arXiv subjects

J. S. Alcaniz

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

At least 145 records · Page 8Linked to original sources

arXiv2005

An old quasar in a young dark energy-dominated universe?

Dark energy is the invisible fuel that seems to drive the current acceleration of the Universe. Its presence, which is inferred from an impressive convergence of high-quality observational results along with some apparently sucessful theoretical predictions, is also supported by the current estimates of the age of the Universe from dating of local and high-$z$ objects. In this paper we test the viability of several dark energy scenarios in the light of the age estimates of the high redshift ($z=3.91$) quasar APM 08279+5255. Using a chemodinamical model for the evolution of spheroids, we first reevaluate its current estimated age, as given by Hasinger et al. (2002). An age of 2.1 Gyr is set by the condition that Fe/O abundance ratio (normalized to solar values) of the model reaches 3.3, which is the best fit value obtained in the above reference. It is shown that for the currently accepted value of the matter density parameter, most of the existing dark energy scenarios cannot accomodate this old high redshift object unless the Hubble parameter is as low as $H_o = 58$ $\rm{km.s^{-1}.Mpc^{-1}}$, as recently advocated by Sandage and collaborators. Even considering less stringent age limits, only cosmological models that predicts a considerably old Universe at high-$z$ can be compatible with the existence of this object. This is the case of the conventional $Λ$CDM scenario and some specific classes of brane world cosmologies.

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arXiv2005

On the Geometry of Dark Energy

Experimental evidence suggests that we live in a spatially flat, accelerating universe composed of roughly one-third of matter (baryonic + dark) and two-thirds of a negative-pressure dark component, generically called dark energy. The presence of such energy not only explains the observed accelerating expansion of the Universe but also provides the remaining piece of information connecting the inflationary flatness prediction with astronomical observations. However, despite of its good observational indications, the nature of the dark energy still remains an open question. In this paper we explore a geometrical explanation for such a component within the context of brane-world theory without mirror symmetry, leading to a geometrical interpretation for dark energy as warp in the universe given by the extrinsic curvature. In particular, we study the phenomenological implications of the extrinsic curvature of a Friedman-Robertson-Walker universe in a five-dimensional constant curvature bulk, with signatures (4,1) or (3,2), as compared with the X-matter (XCDM) model. From the analysis of the geometrically modified Friedman's equations, the deceleration parameter and the Weak Energy Condition, we find a consistent agreement with the presently known observational data on inflation for the deSitter bulk, but not for the anti-deSitter case.

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arXiv2005

Constraints on the Cardassian Expansion from the Cosmic Lens All-Sky Survey Gravitational Lens Statistics

The existence of a dark energy component has usually been invoked as the most plausible way to explain the recent observational results. However, it is also well known that effects arising from new physics (e.g., extra dimensions) can mimic the gravitational effects of a dark energy through a modification of the Friedmann equation. In this paper we investigate some observational consequences of a flat, matter dominated and accelerating/decelerating scenario in which this modification is given by $H^{2} = g(ρ_m, n, q)$ where $g(ρ_m, n, q)$ is a new function of the energy density $ρ_m$, the so-called generalized Cardassian models. We mainly focus our attention on the constraints from the recent Cosmic All Sky Survey (CLASS) lensing sample on the parameters $n$ and $q$ that fully characterize the models. We show that, for a large interval of the $q - n$ parametric space, these models are in agreement with the current gravitational lenses data. The influence of these parameters on the acceleration redshift, i.e., the redshift at which the universe begins to accelerate, and on the age of the universe at high-redshift is also discussed.

