter density quantified by the $α$ Dyer-Roeder parameter. By using two different samples of SNe type Ia data, the $Ω_m$ and $α$ parameters are constrained by applying the Zeldovich-Kantowski-Dyer-Roeder (ZKDR) luminosity distance redshift relation for a flat ($Λ$CDM) model. A $χ^{2}$-analysis using the 115 SNe Ia data of Astier {\it et al.} sample (2006) constrains the density parameter to be $Ω_m=0.26_{-0.07}^{+0.17}$($2σ$) while the $α$ parameter is weakly limited (all the values $\in [0,1]$ are allowed even at 1$σ$). However, a similar analysis based the 182 SNe Ia data of Riess {\it et al.} (2007) constrains the pair of parameters to be $Ω_m= 0.33^{+0.09}_{-0.07}$ and $α\geq 0.42$ ($2σ$). Basically, this occurs because the Riess {\it et al.} sample extends to appreciably higher redshifts. As a general result, even considering the existence of inhomogeneities as described by the smoothness $α$ parameter, the Einstein-de Sitter model is ruled out by the two samples with a high degree of statistical confidence ($11.5σ$ and $9.9σ$, respectively). The inhomogeneous Hubble-Sandage diagram discussed here highlight the necessity of the dark energy, and a transition deceleration/accelerating phase at $z\sim 0.5$ is also required.

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arXiv2007

ZKDR Distance, Angular Size and Phantom Cosmology

The influence of mass inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component. The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is characterized by a smoothness parameter $α(z)$ and a power index $γ$, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. By marginalizing over the characteristic angular size $l$, fixing $Ω_M = 0.26$ and assuming a Gaussian prior on $ω$, i.e., $ω= -1 \pm 0.3$, the best fit values are $ω= -1.03$ and $α= 0.9$. By assuming a Gaussian prior on the matter density parameter, i.e., $Ω_M = 0.3 \pm 0.1$, the best fit model for a phantom cosmology with $ω= -1.2$ occurs at $Ω_M = 0.29$ and $α= 0.9$ when we marginalize over the characteristic size of the compact radio sources. The results discussed here suggest that the ZKDR distance can give important corrections to the so-called background tests of dark energy.

astro-ph↗

arXiv2006

New Constraints on the Variable Equation of State Parameter from X-Ray Gas Mass Fractions and SNe Ia

Recent measurements are suggesting that we live in a flat Universe and that its present accelerating stage is driven by a dark energy component whose equation of state may evolve in time. Assuming two different parameterizations for the function $ω(z)$, we constrain their free parameters from a joint analysis involving measurements from X-Ray luminosity of galaxy clusters and SNe type Ia data.

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arXiv2006

Hamilton-Jacobi Approach for Power-Law Potentials

The classical and relativistic Hamilton-Jacobi approach is applied to the one-dimensional homogeneous potential, $V(q)=αq^n$, where $α$ and $n$ are continuously varying parameters. In the non-relativistic case, the exact analytical solution is determined in terms of $α$, $n$ and the total energy $E$. It is also shown that the non-linear equation of motion can be linearized by constructing a hypergeometric differential equation for the inverse problem $t(q)$. A variable transformation reducing the general problem to that one of a particle subjected to a linear force is also established. For any value of $n$, it leads to a simple harmonic oscillator if $E>0$, an "anti-oscillator" if $E<0$, or a free particle if E=0. However, such a reduction is not possible in the relativistic case. For a bounded relativistic motion, the first order correction to the period is determined for any value of $n$. For $n >> 1$, it is found that the correction is just twice that one deduced for the simple harmonic oscillator ($n=2$), and does not depend on the specific value of $n$.

gr-qc↗

arXiv2004

The Existence of an Old Quasar at $z = 3.91$ and its Implications for $Λ(t)$ Deflationary Cosmologies

We investigate some observational constraints on decaying vacuum cosmologies based on the recently discovered old high redshift quasar APM 08279+5255. This object is located at $z = 3.91$ and has an estimated age of 2-3 Gyr. The class of $Λ(t)$ cosmologies is characterized by a positive $β$ parameter smaller than unity which quantifies the ratio between the vacuum and the total energy density. Assuming the lower limit age (2 Gyr) and that the cold dark matter contributes with $Ω_{\rm M}=0.2$ we show that $β$ is constrained to be $\ge 0.07$ while for an age of 3 Gyr and $Ω_{\rm M}=0.4$ the $β$ parameter must be greater than 0.32. Our analysis includes closed, flat and hyperbolic scenarios, and it strongly suggests that there is no age crisis for this kind of $Λ(t)$ cosmologies. Lower limits to the redshift quasar formation are also briefly discussed to the flat case. For $Ω_{\rm M}=0.4$ we found that the redshift formation is constrained by $z_{f}\ge 8.0$.

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