Showing papers on "Cosmology published in 1994"

Cosmic strings

Abstract: The topic of cosmic strings provides a bridge between the physics of the very small and the very large They are predicted by some unified theories of particle interactions If they exist, they may help to explain some of the largest-scale structures seen in the Universe today They are `topological defects' that may have been formed at phase transitions in the very early history of the Universe, analogous to those found in some condensed-matter systems --- vortex lines in liquid helium, flux tubes in type-II superconductors, or disclination lines in liquid crystals In this review, we describe what they are, why they have been hypothesized and what their cosmological implications would be The relevant background from the standard models of particle physics and cosmology is described in section 1 In section 2, we review the idea of symmetry breaking in field theories, and show how the defects formed are constrained by the topology of the manifold of degenerate vacuum states We also discuss the different types of cosmic strings that can appear in different field theories Section 3 is devoted to the dynamics of cosmic strings, and section 4 to their interaction with other fields The formation and evolution of cosmic strings in the early Universe is the subject of section 5, while section 6 deals with their observational implications Finally, the present status of the theory is reviewed in section 7

False vacuum inflation with Einstein gravity

Abstract: We present a detailed investigation of chaotic inflation models which feature two scalar fields such that one field (the inflaton) rolls while the other is trapped in a false vacuum state. The false vacuum becomes unstable when the magnitude of the inflaton field falls below some critical value, and a first or second order transition to the true vacuum ensues. Particular attention is paid to the case termed ``hybrid inflation'' by Linde, where the false vacuum energy density dominates so that the phase transition signals the end of inflation. We focus mostly on the case of a second order transition, but treat also the first order case and discuss bubble production in that context for the first time. False-vacuum-dominated inflation is dramatically different from the usual true vacuum case, both in its cosmology and in its relation to particle physics. The spectral index of the adiabatic density perturbation originating during inflation can be indistinguishable from 1, or it can be up to ten percent or so higher. The energy scale at the end of inflation can be many orders of magnitude less than the value ${10}^{16}$ GeV, which is ususal in the true vacuum case. Reheating occurs promptly at the end of inflation. Cosmic strings or other topological defects are almost inevitably produced at the end of inflation, and if the inflationary energy scale is near its upper limit they contribute significantly to large scale structure formation and the cosmic microwave background anisotropy.Turning to particle physics, false vacuum inflaton occurs with the inflaton field far below the Planck scale and is therefore somewhat easier to implement in the context of supergravity than true vacuum chaotic inflation. The smallness of the inflaton mass compared with the inflationary Hubble parameter still presents a difficulty for generic supergravity theories. Remarkably, however, the difficulty can be avoided in a natural way for a class of supergravity models that follow from orbifold compactification of superstrings. This opens up the prospect of a truly realistic superstring-derived theory of inflation. One possibility, which we show to be viable at least in the context of global supersymmetry, is that the Peccei-Quinn symmetry is responsible for the false vacuum.

The Cosmon model for an asymptotically vanishing time dependent cosmological 'constant'