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arXiv2004

Complementary Constraints on Brane Cosmology

The acceleration of the expansion of the universe represents one of the major challenges to our current understanding of fundamental physics. In principle, to explain this phenomenon, at least two different routes may be followed: either adjusting the energy content of the Universe -- by introducing a negative-pressure dark energy -- or modifying gravity at very large scales -- by introducing new spatial dimensions, an idea also required by unification theories. In the cosmological context, the role of such extra dimensions as the source of the dark pressure responsable for the acceleration of our Universe is translated into the so-called brane world (BW) cosmologies. Here we study complementary constraints on a particular class of BW scenarios in which the modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. To this end, we use the most recent Chandra measurements of the X-ray gas mass fraction in galaxy clusters, the WMAP determinations of the baryon density parameter, measurements of the Hubble parameter from the \emph{HST}, and the current supernova data. In agreement with other recent results, it is shown that these models provide a good description for these complementary data, although a closed scenario is always favored in the joint analysis. We emphasize that observational tests of BW scenarios constitute a natural verification of the role of possible extra dimensions in both fundamental physics and cosmology.

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arXiv2004

Thermodynamics, Spectral Distribution and the Nature of Dark Energy

Recent astronomical observations suggest that the bulk of energy in the Universe is repulsive and appears like a dark component with negative pressure ($ω\equiv p_x/ρ_x < 0$). In this work we investigate thermodynamic and statistical properties of such a component. It is found that its energy and temperature grow during the evolution of the Universe since work is done on the system. Under the hypothesis of a null chemical potential, the case of phantom energy ($ω< -1$) seems to be physically meaningless because its entropy is negative. It is also proved that the wavelengths of the $ω$-quanta decrease in an expanding Universe. This unexpected behavior explains how their energy may be continuously stored in the course of expansion. The spectrum and the associated Wien-type law favors a fermionic nature with $ω$ naturally restricted to the interval $-1 \leqslant ω< -1/2$. Our analysis also implies that the ultimate fate of the Universe may be considerably modified. If a dark energy dominated Universe expands forever, it will become increasingly hot.

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arXiv2004

Measuring the Chaplygin gas equation of state from angular and luminosity distances

An exotic dark component, named generalized Chaplygin gas (Cg) and parameterized by an equation of state $p = -A/ρ_{Cg}^α$ where $A$ and $α$ are arbitrary constants, is one of the possible candidates for dark energy as well as for a unified scenario of dark matter/energy. In this paper we investigate qualitative and quantitative aspects of the angular size - redshift test in cosmological models driven by such a dark component. We discuss the prospects for constraining the Cg equation of state from measurements of the angular size at low and high redshift radio sources and also from a joint analysis involving angular size and supernova data. A detailed discussion about the influence of the Cg on the minimal redshift at which the angular size of an extragalactic source takes its minimal value is also presented.

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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.

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arXiv2004

Non-extensive statistics and the stellar polytrope index

We use physical constrains imposed from the H-Theorem and from the negative nature of the heat capacity of self-gravitating thermodynamically isolated systems to investigate some possible limits on the stellar polytrope index $n$ within the domain of a classical non-extensive kinetic theory.

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arXiv2004

Cosmological constraints on Chaplygin gas dark energy from galaxy clusters X-ray and supernova data

The recent observational evidences for the present accelerated stage of the Universe have stimulated renewed interest for alternative cosmologies. In general, such models contain an unknown negative-pressure dark component that explains the supernova results and reconciles the inflationary flatness prediction ($Ω_{\rm{T}} = 1$) and the cosmic microwave background measurements with the dynamical estimates of the quantity of matter in the Universe ($Ω_{\rm{m}} \simeq 0.3 \pm 0.1$). In this paper we study some observational consequences of a dark energy candidate, the so-called generalized Chaplygin gas which is characterized by an equation of state $p_{C} = -A/ρ_{C}^α$, where $A$ and $α$ are positive constants. We investigate the prospects for constraining the equation of state of this dark energy component by combining Chandra observations of the X-ray luminosity of galaxy clusters, independent measurements of the baryonic matter density, the latest measurements of the Hubble parameter as given by the HST Key Project and data of the Supernova Cosmology Project. We show that very stringent constraints on the model parameters can be obtained from this combination of observational data.

