arXiv subjects

J. S. Alcaniz

Publications and source records attributed to J. S. Alcaniz.

159 records · Page 9Linked to original sources

arXiv2002

Current and future supernova constraints on decaying $Λ$ cosmologies

We investigate observational constraints from present and future supernova data on a large class of decaying vacuum cosmologies. In such scenarios the present value of the vacuum energy density is quantified by a positive $β$ parameter smaller than unity. By assuming a Gaussian prior on the matter density parameter ($Ω_{\rm{m}} = 0.35 \pm 0.07$) we find $Ω_{\rm{m}} = 0.34^{+0.14}_{-0.12}$ and $β= 0.62^{+0.12}_{-0.24}$ (95% c.l.) as the best fit values for the present data. We show that, while the current data cannot provide restrictive constraints on the $Ω_{\rm{m}} - β$ plane, the future SNe data will limit considerably the allowed space of parameter. A brief discussion about the equivalence between dynamical-$Λ$ scenarios and scalar field cosmologies is also included.

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arXiv2002

Age Constraints on Brane Models of Dark Energy

Inspired by recent developments in particle physics, the so-called brane world cosmology seems to provide an alternative explanation for the present dark energy problem. In this paper, we use the estimated age of high-$z$ objects to constrain the value of the cosmological parameters in some particular scenarios based on this large scale modification of gravity. We show that such models are compatible with these observations for values of the crossover distance between the 4 and 5 dimensions of the order of $r_c \leq 1.67H_o^{-1}$.

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arXiv2002

Deflationary cosmology: constraints from angular size and ages of globular clusters

Observational constraints to a large class of decaying vacuum cosmologies are derived using the angular size data of compact radio sources and the latest age estimates of globular clusters. For this class of deflationary $Λ(t)$ models, the present value of the vacuum energy density is quantified by a positive $β$ parameter smaller than unity. In the case of milliarcsecond compact radio-sources, we find that the allowed intervals for $β$ and the matter density parameter $Ω_m$ are heavily dependent on the value of the mean projected linear size $l$. For $l \simeq 20h^{-1} - 30h^{-1}$ pc, the best fit occurs for $β\sim 0.58$, $Ω_{\rm{m}} \sim 0.58$, and $β\sim 0.76$, $Ω_{\rm{m}} \sim 0.28$, respectively. This analysis shows that if one minimizes $χ^{2}$ for the free parameters $l$, $Ω_{\rm{m}}$ and $β$, the best fit for these angular size data corresponds to a decaying $Λ(t)$ with $Ω_{\rm{m}} = 0.54$ $β=0.6$ and $l = 22.64h^{-1}$ pc. Constraints from age estimates of globular clusters and old high redshift galaxies are not so restrictive, thereby suggesting that there is no age crisis for this kind of $Λ(t)$ cosmologies.

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arXiv2002

Brane World Cosmologies and Statistical Properties of Gravitational Lenses

Brane world cosmologies seem to provide an alternative explanation for the present accelerated stage of the Universe with no need to invoke either a cosmological constant or an exotic \emph{quintessence} component. In this paper we investigate statistical properties of gravitational lenses for some particular scenarios based on this large scale modification of gravity. We show that a large class of such models are compatible with the current lensing data for values of the matter density parameter $Ω_{\rm{m}} \leq 0.94$ ($1σ$). If one fixes $Ω_{\rm{m}}$ to be $\simeq 0.3$, as suggested by most of the dynamical estimates of the quantity of matter in the Universe, the predicted number of lensed quasars requires a slightly open universe with a crossover distance between the 4 and 5-dimensional gravities of the order of $1.76 H_o^{-1}$.

