Coupled quintessence and vacuum decay

We discuss observational consequences of a class of cosmological models characterized by the dilution of pressureless matter attenuated with respect to the usual $a^{-3}$ scaling due to the decay of vacuum energy. We carry out a joint statistical analysis of observational data from the new \emph{gold} sample of 182 SNe Ia, recent estimates of the CMB shift parameter, and BAO measurements from the SDSS to show that such models favor the decay of vacuum only into the dark matter sector, and that the separately conserved baryons cannot be neglected. In order to explore ways to more fundamentally motivated models, we also derive a coupled scalar field version for this general class of vacuum decay scenarios.Comment: 6 pages, 3 figures, LaTe

Can Strong Gravitational Lensing Constrain Dark Energy?

We discuss the ratio of the angular diameter distances from the source to the lens, $D_{ds}$, and to the observer at present, $D_{s}$, for various dark energy models. It is well known that the difference of $D_s$s between the models is apparent and this quantity is used for the analysis of Type Ia supernovae. However we investigate the difference between the ratio of the angular diameter distances for a cosmological constant, $(D_{ds}/D_{s})^{\Lambda}$ and that for other dark energy models, $(D_{ds}/D_{s})^{\rm{other}}$ in this paper. It has been known that there is lens model degeneracy in using strong gravitational lensing. Thus, we investigate the model independent observable quantity, Einstein radius ($\theta_E$), which is proportional to both $D_{ds}/D_s$ and velocity dispersion squared, $\sigma_v^2$. $D_{ds}/D_s$ values depend on the parameters of each dark energy model individually. However, $(D_{ds}/D_s)^{\Lambda} - (D_{ds}/D_{s})^{\rm{other}}$ for the various dark energy models, is well within the error of $\sigma_v$ for most of the parameter spaces of the dark energy models. Thus, a single strong gravitational lensing by use of the Einstein radius may not be a proper method to investigate the property of dark energy. However, better understanding to the mass profile of clusters in the future or other methods related to arc statistics rather than the distances may be used for constraints on dark energy.Comment: 15 pages, 13 figures, Accepted in PR

Cosmology in a String-Dominated Universe

The string-dominated universe locally resembles an open universe, and fits dynamical measures of power spectra, cluster abundances, redshift distortions, lensing constraints, luminosity and angular diameter distance relations and microwave background observations. We show examples of networks which might give rise to recent string-domination without requiring any fine-tuned parameters. We discuss how future observations can distinguish this model from other cosmologies.Comment: 17 pages including 4 figures, of which one is in colo

Supernovae Ia Constraints on a Time-Variable Cosmological "Constant"

The energy density of a scalar field $\phi$ with potential $V(\phi) \propto \phi^{-\alpha}$, $\alpha > 0$, behaves like a time-variable cosmological constant that could contribute significantly to the present energy density. Predictions of this spatially-flat model are compared to recent Type Ia supernovae apparent magnitude versus redshift data. A large region of model parameter space is consistent with current observations. (These constraints are based on the exact scalar field model equations of motion, not on the widely used time-independent equation of state fluid approximation equations of motion.) We examine the consequences of also incorporating constraints from recent measurements of the Hubble parameter and the age of the universe in the constant and time-variable cosmological constant models. We also study the effect of using a non-informative prior for the density parameter.Comment: Accepted for publication in Ap

Black Hole Formation with an Interacting Vacuum Energy Density

We discuss the gravitational collapse of a spherically symmetric massive core of a star in which the fluid component is interacting with a growing vacuum energy density. The influence of the variable vacuum in the collapsing core is quantified by a phenomenological \beta-parameter as predicted by dimensional arguments and the renormalization group approach. For all reasonable values of this free parameter, we find that the vacuum energy density increases the collapsing time but it cannot prevent the formation of a singular point. However, the nature of the singularity depends on the values of \beta. In the radiation case, a trapped surface is formed for \beta<1/2 whereas for \beta>1/2, a naked singularity is developed. In general, the critical value is \beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of state of the fluid component.Comment: 9 pages, 8 figure

