The Long-Term Future of Extragalactic Astronomy

If the current energy density of the universe is indeed dominated by a cosmological constant, then high-redshift sources will remain visible to us only until they reach some finite age in their rest-frame. The radiation emitted beyond that age will never reach us due to the acceleration of the cosmic expansion rate, and so we will never know what these sources look like as they become older. As a source image freezes on a particular time frame along its evolution, its luminosity distance and redshift continue to increase exponentially with observation time. The higher the current redshift of a source is, the younger it will appear as it fades out of sight. For the popular set of cosmological parameters, I show that a source at a redshift z=5-10 will only be visible up to an age of 4-6 billion years. Arguments relating the properties of high-redshift sources to present-day counterparts will remain indirect even if we continue to monitor these sources for an infinite amount of time. These sources will not be visible to us when they reach the current age of the universe.Comment: Phys. Rev. D, in press (2001

A 5D non compact and non Ricci flat Kaluza-Klein Cosmology

A model universe is proposed in the framework of 5-dimensional noncompact Kaluza-Klein cosmology which is not Ricci flat. The 4D part as the Robertson-Walker metric is coupled to conventional perfect fluid, and its extra-dimensional part is coupled to a dark pressure through a scalar field. It is shown that neither early inflation nor current acceleration of the 4D universe would happen if the non-vacuum states of the scalar field would contribute to 4D cosmology.Comment: 13 pages, major revision, published online in GR

Dynamics of Logamediate and Intermediate Scenarios in the Dark Energy Filled Universe

We have considered a model of two component mixture i.e., mixture of Chaplygin gas and barotropic fluid with tachyonic field. In the case, when they have no interaction then both of them retain their own properties. Let us consider an energy flow between barotropic and tachyonic fluids. In both the cases we find the exact solutions for the tachyonic field and the tachyonic potential and show that the tachyonic potential follows the asymptotic behavior. We have considered an interaction between these two fluids by introducing a coupling term. Finally, we have considered a model of three component mixture i.e., mixture of tachyonic field, Chaplygin gas and barotropic fluid with or without interaction. The coupling functions decays with time indicating a strong energy flow at the initial period and weak stable interaction at later stage. To keep the observational support of recent acceleration we have considered two particular forms (i) Logamediate Scenario and (ii) Intermediate Scenario, of evolution of the Universe. We have examined the natures of the recent developed statefinder parameters and slow-roll parameters in both scenarios with and without interactions in whole evolution of the universe.Comment: 28 pages, 20 figure

Cosmological Tracking Solutions

A substantial fraction of the energy density of the universe may consist of quintessence in the form of a slowly-rolling scalar field. Since the energy density of the scalar field generally decreases more slowly than the matter energy density, it appears that the ratio of the two densities must be set to a special, infinitesimal value in the early universe in order to have the two densities nearly coincide today. Recently, we introduced the notion of tracker fields to avoid this initial conditions problem. In the paper, we address the following questions: What is the general condition to have tracker fields? What is the relation between the matter energy density and the equation-of-state of the universe imposed by tracker solutions? And, can tracker solutions explain why quintessence is becoming important today rather than during the early universe

An accelerated closed universe

We study a model in which a closed universe with dust and quintessence matter components may look like an accelerated flat Friedmann-Robertson-Walker (FRW) universe at low redshifts. Several quantities relevant to the model are expressed in terms of observed density parameters, $\Omega_M$ and $\Omega_{\Lambda}$, and of the associated density parameter $\Omega_Q$ related to the quintessence scalar field $Q$.Comment: 11 pages. For a festschrift honoring Alberto Garcia. To appear in Gen. Rel. Gra

