Showing papers on "Big Rip published in 1991"

Ultrarelativistic Bose-Einstein condensation in the Einstein universe and energy conditions

Abstract: We study a relativistic boson gas of particles and antiparticles in the Einstein universe at high temperatures and densities. We obtain the thermodynamic potential of the boson system in curved spacetime. The result shows that ultrarelativistic Bose-Einstein condensation can occur at very high densities in the Einstein universe, and by implication in the early stages of a dynamically changing universe. We show that in a slowly changing universe with a weak self-interaction this ultrarelativistic Bose-Einstein condensate would violate the strong energy condition which enters into the cosmological singularity theorems.

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.

Dark matter in the universe

Abstract: What is the quantity and composition of material in the Universe? This is one of the most fundamental questions we can ask about the Universe, and its answer bears on a number of important issues including the formation of structure in the Universe, and the ultimate fate and the earliest history of the Universe. Moreover, answering this question could lead to the discovery of new particles, as well as shedding light on the nature of the fundamental interactions. At present, only a partial answer is at hand: Most of the material in the Universe does not give off detectable radiation, i.e., is "dark;" the dark matter associated with bright galaxies contributes somewhere between 10% and 30% of the critical density (by comparison luminous matter contributes less than 1%); baryonic matter contributes between 1.1% and 12% of critical. The case for the spatially-flat, Einstein-de Sitter model is supported by three compelling theoretical arguments — structure formation, the temporal Copernican principle, and inflation — and by some observational data. If Ω is indeed unity — or even just significantly greater than 0.1 — then there is a strong case for a Universe comprised of nonbaryonic matter. There are three well motivated particle dark-matter candidates: an axion of mass 10−6 eV to 10−4 eV; a neutralino of mass 10 GeV to about 3 TeV; or a neutrino of mass 20 eV to 90 eV. All three posibilities can be tested by experiments that are either being planned or are underway.

Gravitational lensing by smooth inhomogeneities in the Universe

Abstract: The formalism developed by Gunn J. E. (1967, Astrophys. J., 150) on the gravitational lensing effect of smooth inhomogeneities in the Universe is reviewed and some new simple analytical expressions for the dispersion in amplification are derived. Two models for the distribution and evolution of density fluctuations are considered, namely a universe dominated by cold dark matter, and a scale-free universe with an initial density fluctuation spectrum corresponding to white noise.

Anisotropy dissipation in quantum cosmology.

Abstract: We study the issue of decoherence and dissipation in the wave function of the Universe for a Bianchi type-I universe with classical and quantum matter. We obtain a coarse-grained description by tracing over the matter degrees of freedom. Provided that for small universes the wave function of the universe is concentrated on a neighborhood of the isotropic configuration, then the coarse-grained density matrix of the universe will show an even more marked peak around isotropy for large universes. In this sense we can say that, while decoherence makes the reduced density matrix of the universe diagonal, dissipation causes the universe to be isotropic with a high probability for large radii.

Gravity-wave insights to Bianchi type-IX universes.

Abstract: Every Bianchi type-IX universe can be interpreted as a closed Friedmann universe on which is superimposed circularly polarized gravitational waves with the longest wavelength that will fit into a closed universe. In this paper, I give a new derivation of this result based on the concept of homogeneous tensor fields on the three-sphere. Every homogeneous symmetric traceless tensor field is shown to be a longest-wavelength three-sphere harmonic. Contrary to previous authors, I show that the wavelength of these gravitational waves is one-half the circumference of the universe. In order to maintain homogeneity, the gravitational waves must all have the same polarization. There are five longest-wavelength modes for each polarization. This interpretation is an {ital exact} description that is valid for every Bianchi type-IX universe---it is in no way limited to first-order perturbations of a Friedmann universe.

Nucleation of a universe in (2+1)-dimensional gravity with a negative cosmological constant.

Abstract: We investigate the nucleation of a universe in a (2+1)-dimensional gravity model with a negative cosmological constant. There are a variety of universes born from nothing by quantum tunneling. Utilizing the powerful technique of hyperbolic geometry, we explicitly construct three-manifolds which describe the nucleation of a higher-genus universe. We calculate the wave function of the universe in the WKB approximation.