Abstract: We investigate the coupled system of gravity and a scalar with exponential potential. The energy momentum tensor of the scalar field induces a time-dependent cosmological “constant”. This adjusts itself dynamically to become in the “late” universe (including today) proportional to the energy density of matter and radiation. Possible consequences for the present cosmology are shortly discussed. We also address the question of naturalness of the cosmon model. Whenever cosmology encounters potential difficulties in the description of the present universe cosmologists revive the discussion about the cosmological constant [1]. The discrepancy between the critical energy density expected from inflationary cosmology and lower dynamical estimates of this density has been attributed to the cosmological constant [2]. The discussion also pertains to the age of the universe [2] and the formation of structure [3]. In fact, a cosmological constant λ of the order of today’s critical energy density in the universe (λ ≈ (2 · 10eV )) strongly affects the present universe without altering the successful predictions of the hot big bang model at early stages of the evolution of the universe. Despite many attempts [4] we have at present no satisfactory understanding why λ should be much smaller than typical energy scales of the standard model or even the Planck mass Mp. For a time-independent cosmological constant it seems even harder to explain why it should be of the order of the present energy density. The latter depends on the age of the universe rather than on fundamental constants. It looks then not very natural that a constant λ should have a value which equals the energy density just at a time within the present cosmological epoch. In this work we consider a model where the cosmological “constant” varies with time such that the asymptotic solution for late times is characterized by a constant ratio λ(t)/ρ(t)[5], [6]. We discuss consequences for present cosmology and various alternatives how “early cosmology” could have made a transition to this type of “late cosmology”. We also briefly address the question of naturalness of an asymptotically vanishing cosmological “constant”. We start from the field equations for a scalar field φ coupled to gravity in a homogenous and isotropic universe (with k = 0 and H the Hubble parameter) φ+ 3Hφ+ ∂V ∂φ = q (1) ρ+ 3H(ρ+ p) + qφ = 0 (2) H = 1 6M2 (

Global Aspects in Gravitation and Cosmology

Abstract: The world of particle physics exploring the atom the structure of the atom the extraterrestrials the cosmic rain the challenge of the big machines the particle explosion colliders and image chambers from charm to top to the limits particles at work.

The Three-Point Correlation Function of the Cosmic Microwave Background in Inflationary Models

Abstract: We analyze the temperature three–point correlation function and the skewness of the Cosmic Microwave Background (CMB), providing general relations in terms of multipole coefficients. We then focus on applications to large angular s anisotropies, such as those measured by the COBE DMR, calculating the contribution to these quantities from primordial, inflation generated, scalar perturbations, via the Sachs–Wolfe effect. Using the techniques of stochastic inflation we are able to provide a universal expression for the ensemble averaged three–point function and for the corresponding skewness, which accounts for all primordial second–order effects. These general expressions would moreover apply to any situation where the bispectrum of the primordial gravitational potential has a hierarchical form. Our results are then specialized to a number of relevant models: power–law inflation driven by an exponential potential, chaotic inflation with a quartic and quadratic potential and a particular case of hybrid inflation. In all these cases non–Gaussian effects are small: as an example, the mean skewness is much smaller than the cosmic rms skewness implied by a Gaussian temperature fluctuation field.

Gravitation and spacetime.

Abstract: Now more than ever, Gravitation and Spacetime, Second Edition, by Hans C. Ohanian and new coauthor Remo Ruffini, deserves John Wheeler's praise as "the best book on the market today of 500 pages or less on gravitation and general relativity." Gravitation and Spacetime has been thoroughly updated with the most exciting finds and hottest theoretical topics in general relativity and cosmology. Highlights of the revision include the rise and fall of the fifth force, principles and applications of gravitational lensing, COBE's spectacular confirmation of the blackbody spectrum of the cosmic thermal radiation, theories of dark matter and inflation, and the early universe as a testing ground for particle physicists' unification theories, and much, much more. The ideal choice for a graduate-level introduction to general relativity, Gravitation and Spacetime is also suitable for an advanced undergaduate course.

Deuterium abundance and background radiation temperature in high-redshift primordial clouds

Abstract: Measurements of the deuterium abundance in the early Universe provide a sensitive test of the 'standard' Big Bang cosmology. The probable detection of deuterium absorption by a gas cloud between us and a distant quasar suggests an abundance much greater than estimated from observations in the Milky Way, and consistent with the amount of presently observed luminous matter comprising all the baryons in the Universe. The same spectra imply a cosmic background temperature for the early Universe which is consistent with our expectations from standard Big Bang cosmology.