Particle-Like Description in Quintessential Cosmology

Assuming equation of state for quintessential matter: $p=w(z)\rho$, we analyse dynamical behaviour of the scale factor in FRW cosmologies. It is shown that its dynamics is formally equivalent to that of a classical particle under the action of 1D potential $V(a)$. It is shown that Hamiltonian method can be easily implemented to obtain a classification of all cosmological solutions in the phase space as well as in the configurational space. Examples taken from modern cosmology illustrate the effectiveness of the presented approach. Advantages of representing dynamics as a 1D Hamiltonian flow, in the analysis of acceleration and horizon problems, are presented. The inverse problem of reconstructing the Hamiltonian dynamics (i.e. potential function) from the luminosity distance function $d_{L}(z)$ for supernovae is also considered.Comment: 35 pages, 26 figures, RevTeX4, some applications of our treatment to investigation of quintessence models were adde

Curvature of the universe and the dark energy potential

The flatness of an accelerating universe model (characterized by a dark energy scalar field $\chi$) is mimicked from a curved model that is filled with, apart from the cold dark matter component, a quintessencelike scalar field $Q$. In this process, we characterize the original scalar potential $V(Q)$ and the mimicked scalar potential $V(\chi)$ associated to the scalar fields $Q$ and $\chi$, respectively. The parameters of the original model are fixed through the mimicked quantities that we relate to the present astronomical data, such that the equation state parameter $w_{_{\chi}}$ and the dark energy density parameter $\Omega_{\chi}$.Comment: References 7 and 8 have been corrected: (7) Riess et al. 1998, AJ, 116, 1009 and (8) Perlmutter et al. 1999, ApJ, 517, 56

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 $p_x = \omega \rho_x$ we place the following limits on the cosmological parameters $\omega$ and $\Omega_{\rm{m}}$: (i) $-1 \leq \omega \leq -0.55$ and $\Omega_{\rm m} = 0.32^{+0.027}_{-0.014}$ (1$\sigma$) if the equation of state of the dark energy is restricted to the interval $-1 \leq \omega < 0$ (\emph{usual} quintessence) and (ii) $\omega = -1.29^{+0.686}_{-0.792}$ and $\Omega_{\rm{m}} = 0.31^{+0.037}_{-0.034}$ ($1\sigma$) if $\omega$ 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.Comment: 6 pages, 4 figures, LaTe

Termination of the Phase of Quintessence by Gravitational Back-Reaction

We study the effects of gravitational back-reaction in models of Quintessence. The effective energy-momentum tensor with which cosmological fluctuations back-react on the background metric will in some cases lead to a termination of the phase of acceleration. The fluctuations we make use of are the perturbations in our present Universe. Their amplitude is normalized by recent measurements of anisotropies in the cosmic microwave background, their slope is taken to be either scale-invariant, or characterized by a slightly blue tilt. In the latter case, we find that the back-reaction effect of fluctuations whose present wavelength is smaller than the Hubble radius but which are stretched beyond the Hubble radius by the accelerated expansion during the era of Quintessence domination can become large. Since the back-reaction effects of these modes oppose the acceleration, back-reaction will lead to a truncation of the period of Quintessence domination. This result impacts on the recent discussions of the potential incompatibility between string theory and Quintessence.Comment: 7 pages a few clarifying comments adde

Expanding Universe: Thermodynamical Aspects From Different Models

The pivotal point of the paper is to discuss the behavior of temperature, pressure, energy density as a function of volume along with determination of caloric EoS from following two model: $w(z)=w_{0}+w_{1}\ln(1+z)$ & $w(z)=-1+\frac{(1+z)}{3}\frac{A_{1}+2A_{2}(1+z)}{A_{0}+2A_{1}(1+z)+A_{2}(1+z)^{2}}$. The time scale of instability for this two models is discussed. In the paper we then generalize our result and arrive at general expression for energy density irrespective of the model. The thermodynamical stability for both of the model and the general case is discussed from this viewpoint. We also arrive at a condition on the limiting behavior of thermodynamic parameter to validate the third law of thermodynamics and interpret the general mathematical expression of integration constant $U_{0}$ (what we get while integrating energy conservation equation) physically relating it to number of micro states. The constraint on the allowed values of the parameters of the models is discussed which ascertains stability of universe. The validity of thermodynamical laws within apparent and event horizon is discussed.Comment: 16 pages, 3 figures(Accepted for publication in "Astrophysics and Space Science"