Showing papers on "Cosmology published in 1991"

Dynamical effects of the cosmological constant.

Abstract: The possibility of measuring the density parameter OMEGA-0 and the cosmological constant lambda-0 = LAMBDA/(3H0(2)) using dynamical tests is explored in linear and non-linear theory. In linear theory we find that the rate of growth of the perturbations at the present epoch is approximated by f(z = 0) almost-equal-to-OMEGA-0(0.6) + 1/70 lambda-0(1 + 1/2 OMEGA-0). Therefore, dynamical tests such as infall around clusters and dipoles at the present epoch do not distinguish well between universes with and without a cosmological constant. At higher redshifts, the perturbations also depend mainly on the matter density at a particular epoch, f(z) almost-equal-to OMEGA-0.6(z), which has a strong dependence on lambda-0 at z almost-equal-to 0.5-2.0. Therefore, information on both parameters can be obtained by looking at clustering at different redshifts. In practice, however, the other observables also depend on the cosmology, and in some cases conspire to give a weak dependence on lambda-0. By using the non-linear spherical infall model for a family of Cold Dark Matter (CDM) power-spectra we also find that dynamics at z = 0 does not tell much about lambda-0. At higher redshifts there is unfortunately another conspiracy between conventional observables, which hides information about lambda-0. The final radius of a virialized cluster (relative to the turnaround radius) is approximated by R(f)/R(ta) almost-equal-to (1 - eta/2)/(2 - eta/2), where eta is the ratio of LAMBDA to the density at turn-around. Therefore a repulsive-LAMBDA gives a smaller final radius than a vanishing-LAMBDA.

Primordial nucleosynthesis redux

Abstract: The abundances of D, He-3, He-4, and Li-7, are presently recalculated within the framework of primordial nucleosynthesis in the standard hot big band model, in order to estimate the primordial abundances of the light elements. A comparison between theory and experiment demonstrates the consistency of standard model predictions; the baryon density parameter is constrained on the basis of a nucleon-to-photon ratio of 2.8-4.0. These bounds imply that the bulk of the baryons in the universe are dark, requiring that the universe be dominated by nonbaryonic matter.

Large-scale structure in the Universe

Abstract: A variety of observations constrain models of the origin of large-scale cosmic structures. Enough observational data have accumulated to constrain (and perhaps determine) the power spectrum of primordial density fluctuations over a very large range of scales, independently of the particular cosmogonical theory assumed. Observations in the near future should be able to weed out many such theories.

Reconstructing the primordial spectrum of fluctuations of the universe from the observed nonlinear clustering of galaxies

Abstract: It was discovered that the nonlinear evolution of the two point correlation function in N-body experiments of galaxy clustering with Omega = 1 appears to be described to good approximation by a simple general formula. The underlying form of the formula is physically motivated, but its detailed representation is obtained empirically by fitting to N-body experiments. In this paper, the formula is presented along with an inverse formula which converts a final, nonlinear correlation function into the initial linear correlation function. The inverse formula is applied to observational data from the CfA, IRAs, and APM galaxy surveys, and the initial spectrum of fluctuations of the universe, if Omega = 1.

The distortion of distant galaxy images by large-scale structure

Abstract: Inhomogeneity in the distribution of mass in the Universe on scales ≤ 100 Mpc can generate a coherent shear distortion or polarization of the images of background galaxies. This distortion may be measurable over patches of the sky up to a few square degrees in size. If this distortion is measured, or conversely, if its magnitude is limited, it should help us understand the degree to which luminosity traces the underlying mass over cosmological scales. A prescription is given for quantifying the galaxy distortion and a propagation equation for its evolution in an inhomogeneous universe is derived

Causal Friedmann-Robertson-Walker cosmology

Abstract: The evolution of a Friedmann-Robertson-Walker universe filled with a viscous simple fluid is analysed. At variance with other treatments the authors' approach complies with relativistic causality since dissipative signals travelling at superluminal speeds are forbidden. This is because use is made of the extended thermodynamics theory of irreversible processes instead of the conventional one. As a consequence some novel results arise. In particular, the initial de Sitter phase of the deflationary universe does not occur. Likewise, the generalized second law of thermodynamics is studied within this context.

Inflationary axion cosmology

Abstract: If Peccei-Quinn (PQ) symmetry is broken after inflation, the initial axion angle is a random variable on cosmological scales; based on this fact, estimates of the relic-axion mass density give too large a value if the axion mass is less than about 10 to the -6th eV. This bound can be evaded if the universe underwent inflation after PQ-symmetry breaking and if the observable universe happens to be a region where the initial axion angle was atypically small. Consideration of fluctuations induced during inflation severely constrains the latter alternative is shown.