Dynamics of Cosmic Flows

Abstract: The editors suggested a review entitled "Are There Large-Scale Motions in the Universe?". The answer is "yes," in the sense that the interpretation of the data as motions is the simplest model, so far consistent with all other available data under the current "standard model" of physical cosmology. I review tests that could have ended up falsifying this model and failed, but the scope of this review is much extended as the field has developed far beyond the question of existence of motions. With the motions being accepted as a working hypothesis, the study of large-scale dynamics is becoming a mature scientific field where observation and theory are confronted in a quantitative way. It is this area of major activity in cosmology that is addressed here. I make no attempt to provide a complete reference list, nor do I try to achieve a balanced discussion of all the issues of relevance and authors involved. My goal is to provide a critical account of some of the issues in this field that I find important, with emphasis on theoretical implications. In many cases I quote only a recent paper automatically implying "and references therein." The reader is referred to a comprehensive, observation-oriented review of large­ scale motions in historical perspective by Burstein ( 1990b), a detailed review of distance indicators in a collection of essays by Jacoby et al ( 1992), and to Principles of Physical Cosmology by Peebles ( 1993). The current phase of the field was seeded by two major developments. One was the confirmation of the dipole moment in the Cosmic Microwave Back­ ground (CMB) (Corey & Wilkinson 1976, Smoot et al 1977), indicating via Doppler shift that the Local Group of galaxies (LG) is moving at rv600 Ian S-1 relative to the cosmological frame defined by the CMB. The other was the in­ vention of methods for inferring distances independent of redshifts based on intrinsic relations between galaxy quantities (Section 3; Tully & Fisher 1977,

Power spectrum constraints from spectral distortions in the cosmic microwave background

Abstract: Using recent experimental limits on chemical potential distortions from Cosmic Background Explorer (COBE) Far Infrared Astronomy Satellite (FIRAS), and the large lever-arm spanning the damping of sub-Jeans scale fluctuations to the COBE DMR fluctuations, we set a constraint on the slope of the primordial power spectrum n. It is possible to analytically calculate the contribution over the full range of scales and redshifts, correctly taking into account fluctuation growth and damping as well as thermalization processes. Assuming conservatively that mu is less than 1.76 x 10(exp -4), we find that the 95% upper limit on n is only weakly dependent on other cosmological parameters, e.g., n is less than 1.60 (h=0.5) and n is less than 1.63 (h=1.0) for Omega(sub 0) = 1, with marginally weaker constraints for Omega(sub 0) is less than 1 in a flat model with a cosmological constant.

Dilaton production in string cosmology.

Abstract: We consider the coupled evolution of density, (scalar) metric and dilaton perturbations in the transition from a “stringy” phase of growing curvature and gravitational coupling to the standard radiation-dominated cosmology. We show that dilaton production, with a spectrum tilted towards large frequencies, emerges as a general property of this scenario. We discuss the frame-independence of the dilaton spectrum and of the inflationary properties of the metric background by using, as model of source, a pressureless gas of weakly interacting strings, which is shown to provide an approximate but consistent solution to the full system of background equations and string equations of motion. We combine various cosmological bounds on a growing dilaton spectrum with the bound on the dilaton mass obtained from tests of the equivalence principle, and we find allowed windows compatible with a universe presently dominated by a relic background of dilatonic dark matter.

Fluctuations of the gravitational constant in the inflationary Brans-Dicke cosmology.

Abstract: According to the Brans--Dicke theory, the value of the gravitational constant G which we measure at present is determined by the value of the Brans--Dicke scalar field \phi at the end of inflation. However, due to quantum fluctuations of the scalar fields produced during inflation, the gravitational constant G(\phi) may take different values in different exponentially large parts of the Universe. We investigate the probability distribution P_p to find a domain of a given volume with a given value of the gravitational constant G at a given time. The investigation is performed for a wide class of effective potentials of the scalar field \sigma which drives inflation, and with two different time parametrizations. Our work is based on the analytical study of the diffusion equations for P_p, as well as on the computer simulation of stochastic processes in the inflationary Universe. We have found that in some inflationary models the probability distribution P_p rapidly approaches a stationary regime. The shape of the distribution depends, however, on the choice of the time parametrization. In some other models the distribution P_p is not stationary. An interpretation of our results and of all ambiguities involved is outlined, and a possible role of anthropic considerations in determination of the gravitational constant is discussed.