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arXiv2004

Constraints on Chaplygin quartessence from the CLASS gravitational lens statistics and supernova data

The nature of the dark components (dark matter and dark energy) that dominate the current cosmic evolution is a completely open question at present. In reality, we do not even know if they really constitute two separated substances. In this paper we use the recent Cosmic All Sky Survey (CLASS) lensing sample to test the predictions of one of the candidates for a unified dark matter/energy scenario, the so-called generalized Chaplygin gas (Cg) which is parametrized by an equation of state $p = -A/ρ_{Cg}^α$ where $A$ and $α$ are arbitrary constants. We show that, although the model is in good agreement with this radio source gravitational lensing sample, the limits obtained from CLASS statistics are only marginally compatible with the ones obtained from other cosmological tests. We also investigate the constraints on the free parameters of the model from a joint analysis between CLASS and supernova data.

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arXiv2004

Testing dark energy beyond the cosmological constant barrier

Although well motivated from theoretical arguments, the cosmological constant \emph{barrier}, i.e., the imposition that the equation-of-state parameter of dark energy ($ω_x \equiv p_x/ρ_x$) is $\geq -1$, seems to introduce bias in the parameter determination from statistical analyses of observational data. In this regard, \emph{phantom} dark energy or \emph{superquintessence} has been proposed in which the usual imposition $ω\geq -1$ is relaxed. Here, we study possible observational limits to the \emph{phantom} behavior of the dark energy from recent distance estimates of galaxy clusters obtained from interferometric measurements of the Sunyaev-Zel'dovich effect/X-ray observations, Type Ia supernova data and CMB measurements. We find that there is much \emph{observationally} acceptable parameter space beyond the $Λ$ \emph{barrier}, thus opening the possibility of existence of more exotic forms of energy in the Universe.

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arXiv2003

Negative heat capacity and non-extensive kinetic theory

The negative nature of the heat capacity $C_V$ of thermodynamically isolated self-gravitating systems is rediscussed in the framework of a non-extensive kinetic theory. It is found that the dependence of $C_V$ on the non-extensive parameter $q$ gives rise to a negative branch with the critical value corresponding to $q = 5/3$ ($C_V\to - \infty$).

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arXiv2003

Gravitational lensing constraints on dark energy from modified Friedmann equations

The existence of a dark energy component has usually been invoked as the most plausible way to explain the recent observational results. However, it is also well known that effects arising from new physics (e.g., extra dimensions) can mimic the gravitational effects of a dark energy through a modification of the Friedmann equation. In this paper we investigate some observational consequences of a flat, matter dominated and accelerating/decelerating scenario in which this modification is given by $H^{2} = g(ρ_m, n, q)$ where $g(ρ_m, n, q)$ is a new function of the energy density $ρ_m$, the so-called generalized Cardassian models. We mainly focus our attention on the constraints from statistical properties of gravitationally lensed quasars on the parameters $n$ and $q$ that fully characterize the models. We show that these models are in agreement with the current gravitationally lensed quasar data for a large interval of the $q - n$ parametric space. The dependence of the acceleration redshift (the redshift at which the universe begins to accelerate) on these parameters is also briefly discussed.

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arXiv2003

The angular size - redshift relation in power-law cosmologies

A linear evolution of the cosmological scale factor is a feature in several models designed to solve the cosmological constant problem via a coupling between scalar or tensor classical fields to the space-time curvature as well as in some alternative gravity theories. In this paper, by assuming a general time dependence of the scale factor, $R \sim t^α$, we investigate observational constraints on the dimensionless parameter $α$ from measurements of the angular size for a large sample of milliarcsecond compact radio sources. In particular, we find that a strictly linear evolution, i.e., $α\simeq 1$ is favoured by these data, which is also in agreement with limits obtained from other independent cosmological tests. The dependence of the critical redshift $z_m$ (at which a given angular size takes its minimal value) with the index $α$ is briefly discussed.