The Kalam Cosmological Argument and the Hypothesis of a Quiescent Universe

Abstract: Although Stewart Goetz is correct that the kalam cosmological argument does not rule out the possibility that the finite temporal series of past events was initiated by a distinct personal agent in an eternal, quiescent universe, this does not appreciably mitigate the force of the argument for theism. For the atheist will hardly be inclined to admit the existence of an eternal, changeless, personal Prime Mover rather than the fact that the universe began to exist. Moreover, since the existence of a quiescent universe is physically impossible, it could have existed only by means of a miracle so stupendous that it involved the suspension of all the laws of nature. The Prime Mover would thus have to be Lord over all the universe, a conclusion which even a successful kalam argument for the beginning of the universe does not attain.

Can a man-made Universe be created by quantum tunneling without an initial singularity?

Abstract: Essentially all modern particle theories suggest the possible existence of a false vacuum state – a metastable state with an energy density that cannot be lowered except by means of a very slow phase transition. Inflationary cosmology makes use of such a state to drive the expansion of the big bang, allowing the entire observed universe to evolve from a very small initial mass. A sphere of false vacuum in our present universe, if larger than a certain critical mass, could inflate to form a new universe which would rapidly detach from its parent. A false vacuum bubble of this size, however, cannot be produced classsically unless an initial singularity is present from the outset. We therefore explore the possibility that a bubble of subcritical size, which classically would evolve to a maximum size and collapse, might instead tunnel through a barrier to produce a new universe. We estimate the tunneling rate using semiclassical quantum gravity, and discover some interesting ambiguities in the formalism.

Black-hole mergers and mass inflation in a bouncing universe

Abstract: THE idea that our expanding Universe was born in a 'bounce'—the re-expansion of a previously contracting universe—is an old cosmogonical hypothesis, and continues to resurface1–5 despite being overshadowed recently by hypotheses inspired by Guth's inflationary model. Here we show how recent developments in the physics of black-hole interiors force a major revision of our ideas of the final moments of a contracting universe, and remove a thermodynamic difficulty6,7 which had appeared to rule out any kind of bounce origin for our Universe. As the black hole formed by the collapse of a rotating star settles down, it absorbs part of the gravitational radiation emitted during the last moments of collapse. This radiation, strongly blue-shifted near the inner horizon, enormously increases the mass of the black hole's core. External observers cannot detect this mass, but it manifests itself dramatically when the black holes in a collapsing universe merge, a few minutes before the 'big crunch'. The mass of a rebounding universe is enormously inflated, and its specific entropy correspondingly reduced. This allows the expansion to begin from a state of relatively low disorder.

Kantowski-Sachs Cosmological Model and an Inflationary Era

Abstract: We study an empty Kantowski-Sachs universe model with cosmological constant Λ. The characteristic feature of an inflationary era is found. This universe model emerges from a pointlike, stringlike, membranelike singularity and develops toward an isotropic de Sitter universe.

Cosmological models that describe particle creation in the early universe and evolve into the ‘‘present‐day’’ universe

Abstract: The disappearance of the cosmological constant can be formally treated by means of similarity solutions of general relativity that evolve from a stage with conformal symmetry to a stage with homothetic symmetry. In this work it is assumed that in this transition the universe does not ‘‘lose its memory’’ completely, but it does ‘‘remember’’ some of its past characteristics. Specifically, it is assumed that the equation of state remains the same in both stages. Then, the most general, spherically symmetric, cosmological model compatible with this assumption is developed. It is shown that it can be used to describe, classically, the birth, near the center of a spherical domain (a ‘‘bubble’’) of positive density and pressure from an early universe with particle production. As a consequence of the difference of pressures, the bubble grows in size and mass and evolves into a present‐day FLRW universe with p=nρ. The model, therefore, is of relevance to the description of phase changes typical of inflationary universe models.