Origin of Quasar Progenitors from the Collapse of Low-Spin Cosmological Perturbations

Abstract: We show that seeds for quasar black holes could have originated from the initial cosmological collapse of overdense regions with unusually small rotation. The gas in these rare regions collapses into a compact disk that shrinks on a short viscous time scale. Using an analytical model, we calculate the low-spin tail of the probability distribution of angular momenta for objects that collapse out of a Gaussian random field of initial density perturbations. The population of low-spin systems is significant for any viable power spectrum of primordial density perturbations. Most objects form just above the cosmological Jeans mass (\sim 10^5 M_sun) at high redshifts z>10. In the standard cold dark matter cosmology, the comoving density of 10^{6-7} M_sun objects with viscous evolution times shorter than 10^{6-7} years is about 10^{-3} (h/0.5)^3 Mpc^{-3}, comparable to the local density of bright galaxies. The seed black holes tend to reside within larger mass systems that collapse later and supply the gas needed for the bright quasar activity.

Measurement of the microwave background temperature at a redshift of 1.776

Abstract: HOT Big Bang cosmology predicts that the temperature of the cosmic microwave background radiation will increase linearly with increasing redshift to early in the history of the Universe The local background temperature (27 K) is known very accurately from direct measurements1–3, but other techniques must be used to estimate it at non-zero redshifts One way is to determine the excitation of atomic transitions in absorbing clouds along the lines-of-sight to distant quasars4 When the transitions are in equilibrium with the microwave background radiation, the radiation will populate the fine-structure levels of the ground states of certain atoms, and the relative populations of the levels can be used to calculate its temperature Here we report the detection of absorption from the first fine-structure level of neutral carbon atoms in a cloud at a redshift of 1776, towards the quasar Q1331 + 170 The population ratio yields a temperature of 74 ± 08 K, assuming that no other significant sources of excitation are present This agrees with the theoretical prediction of 758 K

Cosmological models with constant deceleration parameter in Brans-Dicke theory

Abstract: A detailed study of cosmological models with constant deceleration parameterq is undertaken in the framework of Brans-Dicke theory. These models are divided into two categories: (i) singular models with expansion driven by big-bang impulse, (ii) non-singlar models with expansion driven by creation of matter particles. Prigogine's hypothesis of creation of matter out of gravitational energy is analysed and extended to BD cosmology. To accommodate the creation of new particles, the universe is regarded as an open thermodynamical system and the energy conservation equation is modified with the incorporation of a creation pressure termp c in the energy-momentum tensor $$\tilde T_{ab} $$ . The exact solutions of the field equations of BD theory with $$\tilde T_{ab} $$ are obtained using the power law relationΦ=KR α, which leads to models with constantq. The behaviour of the solutions is investigated for different range of values ofa. The role played by the BD scalar fieldΦ and creation of matter particles in the expansion of the universe is investigated. It is found that one particular model with constantq has exponential expansion.

Production of Heavy Elements in Inhomogeneous Cosmologies

Abstract: The incorporation of quantum chromodynamics into big bang cosmology predicts that the universe shifted from a quark-gluon plasma to a meson gas when it was roughly 10 μs old. If this change is due to a first order phase transition, baryon density inhomogeneities can be produced and survive until nucleosynthesis. At that point they can affect the primordial abundances resulting from element formation in the early universe. In this article we undertake a detailed study of one of the proposed signatures of inhomogeneous nucleosynthesis — the production of heavy elements by neutron capture — and find that heavy elements are produced, but the level of production is probably too low to be observable.

