Showing papers on "Particle horizon 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.

Hubble Spheres and Particle Horizons

Abstract: Isotropy of the cosmic microwave background radiation highlights the horizon problem. Analysis of the horizon problem and its inflationary solution requires that we study the properties of the Hubble sphere and distinguish between the Hubble surface and the particle horizon. All regions in the visible universe become causally connected when inflation increases the distance of the particle horizon by a factor of 3. If exponential inflation occurs because of a phase transition at temperature ∼ 10 15 GeV, the number of e-foldings to effect this threefold increase in the particle horizon distance is N ∼ 60

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.

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.

Inflation in curved model universes with noncritical density

Abstract: Non.flat Friedmann-Lemaitre models solving the horizon problem in which, prior to and during inflation, cumat~re, cosmological constant and radiation are taken into account, are constructed Tor present density parameter values in the range 0.360,s 1.5. The presentation is based on an exact solution 10 the Lemaitre equation containing the three abave.mentioned contributions. Then the influence of matter different from radiation is discussed. We also Sive an example For what happens when the expansion is anisotropic. The large-scale structure of the universe is successfully represented by a Robertson- Walker ( RW) spacetime filled with matter and radiation obeying Einstein's field equation. Such models, being based on the a priori assumptions of spatial homogeneity and isotropy, cannot explain these two fundamental properties which are well supported particularly by the observed isotropy of the 3 K background radiation. Even if one assumes homogeneity and isotropy of spacetime, an explanation of the corresponding properties of marter and radialion in terms of transport processes fails because the comoving radius ud of the particle horizon at the time of decoupling Id, the 'primeval particle horizon', is much smaller than the comoving radius U, of that part of the universe which became visible at decoupling, the 'visual horizon'. Elementary particle theories assume the existence of scalar fields in the very early phases of the universe. These scalar fields can simulate a cosmological constant ( 11. If these scalar fields decay long before decoupling, the horizon problem can be solved, as first pointed out by Guth (?I. The solution of the horizon problem and related problems make inflationary cosmology attractive. It has been claimed that inflation yields a very small spatial curvature. In fact most calculations in inflationary cosmology have been done with a flat RW metric. Also it has been predicted that this vanishing spatial curvature can be used to test inflation (3, 41. Ellis has shown, however, with a simple model that it is possible to solve the horizon problem in cosmological models with an inflationary phase and to get a non-negligible curvature today. In these models the present value of the density parameter n= ~p,,,,/3H' does not have to equal 1 to high accuracy, either. Below we present an improved model supporting the essential conclusions of Ellis. Our model takes into account the effects of radiation and curvature at all times. Moreover, we avoid the unphysical discontinuity of source terms at the onset of inflation used by Ellis for computational simplicity. In contrast to Ellis', our model yields only a lower bound for the expansion during the inflationary phase. If used as a background model for fluctuations of the scalar field, it can therefore be adjusted even if the fluctuation calculations lead to larger lower bounds for the amount of inflationary expansion.

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.

Particle creation, inflation, and cosmic isotropy.

Abstract: Within the framework of homogeneous models of the Universe, inflation provides the simplest explanation for the present cosmic isotropy, and a Bianchi type-IX (mixmaster) model is the least prejudiced guess we can make about the state of the Universe before the inflationary phase. However, a mixmaster model would not inflate unless either shear or the radiation energy density are large enough. Particle creation enhances the radiation energy density and therefore enlarges the set of inflating initial conditions for the Universe.

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.

Gauge-invariant cosmological perturbations from a thermodynamical point of view

Abstract: A thermodynamically oriented gauge-invariant description of scalar cosmological perturbations around a flat Friedman universe is given. All basic variables have their physical meaning on comoving hypersurfaces, i.e. on hypersurfaces orthogonal to the matter worldlines. Several well known growth and decay rates of cosmological perturbation modes are rediscovered in a transparent way. The formalism is extended to multicomponent systems and applied to a mixture of radiation and non-relativistic particles for perturbations both smaller and larger than the particle horizon. Thermodynamical perturbation relations in the Minkowski spacetime turn out to be valid in the expanding universe, provided all perturbed quantities are replaced by their gauge-invariant counterparts.

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.

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.

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.

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

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.

The cosmological constant and the pulsating universe

Abstract: The author's recently described method for obtaining an upper bound for a recollapsing, closed universe with a positive cosmological constant is extended to obtain a lower bound for a negativeA. The bounds are readily generalized to cosmologies of higher dimensions. As the radius of recollapse becomes infinite and the metric Minkowskian, the bounds shrink to zero. It is inferred that in a special relativistic theory, the cosmological term should vanish. A special relativistic proof of this is given based on requiring invariance under the «tilting» of the spacelike hypersurface. There is a discussion of the significance of this «guardian symmetry» for the self-stress problems of classical electron theory as well as the possibility of breaking supersymmetry without engendering a cosmological term. A brief application is given to the regularization of the Casimir effect. Some of the remaining problems and possible successes of the pulsating universe are also described. In the appendix, the well-known instability of the Einstein universe is briefly treated by the method used here to obtain bounds on the cosmological constant.