Torsion, Wormholes, and the Problem of the Cosmological Constant

Abstract: We consider the effect of torsion in the early universe to see if it is possible to explain the small value (if not zero) of the Cosmological constant at the present time. For the gauge-theoretic formulation of the Einstein-Cartan theory, we find a wormhole instanton solution which has a minimum (baby universe) radius of the Planck length. The basic difficulty with the wormhole approach is stressed. Finally, we give an explicit calculation from the expression for the evolution of the scale factor, which shows that the spin-dominated interaction term in the very early universe can cancel the Cosmological constant term at that epoch.

A Simple Model of the Universe without Singularities

Abstract: For a long time the “standard” model of the universe [1] was popular among physicists. However, this model presents certain difficulties, such as the flatness problem, the homogeneity and isotropy problem, and the horizon (or causality) problem [2]. To overcome them the standard model has been modified by the addition of inflationary scenarios, generally based on theories the validity of which has not yet been established [2,3]. However, even with these modifications one difficulty remains, the initial singularity, or big bang, of the standard model. It is hoped that the introduction of the quantum theory to describe the early behavior of the universe will remove this singularity, but so far no satisfactory way of doing this has been found.

Cosmological models of the Universe with reversal of time's arrow

Abstract: Cosmological models of the universe with reversal of time's arrow are considered. Formulations are given of the hypothesis of Cosmological CPT symmetry suggested earlier by the writer, and of the hypothesis of an open model with many sheets, with negative spatial curvature, and with possible violation of CTT symmetry by an invariant combined charge. The statistical paradox of reversibility is discussed for these models. The small dimensionless parameter δ2/α2, which characterizes the mean spatial curvature of the universe, is explained as the result of the evolution of the universe through many successive cycles of expansion and contraction.

Quark-hadron phase transition of the early Universe in the nontopological soliton model.

Abstract: Based on the nontopological soliton model, we discuss the quark-hadron phase transition of the early Universe in the hot big-bang model. We extract the parameter spaces which are relevant for small supercooling of the phase transition and the inhomogeneous nucleosynthesis. In particular, we explore the possibility that the early Universe is a cold universe filled with very dense and degenerate baryons. This cold universe is then reheated by the latent heat released from the quark-hadron phase transition. Without fine-tuning any parameter in the soliton model, we find that a typical potential in the model would dilute the dense baryons to an acceptable baryon asymmetry of the Universe. We also discuss the generation of density fluctuations during the phase transition in the cold universe.

The Best-Fit Universe

Abstract: Inflation provides very strong motivation for a flat Universe, Harrison-Zel’dovich (constant-curvature) density perturbations, and cold dark matter. However, there are a number of cosmological observations that conflict with the predictions of the simplest such model—one with zero cosmological constant. They include the age of the Universe, dynamical determinations of Ω, galaxy-number counts, and the apparent abundance of large-scale structure in the Universe. While the discrepancies are not yet serious enough to rule out the simplest and “most well motivated” model, the current data point to a “best-fit model” with the following parameters: ΩB Ω 0.03, QCDM Ω 0.17, ΩΛ Ω 0.8, and H o ≃ 70 km sec-1 Mpc-1, which improves significantly the concordance with observations. While there is no good reason to expect such a value for the cosmological constant, there is no physical principle that would rule such out.

The First Moments of the Universe: The Limits of Knowledge

Abstract: The contemporary astrophysicist today deals with questions that bear on the area known to traditional philosophy as &dquo;metaphysics.&dquo; Consequently, it is tempting to cross the threshold. One can allow oneself to be tempted by the idea that science is in a position to provide solutions to ancient and venerable metaphysical quests. One can even imagine, according to the wish expressed two thousand years ago by Epicurus, that it can calm our &dquo;metaphysical anxieties.&dquo;