Large-scale structure after COBE: Peculiar velocities and correlations of cold dark matter halos

Abstract: Large N-body simulations on parallel supercomputers allow one to simultaneously investigate large-scale structure and the formation of galactic halos with unprecedented resolution. Our study shows that the masses as well as the spatial distribution of halos on scales of tens of megaparsecs in a cold dark matter (CDM) universe with the spectrum normalized to the anisotropies detected by Cosmic Background Explorer (COBE) is compatible with the observations. We also show that the average value of the relative pairwise velocity dispersion sigma(sub v) - used as a principal argument against COBE-normalized CDM models-is significantly lower for halos than for individual particles. When the observational methods of extracting sigma(sub v) are applied to the redshift catalogs obtained from the numerical experiments, estimates differ significantly between different observation-sized samples and overlap observational estimates obtained following the same procedure.

Modern Cosmology and the Dark Matter Problem

Abstract: Preface Part I. Dark Matter in Astronomy and Cosmology: 1. Dark matter in galaxies 2. Dark matter in clusters of galaxies 3. Dark matter in intergalactic space 4. The identity of the dark matter Part II. Ionisation Problems in Astronomy and Cosmology: 5. Diffuse ionisation in the Milky Way 6. Diffuse ionisation in spiral galaxies 7. The intergalactic flux of hydrogen-ionising photons Part III. Neutrino Decay and Ionisation in the Universe: 8. The radiative decay of massive neutrinos 9. Neutrino decay and the ionisation of the Milky Way 10. Neutrino decay and the ionisation of spiral galaxies 11. The intergalactic flux of ionising decay photons 12. The reionisation of the Universe Part IV. Observational Searches for the Neutrino Decay Line: 13. Observational searches for the neutrino decay line References Subject index.

Astrophysical deductions from the quasi-steady-state cosmology

Abstract: The numerical consequences of the theory developed in two previous papers are examined in more detail. In particular, it is shown that the radio source count data can be explained to high accuracy, including the steeper-than-Euclidean slope of the count distribution at its high-flux end. By specifying the parameters of the theory, we obtain reasonable values for the Hubble constant and the ages of the globular clusters, a maximum redshift for any object observed from the present oscillatory cycle of the Universe, a minimum average density of ∼2×10 −27 g cm −3 for clusters of galaxies, and a temperature close to 2.7 K for the microwave background. The theory predicts a near-blackbody spectrum for the microwave background, and also explains the observed anisotropy

The cosmic no-hair theorem and the non-linear stability of homogeneous Newtonian cosmological models

Abstract: The validity of the cosmic no-hair theorem is investigated in the context of Newtonian cosmology with a perfect fluid matter model and a positive cosmological constant. It is shown that if the initial data, for an expanding cosmological model of this type, is subjected to a small perturbation then the corresponding solution exists globally in the future and the perturbation decays in a way which can be described precisely. It is emphasized that no linearization of the equations or special symmetry assumptions are needed. The result can also be interpreted as a proof of the non-linear stability of the homogeneous models. In order to prove the theorem we write the general solution as the sum of a homogeneous background and a perturbation. As a by-product of the analysis it is found that there is an invariant sense in which an inhomogeneous model can be regarded as a perturbation of a unique homogeneous model. A method is given for associating uniquely to each Newtonian cosmological model with compact spatial sections a spatially homogeneous model which incorporates its large-scale dynamics. This procedure appears very naturally in the Newton--Cartan theory which we take as the starting point for Newtonian cosmology.

Dirac's Cosmology with Varying Cosmological Constant

Abstract: The phenomenological approach to investigating the decay of the vacuum energy density is generalized in the spirit of Dirac's large number hypothesis. Different arguments are outlined to justify an universal dependence of the type λ=ΒH 2 (Β is a pure number andH is the Hubble parameter). Such time-varying A does change the predictions of the earlier Dirac's cosmology. The deceleration parameter (q 0=(2−Β)/(1+Β)) and the age parameter (H 0t0=(1+Β)/3) are now compatible with the observations. The model also allows a “power-law” inflationary phase and a rate of matter creation smaller than the one present in the steady state universe.

