Bharat Ratra

The Cosmological constant and dark energy

Physics invites the idea that space contains energy whose gravitational effect approximates that of Einsteins cosmological constant, Lambda: nowadays the concept is termed dark energy or quintessence. Physics also suggests the dark energy could be dynamical, allowing the arguably appealing picture that the dark energy density is evolving to its natural value, zero, and is small now because the expanding universe is old. This alleviates the classical problem of the curious energy scale of order a millielectronvolt associated with a constant Lambda. Dark energy may have been detected by recent advances in the cosmological tests. The tests establish a good scientific case for the context, in the relativistic Friedmann-Lemaitre model, including the gravitational inverse square law applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one quarter of the critical Einstein-de Sitter value. The case for detection of dark energy is serious but not yet as convincing: we await more checks that may come out of work in progress. Planned observations might be capable of detecting evolution of the dark energy density: a positive result would be a considerable stimulus to attempts to understand the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

Hubble parameter measurement constraints on the cosmological deceleration-acceleration transition redshift

We compile a list of 28 independent measurements of the Hubble parameter between redshifts 0.07 ≤ z ≤ 2.3 and use this to place constraints on model parameters of constant and time-evolving dark energy cosmologies. These H(z) measurements by themselves require a currently accelerating cosmological expansion at about, or better than, 3σ confidence. The mean and standard deviation of the six best-fit model deceleration-acceleration transition redshifts (for the three cosmological models and two Hubble constant priors we consider) are z da = 0.74 ± 0.05, in good agreement with the recent Busca et al. determination of z da = 0.82 ± 0.08 based on 11 H(z) measurements between redshifts 0.2 ≤ z ≤ 2.3, almost entirely from baryon-acoustic-oscillation-like data.

Cosmological Constraints from Hubble Parameter versus Redshift Data

We use the Simon et al. determination of the redshift dependence of the Hubble parameter to constrain cosmological parameters in three dark energy cosmological models. We consider the standard ΛCDM model, the XCDM parameterization of the dark energy equation of state, and a slowly rolling dark energy scalar field with an inverse power-law potential. The constraints are restrictive, consistent with those derived from Type Ia supernova redshift-magnitude data, and complement those from galaxy cluster gas mass fraction versus redshift data.

Supernova Ia Constraints on a Time-variable Cosmological “Constant”

The energy density of a scalar field with potential V() ∝ -α, α > 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 supernova 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 noninformative prior for the density parameter.

Non‐Gaussian Error Distribution of Hubble Constant Measurements

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HUBBLE PARAMETER MEASUREMENT CONSTRAINTS ON THE REDSHIFT OF THE DECELERATION–ACCELERATION TRANSITION, DYNAMICAL DARK ENERGY, AND SPACE CURVATURE

H_{0}

The Beginning and Evolution of the Universe

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Cosmological Constraints from Compact Radio Source Angular Size versus Redshift Data

We compile an updated list of 38 measurements of the Hubble parameter

Hubble parameter measurement constraints on dark energy

H(z)

Faraday rotation of the Cosmic Microwave Background polarization by a stochastic magnetic field

between redshifts