Cosmic Concordance and Quintessence

We present a comprehensive study of the observational constraints on spatially flat cosmological models containing a mixture of matter and quintessence --- a time varying, spatially inhomogeneous component of the energy density of the universe with negative pressure. Our study also includes the limiting case of a cosmological constant. Low red shift constraints include the Hubble parameter, baryon fraction, cluster abundance, age of the universe, bulk velocity and shape of the mass power spectrum; intermediate red shift constraints are due to type 1a supernovae, gravitational lensing, the Ly-a forest, and the evolution of large scale structure; high red shift constraints are based on cosmic microwave background temperature anisotropy. Mindful of systematic errors, we adopt a conservative approach in applying these constraints. We determine that quintessence models in which the matter density parameter is 0.2 \ls \Omega_m \ls 0.5 and the effective, density-averaged equation of state is -1 \le w \ls -0.2, are consistent with the most reliable, current low red shift and CMB observations at the $2\sigma$ level. Factoring in the constraint due to type 1a SNe, the range for the equation of state is reduced to -1 \le w \ls -0.4, where this range represents models consistent with each observational constraint at the 2$\sigma$ level or better (concordance analysis). A combined maximum likelihood analysis suggests a smaller range, -1 \le w \ls -0.6. We find that the best-fit and best-motivated quintessence models lie near $\Omega_m \approx 0.33$, $h \approx 0.65$, and spectral index $n_s=1$, with an effective equation of state $w \approx -0.65$ for ``tracker'' quintessence and $w=-1$ for ``creeper'' quintessence. (abstract shortened)Comment: revised to match ApJ version; 33 pages; 20 figures, 4 in color; uses emulateapj.st

Perturbation evolution in cosmologies with a decaying cosmological constant

Structure formation models with a cosmological constant are successful in explaining large-scale structure data, but are threatened by the magnitude-redshift relation for Type Ia supernovae. This has led to discussion of models where the cosmological `constant' decays with time, which might anyway be better motivated in a particle physics context. The simplest such models are based on scalar fields, and general covariance demands that a time-evolving scalar field also supports spatial perturbations. We consider the effect of such perturbations on the growth of adiabatic energy density perturbations in a cold dark matter component. We study two types of model, one based on an exponential potential for the scalar field and the other on a pseudo-Nambu Goldstone boson. For each potential, we study two different scenarios, one where the scalar field presently behaves as a decaying cosmological constant and one where it behaves as dust. The initial scalar field perturbations are fixed by the adiabatic condition, as expected from the inflationary cosmology, though in fact we show that the choice of initial condition is of little importance. Calculations are carried out in both the zero-shear (conformal newtonian) and uniform-curvature gauges. We find that both potentials allow models which can provide a successful alternative to cosmological constant models.Comment: 14 pages RevTeX file with three figures incorporated (uses RevTeX and epsf). Also available by e-mailing ARL, or by WWW at http://star-www.maps.susx.ac.uk/papers/lsstru_papers.html Revised version corrects an error in Eq10; results unchange

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 constraints from lensing statistics and supernovae on the cosmic equation of state

We investigate observational constraints from lensing statistics and high-z type Ia supernovae on flat cosmological models with nonrelativistic matter and an exotic fluid with equation of state, $p_x=(m/3 -1)\rho_x$. We show that agreement with both tests at the 68% confidence level is possible if the parameter $m$ is low ($m \lesssim 0.85$) and $0.24 \lesssim \Omega_{m0} \lesssim 0.38$ with lower values of $\Omega_{m0}$ corresponding to higher $m$. We find that a conventional cosmological constant model with $\Omega_{m0}\simeq 0.33$ is the best fit model of the combined likelihood.Comment: 7 pages, 4 postscript figures, revtex, submitted to Phys. Rev.

New Constraints from High Redshift Supernovae and Lensing Statistics upon Scalar Field Cosmologies

We explore the implications of gravitationally lensed QSOs and high-redshift SNe Ia observations for spatially flat cosmological models in which a classically evolving scalar field currently dominates the energy density of the Universe. We consider two representative scalar field potentials that give rise to effective decaying $\Lambda$ (``quintessence'') models: pseudo-Nambu-Goldstone bosons ($V(\phi)=M^4(1+\cos (\phi /f))$) and an inverse power-law potential ($V(\phi)=M^{4+\alpha}\phi ^{-\alpha}$). We show that a large region of parameter space is consistent with current data if $\Omega_{m0} > 0.15$. On the other hand, a higher lower bound for the matter density parameter suggested by large-scale galaxy flows, $\Omega_{m0} > 0.3$, considerably reduces the allowed parameter space, forcing the scalar field behavior to approach that of a cosmological constant.Comment: 6 pages, 2 figures, submitted to PR