Support of dS/CFT correspondence from space-time perturbations

We analyse the spectrum of perturbations of the de Sitter space on the one hand, while on the other hand we compute the location of the poles in the Conformal Field Theory (CFT) propagator at the border. The coincidence is striking, supporting a dS/CFT correspondence. We show that the spectrum of thermal excitations of the CFT at the past boundary $I^{-}$ together with that spectrum at the future boundary $I^{+}$ is contained in the quasi-normal mode spectrum of the de Sitter space in the bulk.Comment: Modified version, appearing in Phys. Rev. D66 (2002) 10401

The Age-Redshift Relation for Standard Cosmology

We present compact, analytic expressions for the age-redshift relation $\tau(z)$ for standard Friedmann-Lema\^ \itre-Robertson-Walker (FLRW) cosmology. The new expressions are given in terms of incomplete Legendre elliptic integrals and evaluate much faster than by direct numerical integration.Comment: 13 pages, 3 figure

Phenomemology of a Realistic Accelerating Universe Using Tracker Fields

We present a realistic scenario of tracking of scalar fields with varying equation of state. The astrophysical constraints on the evolution of scalar fields in the physical universe are discussed. The nucleosynthesis and the galaxy formation constraints have been used to put limits on $\Omega_\phi$ and estimate $\epsilon$ during cosmic evolution. Interpolation techniques have been applied to estimate $\epsilon\simeq0.772$ at the present epoch. The epoch of transition from matter to quintessence dominated era and consequent onset of acceleration in cosmic expansion is calculated and taking the lower limit $\Omega_n^0 = 0.2$ as estimated from $SN_e I_a$ data, it is shown that the supernova observations beyond redshift $z=1$ would reveal deceleration in cosmic expansion.Comment: 10 pages, 4 figures, late

Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications

The ultimate fate of the universe, infinite expansion or a big crunch, can be determined by measuring the redshifts, apparent brightnesses, and intrinsic luminosities of very distant supernovae. Recent developments have provided tools that make such a program practicable: (1) Studies of relatively nearby Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be accurately determined; (2) New research techniques have made it possible to schedule the discovery and follow-up observations of distant supernovae, producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These distant supernovae provide a record of changes in the expansion rate over the past several billion years. By making precise measurements of supernovae at still greater distances, and thus extending this expansion history back far enough in time, we can distinguish the slowing caused by the gravitational attraction of the universe's mass density Omega_M from the effect of a possibly inflationary pressure caused by a cosmological constant Lambda. We report here the first such measurements, with our discovery of a Type Ia supernova (SN 1997ap) at z = 0.83. Measurements at the Keck II 10-m telescope make this the most distant spectroscopically confirmed supernova. Over two months of photometry of SN 1997ap with the Hubble Space Telescope and ground-based telescopes, when combined with previous measurements of nearer SNe Ia, suggests that we may live in a low mass-density universe. Further supernovae at comparable distances are currently scheduled for ground and space-based observations.Comment: 12 pages and 4 figures (figure 4 is repeated in color and black and white) Nature, scheduled for publication in the 1 January, 1998 issue. Also available at http://www-supernova.lbl.go

Scaling solutions in general non-minimal coupling theories

A class of generalized non-minimal coupling theories is investigated, in search of scaling attractors able to provide an accelerated expansion at the present time. Solutions are found in the strong coupling regime and when the coupling function and the potential verify a simple relation. In such cases, which include power law and exponential functions, the dynamics is independent of the exact form of the coupling and the potential. The constraint from the time variability of $G$, however, limits the fraction of energy in the scalar field to less than 4% of the total energy density, and excludes accelerated solutions at the present.Comment: 10 pages, 3 figures, accepted for publication in Phys. Rev.

An Interacting Two-Fluid Scenario for Dark Energy in FRW Universe

We study the evolution of the dark energy parameter within the scope of a spatially flat and isotropic Friedmann-Robertson-Walker (FRW) model filled with barotropic fluid and dark energy. To obtain the deterministic solution we choose the scale factor $a(t) = \sqrt{t e^{t}}$ which yields a time dependent deceleration parameter (DP). In doing so we consider the case minimally coupled with dark energy to the perfect fluid as well as direct interaction with it.Comment: 11 pages, accepted for publication in Chinese Physics Letters. Unlike the previous version the new one contains the time depending deceleration paramete

Perturbation evolution with a non-minimally coupled scalar field

We recently proposed a simple dilaton-derived quintessence model in which the scalar field was non-minimally coupled to cold dark matter, but not to `visible' matter. Such couplings can be attributed to the dilaton in the low energy limit of string theory, beyond tree level. In this paper we discuss the implications of such a model on structure formation, looking at its impact on matter perturbations and CMB anisotropies. We find that the model only deviates from $\Lambda$CDM and minimally coupled theories at late times, and is well fitted to current observational data. The signature left by the coupling, when it breaks degeneracy at late times, presents a valuable opportunity to constrain non-minimal couplings given the wealth of new observational data promised in the near future.Comment: Version appearing in Physical Review D. 10 pages, 9 figs. Comparison with SN1a and projected MAP results, and appendix adde

Distinguishing among Scalar Field Models of Dark Energy

We show that various scalar field models of dark energy predict degenerate luminosity distance history of the Universe and thus cannot be distinguished by supernovae measurements alone. In particular, models with a vanishing cosmological constant (the value of the potential at its minimum) are degenerate with models with a positive or negative cosmological constant whose magnitude can be as large as the critical density. Adding information from CMB anisotropy measurements does reduce the degeneracy somewhat but not significantly. Our results indicate that a theoretical prior on the preferred form of the potential and the field's initial conditions may allow to quantitatively estimate model parameters from data. Without such a theoretical prior only limited qualitative information on the form and parameters of the potential can be extracted even from very accurate data.Comment: 15 pages, 5 figure

Properties of cosmologies with dynamical pseudo Nambu-Goldstone bosons

We study observational constraints on cosmological models with a quintessence field in the form of a dynamical pseudo Nambu-Goldstone boson. After reviewing the properties of the solutions, from a dynamical systems phase space analysis, we consider the constraints on parameter values imposed by luminosity distances from the 60 Type Ia supernovae published by Perlmutter et al., and also from gravitational lensing statistics of distant quasars. In the case of the Type Ia supernovae we explicitly allow for the possibility of evolution of the peak luminosities of the supernovae sources, using simple empirical models which have been recently discussed in the literature. We find weak evidence to suggest that the models with supernovae evolution fit the data better in the context of the quintessence models in question. If source evolution is a reality then the greatest challenge facing these models is the tension between current value of the expansion age, H_0 t_0, and the fraction of the critical energy density, Omega_{phi0}, corresponding to the scalar field. Nonetheless there are ranges of the free parameters which fit all available cosmological data.Comment: 22 pages, RevTeX, 13 figures, epsf. v3: References added, plus a few sentences to clarify some small points; v4: Typos fixe

Cosmological dynamics in six-order gravity

We consider cosmological dynamics in generalized modified gravity theory with the $R\Box R$ term added to the action of the form $R+R^N$. Influence of $R \Box R$ term to the known solutions of modified gravity is described. We show that in particular case of $N=3$ these two non-Einstein terms are equally important on power-law solutions. These solutions and their stability have been studied using dynamical system approach. Some results for the case of $N \ne 3$ (including stability of de Sitter solution in the theory under investigation) have been found using other methods