P. J. E. Peebles

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.

THE COSMIC BARYON BUDGET

We present an estimate of the global budget of baryons in all states, with conservative estimates of the uncertainties, based on all relevant information we have been able to marshal. Most of the baryons today are still in the form of ionized gas, which contributes a mean density uncertain by a factor of about 4. Stars and their remnants are a relatively minor component, comprising for our best-guess plasma density only about 17% of the baryons, while populations contributing most of the blue starlight comprise less than 5%. The formation of galaxies and of stars within them appears to be a globally inefficient process. The sum over our budget, expressed as a fraction of the critical EinsteinEde Sitter density, is in the range with a best guess of (at Hubble constant 70 km 0.007 ( ) B ( 0.041, ) B D 0.021 s~1 Mpc~1). The central value agrees with the prediction from the theory of light element production and with measures of the density of intergalactic plasma at redshift z D 3. This apparent concordance suggests that we may be close to a complete survey of the major states of the baryons. Subject headings: cosmology: observations E elementary particles E galaxies: fundamental parameters

A Survey of galaxy redshifts. 5. The Two point position and velocity correlations

We describe the results of a study of the two-point correlations in the 14.5 m/sub B/ CfA redshift survey. We use the distance information provided by the redshifts to estimate the two-point spatial correlation function zeta(r) in a way that is designed to be unbiased by peculiar velocities. The results agree well with what has been found from the deeper angular distributions. In the fiducial model zeta(r) = (r/sub 0//r)/sup ..gamma../ with ..gamma.. = 1.77 we find r/sub 0/ = 5.4 +- 0.3 h/sup -1/ Mpc (H/sub 0/ = 100 h km s/sup -1/ Mpc/sup -1/). At r> or approx. =10 h/sup -1/ Mpc, zeta(r) seems to steepen and may in fact be negative at 20< or approx. =hr< or approx. =40 Mpc. In existing n-body simulations zeta(r) is poorly modeled by a power law, with more power on small scales and less power on large scales than the data. This confirms the visual impressions that the n-body clusters are too compact and the clusters too homogeneously distributed relative to the data.

The Cosmic Energy Inventory

We present an inventory of the cosmic mean densities of energy associated with all the known states of matter and radiation at the present epoch. The observational and theoretical bases for the inventory have become rich enough to allow estimates with observational support for the densities of energy in some 40 forms. The result is a global portrait of the effects of the physical processes of cosmic evolution.

Large scale background temperature and mass fluctuations due to scale invariant primeval perturbations

The large-scale anisotropy of the microwave background and the large-scale fluctuations in the mass distribution are discussed under the assumptions that the universe is dominated by very massive weakly interacting particles and that the primeval density fluctuations were adiabatic with the scale-invariant spectrum proportional wavenumber. This model yields a characteristic mass comparable to that of a large galaxy independent of the particle mass, m/sub x/, if m/sub x/> or approx. =1 keV. The expected background fluctuations are well below present observational limits.

The Void Phenomenon

Advances in theoretical ideas on how galaxies formed have not been strongly influenced by the advances in observations of what might be in the voids between the concentrations of ordinary optically selected galaxies. The theory and observations are maturing, and the search for a reconciliation offers a promising opportunity to improve our understanding of cosmic evolution. I comment on the development of this situation and present an update of a nearest neighbor measure of the void phenomenon that may be of use in evaluating theories of galaxy formation.

Tests of Cosmological Models Constrained by Inflation

The inflationary scenario requires that the universe have negligible curvature along constant-density surfaces. In the Friedmann-Lemaitre cosmology that leaves us with two free parameters, Hubbles constant H/sub 0/ and the density parameter ..cap omega../sub 0/ (or, equivalently, the cosmological constant ..lambda..). I discuss here tests of this set of models from local and high-redshift observations. The data agree reasonably well with ..cap omega../sub 0/approx.0.2.

Interacting Dark Matter and Dark Energy

We discuss models for the cosmological dark sector in which the energy density of a scalar field approximates Einsteins cosmological constant and the scalar field value determines the dark matter particle mass by a Yukawa coupling. A model with one dark matter family can be adjusted so the observational constraints on the cosmological parameters are close to but different from what is predicted by the ΛCDM model. This may be a useful aid to judging how tightly the cosmological parameters are constrained by the new generation of cosmological tests that depend on the theory of structure formation. In a model with two families of dark matter particles the scalar field may be locked to near zero mass for one family. This can suppress the long-range scalar force in the dark sector and eliminate evolution of the effective cosmological constant and the mass of the nonrelativistic dark matter particles, making the model close to ΛCDM, until the particle number density becomes low enough to allow the scalar field to evolve. This is a useful example of the possibility for complexity in the dark sector.

The Time Evolution of Bias

We study the evolution of the bias factor b and the mass-galaxy correlation coefficient r in a simple analytic model for galaxy formation and the gravitational growth of clustering. The model shows that b and r can be strongly time dependent but tend to approach unity even if galaxy formation never ends as the gravitational growth of clustering debiases the older galaxies. The presence of random fluctuations in the sites of galaxy formation relative to the mass distribution can cause large and rapidly falling bias values at high redshift.

Fluid Dark Matter

Dark matter modeled as a classical scalar field that interacts only with gravity and with itself by a potential that is close to quartic at large field values and approaches a quadratic form when the field is small would be gravitationally produced by inflation and, at the present epoch, could act like an ideal fluid with pressure that is a function only of the mass density. This could have observationally interesting effects on the core radii and solid-body rotation of dark matter halos and on the low-mass end of the primeval mass fluctuation power spectrum.