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arXiv2003

Constraining the dark energy with galaxy clusters X-ray data

The equation of state characterizing the dark energy component is constrained by combining Chandra observations of the X-ray luminosity of galaxy clusters with independent measurements of the baryonic matter density and the latest measurements of the Hubble parameter as given by the HST key project. By assuming a spatially flat scenario driven by a "quintessence" component with an equation of state $p_x = ωρ_x$ we place the following limits on the cosmological parameters $ω$ and $Ω_{\rm{m}}$: (i) $-1 \leq ω\leq -0.55$ and $Ω_{\rm m} = 0.32^{+0.027}_{-0.014}$ (1$σ$) if the equation of state of the dark energy is restricted to the interval $-1 \leq ω< 0$ (\emph{usual} quintessence) and (ii) $ω= -1.29^{+0.686}_{-0.792}$ and $Ω_{\rm{m}} = 0.31^{+0.037}_{-0.034}$ ($1σ$) if $ω$ violates the null energy condition and assume values $< -1$ (\emph{extended} quintessence or ``phantom'' energy). These results are in good agreement with independent studies based on supernovae observations, large-scale structure and the anisotropies of the cosmic background radiation.

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arXiv2003

Cosmological implications of APM 08279+5255, an old quasar at z = 3.91

The existence of old high-redshift objects provides an important tool for constraining the expanding age of the Universe and the formation epoch of the first objects. In a recent paper, Hasinger {\it et al.} (2002) reported the discovery of the quasar APM 08279+5255 at redshift $z=3.91$ with an extremely high iron abundance, and estimated age of 2 - 3Gyr. By assuming the lower limit for this age estimate and the latest measurements of the Hubble parameter as given by the HST key project, we study some cosmological implications from the existence of this object. In particular, we derive new limits on the dark matter and vacuum energy contribution. Our analysis is also extended to quintessence scenarios in which the dark energy is parameterized by a smooth component with an equation of state $p_x = ω_x ρ_x$ ($-1\leq ω_x < 0$). For flat models with a relic cosmological constant we show that the vacuum energy density parameter is constrained to be $Ω_Λ\geq 0.78$, a result that is marginally compatible with recent observations from type Ia supernovae (SNe Ia) and cosmic microwave background (CMB). For quintessence scenarios the same analysis restricts the cosmic parameter to $ω_x \leq -0.22$. Limits on a possible first epoch of quasar formation are also briefly discussed. The existence of this object pushes the formation era back to extremely high redshifts.

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arXiv2002

High-redshift objects and the generalized Chaplygin gas

Motivated by recent developments in particle physics and cosmology, there has been growing interest in an unified description of dark matter and dark energy scenarios. In this paper we explore observational constraints from age estimates of high-$z$ objects on cosmological models dominated by an exotic fluid with equation of state $p = -A/ρ^α$ (the so-called generalized Chaplygin gas) which has the interesting feature of interpolating between non-relativistic matter and negative-pressure dark energy regimes. As a general result we find that, if the age estimates of these objects are correct, they impose very restrictive limits on some of these scenarios.

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arXiv2002

Cosmological consequences of a Chaplygin gas dark energy

A combination of recent observational results has given rise to what is currently known as the dark energy problem. Although several possible candidates have been extensively discussed in the literature to date the nature of this dark energy component is not well understood at present. In this paper we investigate some cosmological implications of another dark energy candidate: an exotic fluid known as the Chaplygin gas, which is characterized by an equation of state $p = -A/ρ$, where $A$ is a positive constant. By assuming a flat scenario driven by non-relativistic matter plus a Chaplygin gas dark energy we study the influence of such a component on the statistical properties of gravitational lenses. A comparison between the predicted age of the universe and the latest age estimates of globular clusters is also included and the results briefly discussed. In general, we find that the behavior of this class of models may be interpreted as an intermediary case between the standard and $Λ$CDM scenarios.

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