Astrophysical applications of quantum corrections to the equation of state of a plasma.

Abstract: The quantum electrodynamic correction to the equation of state of a plasma at finite temperature is applied to the areas of solar physics and cosmology. A previously neglected, purely quantum term in the correction is found to change the equation of state in the solar core by -0.37%, which is roughly estimated to decrease the calculated high energy neutrino flux by about 2.2%. We also show that a previous calculation of the effect of this correction on big bang nucleosynthesis is incomplete, and we estimate the correction to the primordial helium abundance Y to be Delta A= 1.4 x 10(exp -4). A physical explanation for the correction is found in terms of corrections to the dispersion relation of the electron, positron, and photon.

The Origin of the Universe

Abstract: There was immense excitement in the scientific community and among the general public when the COBE space probe sent back data that proved not only that the Big Bang had happened but also that it had happened at more or less exactly the time that astronomers had calculated. Barrow describes these finds and then goes on to explain how they allow us to reach back and shed light upon events at the dawn of time. What does it mean to say that the universe appeared out of nothing? Did it need a beginning, and will it ever end? Why do we think that most of the universe is invisible? The ideas that cosmologists are wrestling with are challenging and extraordinary: here they are explained with unfailing fluency.

MULTIDIMENSIONAL COSMOLOGY WITH m-COMPONENT PERFECT FLUID

Abstract: A cosmological model describing the evolution of n Einstein spaces (n>1) with m- component perfect-fluid matter is considered. When all spaces are Ricci-flat and for any α-th component the pressures in all spaces are proportional to the density: , the Einstein and Wheeler-DeWitt equations are integrated in the cases: (i) m=1, for all ; (ii) m>1, for some special sets of . For m=1 the quantum wormhole solutions are also obtained.

A two-fluid approximation for calculating the cosmic microwave background anisotropies

Abstract: We present a simplified treatment for calculating the cosmic microwave background anisotropy power spectrum in adiabatic models. It consists of solving for the evolution of a two-fluid model until the epoch of recombination and then integrating over the sources to obtain the cosmic microwave background (CMB) anisotropy power spectrum. The approximation is useful both for a physical understanding of CMB anisotropies as well as for a quantitative analysis of cosmological models. Comparison with exact calculations shows that the accuracy is typically 10%-20% over a large range of angles and cosmological models, including those with curvature and cosmological constant. Using this approximation we investigate the dependence of the CMB anisotropy on the cosmological parameters. We identify six dimensionless parameters that uniquely determine the anisotropy power spectrum within our approximation. CMB experiments on different angular scales could in principle provide information on all these parameters. In particular, mapping of the Doppler peaks would allow an independent determination of baryon mass density, matter mass density, and the Hubble constant.

Time symmetry and asymmetry in quantum mechanics and quantum cosmology.

Abstract: We investigate the origin of the arrow of time in quantum mechanics in the context of quantum cosmology. The ``Copenhagen'' quantum mechanics of measured subsystems incorporates a fundamental arrow of time. Extending discussions of Aharonov, Bergmann and Lebovitz, Griffiths, and others we investigate a generalized quantum mechanics for cosmology that utilizes both an initial and a final density matrix to give a time-neutral formulation without a fundamental arrow of time. Time asymmetries can arise for particular universes from differences between their initial and final conditions. Theories for both would be a goal of quantum cosmology. A special initial condition and a final condition of indifference would be sufficient to explain the observed time asymmetries of the universe. In this essay we ask under what circumstances a completely time symmetric universe, with T-symmetric initial and final condition, could be consistent with the time asymmetries of the limited domain of our experience. We discuss the approach to equilibrium, the electromagnetic system, and T-violation in the weak interactions in such universes. The decoherence of alternatives that is necessary for the prediction of probabilities in the quantum mechanics of closed systems is the origin of a fundamental limitation on quantum cosmologies with initial and final conditions. The initial and final density matrices cannot both be pure; if they are there is no decoherence and no prediction.