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arXiv2002

Dark energy and the angular size - redshift diagram for milliarcsecond radio sources

We investigate observational constraints on the cosmic equation of state from measurements of angular size for a large sample of milliarcsecond compact radio sources. The results are based on a flat Friedmann-Robertson-Walker (FRW) type models driven by non-relativistic matter plus a smooth dark energy component parametrized by its equation of state $p_x = ωρ_x$ ($-1 \leq ω< 0$). The allowed intervals for $ω$ and $Ω_{\rm{m}}$ are heavily dependent on the value of the mean projected linear size $l$. For $l \simeq 20h^{-1} - 30h^{-1}$ pc, we find $Ω_{\rm{m}} \leq 0.62$, $ω\leq -0.2$ and $Ω_{\rm{m}} \leq 0.17$, $ω\leq -0.65$ (68% c.l.), respectively. As a general result, this analysis shows that if one minimizes $χ^{2}$ for the parameters $l$, $Ω_{\rm{m}}$ and $ω$, the conventional flat $Λ$CDM model ($ω= -1$) with $Ω_{\rm{m}} = 0.2$ and $l = 22.6 h^{-1}$pc is the best fit for these angular size data.

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arXiv2002

Some Observational Consequences of Brane World Cosmologies

The presence of dark energy in the Universe is inferred directly and indirectly from a large body of observational evidence. The simplest and most theoretically appealing possibility is the vacuum energy density (cosmological constant). However, although in agreement with current observations, such a possibility exacerbates the well known cosmological constant problem, requiring a natural explanation for its small, but nonzero, value. In this paper we focus our attention on another dark energy candidate, one arising from gravitational \emph{leakage} into extra dimensions. We investigate observational constraints from current measurements of angular size of high-$z$ compact radio-sources on accelerated models based on this large scale modification of gravity. The predicted age of the Universe in the context of these models is briefly discussed. We argue that future observations will enable a more accurate test of these cosmologies and, possibly, show that such models constitute a viable possibility for the dark energy problem.

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arXiv2002

Cosmological implications of old galaxies at high redshifts

Old high-$z$ galaxies are important tools for understanding the structure formation problem and may become the key to determine the ultimate fate of the Universe. In this {\it letter}, the inferred ages of the three oldest galaxies at high redshifts reported in the literature are used to constrain the first epoch of galaxy formation and to reanalyse the high-z time scale crisis. The lower limits on the formation redshift $z_f$ depends on the quantity of cold dark matter in the Universe. In particular, if $Ω_m\geq 0.37$ these galaxies are not formed in FRW cosmologies with no dark energy. This result is in line with the Supernovae type Ia measurements which suggest that the bulk of energy in the Universe is repulsive and appears like an unknown form of dark energy component. In a complementar analysis, unlike recent claims favoring the end of the age problem, it is shown that the Einstein-de Sitter model is excluded at high-z by $\sim 3σ$.

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arXiv2001

Dark energy and the epoch of galaxy formation

The influence of a dark component on the first epoch of galaxy formation is analysed by using the ages of the three oldest high-redshift galaxies known in the literature. Our results, based on a spatially flat accelerated Universe driven by a "quintessence" component ($p_x = ωρ_x$), show that if the inferred ages of these objects are correct the first formation era is pushed back to extremely high redshifts. For the present best-fit quintessence model ($Ω_{x}= 0.7$, $ω< -0.6$), we find a lower bound of $z_f \geq 7.7$, whereas in the extreme case of $Λ$CDM model ($ω= -1$) the limit is slightly smaller ($z_f \geq 5.8$). The case for open cold dark matter models (OCDM) has also been discussed. For $Ω_m \simeq 0.3$, the formation redshift is restricted by $z_f \geq 18$. As a general result, if $Ω_m \geq 0.37$, these galaxies are not formed in FRW cosmologies with no dark energy since for all these cases $z_f \to \infty$.

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arXiv2000

Constraining the cosmic equation of state from old galaxies at high redshift

New limits on the cosmic equation of state are derived from age measurements of three recently reported old high redshift galaxies (OHRG). The results are based on a flat FRW type cosmological model driven by nonrelativistic matter plus a smooth component parametrized by its equation of state $p_{x} = ωρ_{x}$ ($ω\geq -1$). The range of $ω$ is strongly dependent on the matter density parameter. For $Ω_{M} \sim 0.3$, as indicated from dynamical measurements, the age estimates of the OHRG restricts the cosmic parameter to $ω\leq - 0.27$. However, if $Ω_{M}$ is the one suggested by some studies of field galaxies, i.e, $Ω_{M} \simeq 0.5$, only a cosmological constant ($ω=-1$) may be compatible with these data.