Topics in particle physics and cosmology

Abstract: The Standard Model of particle physics, together with the Big Bang model of the early universe, constitute a framework which encompasses our current understanding of fundamental laws and beginning of our universe. Despite recent speculative trends, quantum field theory remains the theoretical tool of choice for investigating new physics either at high energy colliders, or in the early universe. In this dissertation, several field theoretic phenomena relevant to cosmology or particle physics are explored. A common theme in these explorations is the structure of the vacuum state in quantum field theory. First, we discuss first-order phase transitions in the early universe, in which the effective vacuum state of the universe shifts discontinuously as the temperature drops below some critical point. We find that the dynamics of a certain type of first-order phase transition can lead to production of primordial black holes, which could constitute the dark matter of our universe. Alternatively, supercooled first-order phase transitions may be the cause of an extended inflationary epoch in the early universe, which is generally regarded as necessary to solve several cosmological puzzles. We derive limits on such scenarios based on nearly model-independent percolation properties of the transition. We also study some nonperturbative aspectsmore » of the field theory vacuum. We show that non-topological solitons of a single fermion and Higgs fields can only exist in strongly coupled theories. In particular, we find that at the lowest fermionic excitations in the Standard Model are single fermions, and not bound states of fermion plugs Higgs. Finally, we investigate the intriguing behavior of instanton-induced cross sections. We discover Higgs-Higgs cross sections which increase exponentially with center of mass energy due to the presence of instanton solutions related to vacuum instability.« less

LEP Physics and the Early Universe

Abstract: I review the implications of the Z decay measurements at LEP (and SLC) for the early universe: (a) The Z width measurements, when combined with non-accelerator data rule out GeV range Dirac neutrinos, Majorana neutrinos, and sneutrinos as WIMP dark matter, rule out most explicit “cosmions”, and strongly constrain neutralinos When combined with data from other experiments, this implies that any WIMP is likely to be heavier than 10-20 GeV, and could easily be in the O(100 GeV-1 TeV) region; (b) The limit on Nv, when combined with big-bang nucleosynthesis (BBN) estimates, is consistent with both baryonic and non-baryonic dark matter and allows one to probe the consistency of BBN itself; (c) direct searches for WIMPs will probably require new detectors, sensitive to spin dependent interactions on possibly heavy targets, with event rates which could be 3-4 orders of magnitude smaller than those expected at present detectors; (d) limits on the Higgs and the top quark also have an impact on possible new processes in the early universe

Generating a hot big bang as a quantum fluctuation

Abstract: This brief review provides an overview of work exploring the hypothesis that the Universe “tunneled into being” as a quantum fluctuation. The basic picture is formulated within the context of a semiclassical description, in which the tunneling is interpreted as a gravitational instanton interpolating between some Planck-sized, metrically and topologically complicated, pretunneling entity and a rapidly expanding, topologically trivial, post-tunneling Universe. It is argued that the tunneling hypothesis may provide a simple explanation of gross observed features of the current Universe, such as the fact that it is big, nearly homogeneous and nearly flat, and that the massless particles contained therein seem characterized by a near-thermal distribution.

Perturbations of the One-Dimensional Structure in the Universe

Abstract: Linear perturbation theory on the background of a one-dimensional, non-linear inhomogeneous universe is considered. For an Einstein-de Sitter behaviour of the scale factor, the exact solution of the perturbation equations is given. The non-linear effect of the density inhomogeneity on the evolution of the peculiar velocity is written in form of a relation between the peculiar velocity and the density parameter Ω including the correction due to the density contrast δ0 of the Zeldovich solution. It is shown that the component parallel to the walls is characterized by Ω 0.6 (1 + δ0 )-0.57.

Thermal spectra in the early Universe

Abstract: It is shown that curvature effects, though completely negligible at the present day, can drastically alter thermal spectra at early times in some inflationary cosmological models. This can lead to a modification of the thermal history of the universe before the entropy production. The effects are only present for spatially curved k not=0 Robertson-Walker models, not for spatially flat k=0 models. If the entropy increase is >> 1087 and k not=0 then the energy density in the quantum fields is much greater, and hence the universe is much younger, at the onset of inflation than would be the case in a spatially flat universe. The effect is not relevant for chaotic inflation.

Catastrophe of Spacetime in the Early Universe

Abstract: According to the some interesting subjects in the process of evolution of the universe, it is discussed that the catastrophe nature of the Finsler spacetime and its cosmological meaning. It is shown that the nature of evolution of the universe could be attributed to the geometric feature of the Finsler spacetime.