The case for the relativistic hot Big Bang cosmology

Abstract: What has become the standard model in cosmology is described, and some highlights are presented of the now substantial range of evidence that most cosmologists believe convincingly establishes this model, the relativistic hot big bang cosmology. It is shown that this model has yielded a set of interpretations and successful predictions that substantially outnumber the elements used in devising the theory, with no well-established empirical contradictions. Brief speculations are made on how the open puzzles and work in progress might affect future developments in this field.

Precollapse Scale Invariance in Gravitational Instability

Abstract: We numerically and analytically investigate the transition to the very nonlinear regime during the gravitational collapse of collisionless matter in an Ω=1 expanding universe. It is found numerically that the formation of halos from isolated and smooth initial overdensities leads to the progressive establishment, complete at the collapse time, of a power-law density profile. The evolutions from various types of initial perturbations are shown to all produce such power laws. The slopes are different, but the scale invariance itself appears quite generic

Large-scale structure from wiggly cosmic strings.

Abstract: Recent simulations of the evolution of cosmic strings indicate the presence of small-scale structure on the strings We show that wakes produced by such wiggly cosmic strings can result in the efficient formation of large-scale structure and large streaming velocities in the Universe without significantly affecting the microwave-background isotropy We also argue that the motion of strings will lead to the generation of a primordial magnetic field The most promising version of this scenario appears to be the one in which the Universe is dominated by light neutrinos

Quasars and galaxy formation. I - The z greater than 4 objects

Abstract: The physical properties of the known quasars with z greater than 4 are examined with particular reference to theories of cosmic structure formation. A 'standard' massive accreting black hole model for quasars is used to calculate the masses, radiative efficiencies, and accretion rates of the observed objects. The masses, densities, cosmological overdensities, and sizes of the accretion fuel reservoirs associated with the quasar black holes are considered, and several indirect arguments are used to connect these quantities to the masses and overdensities of the quasar host objects. Finally, constraints on the epoch of formation of the host objects are discussed. The general conclusion is that the observed quasars with z greater than 4 suggest that the cosmic structure formation was already well advanced at z = 5, when the universe was a small fraction of its present age.

Quantum creation of topological defects during inflation.

Abstract: Circular loops of string and spherical domain-wall bubbles of radius equal to the horizon can spontaneously nucleate in de Sitter space. These objects are expanded by the subsequent inflation, and by the end of the inflationary era they have a spectrum of sizes extending well beyond the present Hubble length. Monopole-antimonopole pairs with an initial separation equal to the horizon can also be produced. The cosmological implications of these effects are briefly discussed.

Theism, atheism, and big bang cosmology

Abstract: Contemporary science presents us with the remarkable theory that the universe began to exist about fifteen billion years ago with a cataclysmic explosion called `the Big Bang'. The question of whether Big Bang cosmology supports theism or atheism has long been a matter of discussion among the general public and in popular science books, but has received scant attention from philosophers. This book sets out to fill this gap by means of a sustained debate between two philosophers, William Lane Craig and Quentin Smith, who defend opposing positions. Craig argues that the Big Bang that began the universe was created by God, while Smith argues that the Big Bang has no cause. The book consists of alternating chapters by Craig and Smith, with each chapter being either a criticism of a preceding chapter or being criticized by a subsequent chapter. Part One consists of Craig's arguments that the past is necessarily finite and that God created the Big Bang, and Smith's criticisms of these arguments. Part Two presents Smith's arguments that Big Bang cosmology is inconsistent with theism and Craig's criticisms of Smith's argument. The authors' arguments are based on Einstein's theory of relativity, and there is also a discussion of Stephen Hawking's new quantum cosmology.

Gauge-invariant Joining Conditions for Cosmological Perturbations

Abstract: Two independent gauge-invariant variables which are continuous at any transition where there is a discontinuous change in pressure are derived. This is valid on any scale and for a general background, subject only to the assumption that the contents of the universe are a perfect fluid. Separately, the continuous variables for a transition from one scalar field to another are shown. The physical significance of these variables are discussed and compared with other, previously suggested, joining conditions. The evolution of large-scale cosmological perturbations is examined, assuming that the pressure is a function only of energy density, together with these joining conditions. It is noted that in inflationary models it is not necessary to consider the evolution of the universe between the time a perturbation expands beyond the Hubble horizon, and the time it comes within the horizon again to predict the final amplitude of the fluctuations.