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arXiv2000

Angular size in quintessence cosmology

We investigate the influence of an exotic fluid component ("quintessence") on the angular size-redshift relation for distant extragalactic sources. Particular emphasis is given for the redshif $z_{m}$ at which the angular size takes its minimal value. We derive an analytical closed form which determines how $z_m$ depends on the parameter of the equation of state describing the exotic component. The results for a flat model dominated by a "quintessence" are compared in detail with the ones for the standard open model dominated by cold dark matter. Some consequences of systematic evolutionary effects on the values of $z_{m}$ are also briefly discussed. It is argued that the critical redshift, for all practical purposes, may completely be removed if such effects are taken into account.

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arXiv1999

Minimal Angular Size of Distant Sources in Open, $Λ$CDM, and Scalar Field Cosmologies

We propose a simple method for determining the redshift $z_{m}$ at which the angular size of an extragalactic source with fixed proper diameter takes its minimal value. A closed analytical expression, which is quite convenient for numerical evaluation is derived. The method is exemplified with the following FRW type expanding universes: the open matter dominated models ($Ω_Λ= 0$), a critical density model with cosmological constant ($Ω_Λ \neq 0$), and the class of scalar field cosmologies proposed by Ratra and Peebles. The influence of systematic evolutionary effects is briefly discussed.

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arXiv1999

Closed and open FRW cosmologies with matter creation: Kinematic tests

The limits imposed by the classical cosmological tests on closed and open FRW universes driven by adiabatic matter creation are investigated. Exact 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. An interesting consequence of these cosmological models is the possibility of an accelerated expansion today (as indicated from supernovae observations) with no need to invoke either a cosmological constant or an exotic "quintessence" component.

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arXiv1999

New limits on $Ω_Λ$ and $Ω_M$ from old galaxies at high redshift

The ages of two old galaxies (53W091, 53W069) at high redshifts are used to constrain the value of the cosmological constant in a flat universe ($Λ$CDM) and the density parameter $Ω_M$ in Friedmann-Robertson-Walker (FRW) models with no $Λ$-term. In the case of $Λ$CDM models, the quoted galaxies yield two lower limits for the vacuum energy density parameter, $Ω_Λ\geq 0.42$ and $Ω_Λ\geq 0.5$, respectively. Although compatible with the limits from statistics of gravitational lensing (SGL) and cosmic microwave background (CMB), these lower bounds are more stringent than the ones recently determined using SNe Ia as standard candles. For matter dominated universes ($Ω_Λ=0$), the existence of these galaxies imply that the universe is open with the matter density parameter constrained by $Ω_M \leq 0.45$ and $Ω_M \leq 0.37$, respectively. In particular, these results disagree completely with the analysis of field galaxies which gives a lower limit $Ω_M \geq 0.40$.

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arXiv1999

Flat FRW Cosmologies with Adiabatic Matter Creation: Kinematic tests

Some observational consequences of a cosmological scenario driven by adiabatic matter creation are investigated. Exact expressions for the lookback time, age of the universe, luminosity distance, angular diameter, and galaxy number counts redshift relations are derived and their meaning discussed in detail. The expressions of the conventional FRW models are significantly modified and provide a powerful method to limit the parameters of the models.

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arXiv1999

Cosmological gravitons and the expansion dynamics during the matter age

Extending previous results [Phys. Rev.D {\bf 56}, 6351 (1997)], we estimate the back reaction of cosmological gravitons in the expansion dynamics, during the matter age. Tensor perturbations with scales larger than the Hubble radius are created due to the inequivalence of vacua at different times. During noninflationary phases these perturbations become effective gravitational waves as they enter the Hubble radius, adding new contributions to the energy density of the subhorizon waves. During the radiation epoch the creation of these effective gravitons may lead to a departure from the standard behaviour $a(t)\propto t^{1/2}$, with possible consequences to several cosmic processes, such as primordial nucleosynthesis. We examine the implications of this phenomenom during the matter dominated era, assuming an initial inflationary period. The dynamical equation obeyed by the scale factor is derived and numerically solved for different values of the relevant parameters involved.

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