arXiv subjects

J. V. Cunha

Publications and source records attributed to J. V. Cunha.

26 records · Page 2Linked to original sources

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

A simplified approach for Chaplygin-type cosmologies

A new class of accelerating cosmological models driven by a one-parameter version of the general Chaplygin-type equation of state is proposed. The simplified version is naturally obtained from causality considerations with basis on the adiabatic sound speed $v_S$ plus the observed accelerating stage of the universe. We show that very stringent constraints on the unique free parameter $α$ describing the simplified Chaplygin model can be obtained from a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations (BAO). In our analysis we have considered separately the SNe type Ia gold sample measured by Riess et al. (2004) and the Supernova Legacy Survey (SNLS) from Astier et al. (2006). At 95.4% (c.l.), we find for BAO + \emph{gold} sample, $0.91 \leq α\leq 1.0$ and $Ω_{\rm{M}}= 0.28^{+0.043}_{-0.048}$ while BAO + SNLS analysis provides $0.94 \leq α\leq 1.0$ and $Ω_{\rm M}=0.27^{+0.048}_{-0.045}$.

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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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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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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

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

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.

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