Non-Gaussian initial conditions in cosmological N-body simulations. II, Cold Dark Matter models

Abstract: Results are reported on N-body simulations of the large-scale structure of the Universe starting from non-Gaussian initial conditions in a cold dark matter cosmogony. Three multiplicative models are condidered where the peculiar gravitational potential is obtained by performing a non-linear transformation on a Gaussian random field: while keeping the standard form for the initial power spectrum, this procedure provides highly non-random phases. The resulting distributions in space and velocity are analysed by different statistical tests and compared with the evolution of a standard Gaussian model with the same initial amplitude.

Cosmology withG and Λ coupling scalars

Abstract: Cosmology with the gravitational and cosmological constants generalized as coupling scalars in Einstein's theory is considered. A general method of solving the field equations is given. Fifteen exact solutions for zero pressure models satisfyingG=G0(R/R0) n are given in the Appendix; they are briefly discussed.

Inflation and singularity prevention in a model for extended-object-dominated cosmology

Abstract: The authors present a geometric procedure which generates a class of cosmological models of deflationary type, i.e. models in which a primordial phase of accelerated expansion evolves smoothly towards the final decelerating state of standard cosmology. They also discuss the possibility that the initial de Sitter geometry, obtained in the context of these models, may be physically interpreted as the consequences of an early phase in which the contribution of finite-size objects becomes dominant.

Gravitational microlensing of high-redshift supernovae by compact objects

Abstract: An analysis of the effect of microlensing by a cosmologically dominant density of compact objects is performed, using high-redshift Type Ia supernovae (SN Ia's) as probes. The compact objects are modeled as a three-dimensional distribution of point masses, and Monte Carlo simulations are done to calculate the resulting amplification probability distributions for several column densities and cosmologies. By combining these distributions with the intrinsic SN Ia luminosity function and comparing with the results for a perfectly smooth universe, estimates are made of the number of supernovae that would need to be observed to confirm or rule out this lensing scenario. It is found that about 1000 SN Ia's with redshifts of z = 1 would be needed to perform this test, which is beyond what current searches can hope to accomplish. Observations of many fewer high-redshift supernovae, used merely as standard candles, appears a promising way of distinguishing between different cosmological models.

Some cosmological models with constant deceleration parameter

Abstract: Berman (1983) presented a law of variation of Hubble’s parameter that yields constant deceleration parameter models of the Universe. In this paper, Robertson-Walker models have been studied in Einstein’s theory with cosmological term and in cosmological theory based on Lyra’s geometry. Some simple models have been obtained. The necessary relations in each model have been derived, considering a perfect fluid.

Microwave background constraints on extended inflation voids

Abstract: Extended inflation ends via the nucleation of bubbles of true vacuum, and in principle the earliest bubbles to nucleate can be expanded by the last stages of inflation to astrophysically interesting sizes, giving large voids empty of any matter. Several authors have suggested a link with the observed bubbly large-scale structure in the Universe. We examine in detail the constraints on the bubble distribution from the microwave background observations, particularly COBE, for different types of dark matter.

Did the universe recombine

Abstract: The Zel'dovich-Sunyaev model-independent arguments for the existence of a neutral hydrogen phase is reviewed in light of new limits on the Compton y parameter from COBE. It is concluded that with baryon densities compatible with standard cosmological nucleosynthesis, the universe could have remained fully ionized throughout its history without producing a detectable spectral distortion. It is argued that it is unlikely that spectral observations of the cosmic microwave background will ever require the universe to have recombined for flat cosmologies.

Constraints on the thermal history of the universe from the cosmic microwave background spectrum

Abstract: We have carried out an extensive numerical analysis of distortions of the microwave background spectrum that can arise over a broad interval of cosmic times. The range of validity of existing analytic approximation formulae, holding for large and very small values of the Comptonization parameter y, has been assessed and the shape of distortions that can arise for intermediate values of y has been determined. From a comparison with existing spectral data we have derived upper limits to the amount of energy that could have been dissipated at each cosmic time. The effectiveness of future measurements in tightening such constraints is discussed

Gauge invariant perturbations in multi-component fluid cosmologies

Abstract: An alternative approach to the study of cosmological density perturbations in cosmology has been presented. Covariant and gauge invariant quantities were defined that characterize density inhomogeneities in an almost-uniform model universe and propagation equations were derived for these quantities in the case of a general perfect fluid. The author extends this work to the case of an imperfect fluid and derives generalizations of these equations. The author then applies the theory to a multi-component fluid, in particular to the case where the background is described by a spatially flat Friedmann-Lemaitre-Robertson-Walker universe model containing a mixture of non-interacting dust and radiation. Solutions are obtained for the comoving fractional density gradient, the energy flux and the relative velocity in the centre of mass frame which include new modes linking the density gradient to the vorticity.