Showing papers on "Particle horizon published in 1997"

Cosmology in a String-Dominated Universe

Abstract: The string-dominated universe is a flat universe that locally resembles an open universe and fits dynamical measures of power spectra, cluster abundances, redshift distortions, lensing constraints, luminosity, angular diameter distance relations, and microwave background observations. We show examples of networks that might give rise to recent string-domination when formed by a phase transition near the electro-weak symmetry breaking scale. We discuss how future observations can distinguish this model from other cosmologies.

Getting around cosmic variance

Abstract: Cosmic microwave background (CMB) anisotropies probe the primordial density field at the edge of the observable Universe. There is a limiting precision (“cosmic variance”) with which anisotropies can determine the amplitude of primordial mass fluctuations. This arises because the surface of last scatter (SLS) probes only a finite two-dimensional slice of the Universe. Probing other SLS's observed from different locations in the Universe would reduce the cosmic variance. In particular, the polarization of CMB photons scattered by the electron gas in a cluster of galaxies provides a measurement of the CMB quadrupole moment seen by the cluster. Therefore, CMB polarization measurements toward many clusters would probe the anisotropy on a variety of SLS's within the observable Universe, and hence reduce the cosmic-variance uncertainty.

Inhomogeneous string cosmologies

Abstract: We present exact inhomogeneous and anisotropic cosmological solutions of low-energy string theory containing dilaton and axion fields. The spacetime metric possesses cylindrical symmetry. The solutions describe ever-expanding universes with an initial curvature singularity and contain known homogeneous solutions as subcases. The asymptotic form of the solution near the initial singularity has a spatially varying Kasner-like form. The inhomogeneous axion and dilaton fields are found to evolve quasihomogeneously on scales larger than the particle horizon. When the inhomogeneities enter the horizon they oscillate as nonlinear waves and the inhomogeneities attentuate. When the inhomogeneities are small they behave as small perturbations of homogeneous universes. The manifestation of duality and the asymptotic behavior of the solutions are investigated. {copyright} {ital 1997} {ital The American Physical Society}

Old Galaxies at High Redshift and the Cosmological Constant

Abstract: In a recent striking discovery, Dunlop et al. (1996) observed a galaxy at redshift z = 1.55 with an estimated age of 3.5 Gyr. This is incompatible with age estimates for a flat matter-dominated universe unless the Hubble constant is less than 45 km s-1 Mpc-1. While both an open universe and a universe with a cosmological constant alleviate this problem, I argue here that this result favors a nonzero cosmological constant, especially when it is considered in light of other cosmological constraints: (1) for the favored range of matter densities, the constraint is more stringent than the globular cluster age constraint, which already favors a nonzero cosmological constant; (2) the age-redshift relation for redshifts of order unity implies that the ratio between the age associated with redshift 1.55 and the present age is also generally larger for a cosmological constant-dominated universe than for an open universe; (3) structure formation is generally suppressed in low-density cosmologies, arguing against early galaxy formation. The additional constraints imposed by the new observation on the parameter space of h versus Ωmatter (where H = 100 h km s-1 Mpc-1) are derived for both cosmologies.

The Limits on Cosmological Anisotropies and Inhomogeneities from COBE Data

Abstract: Assuming that the cosmological principle holds, Maartens, Ellis, & Stoeger (MES) recently constructed a detailed scheme linking anisotropies in the cosmic background radiation (CMB) with anisotropies and inhomogeneities in the large-scale structure of the universe and showed how to place limits on those anisotropies and inhomogeneities simply by using CMB quadrupole and octupole limits. First, we indicate and discuss the connection between the covariant multiple moments of the temperature anisotropy used in the MES scheme and the quadrupole and octupole results from COBE. Then we introduce those results into the MES limit equations to obtain definite quantitative limits on the complete set of cosmological measures of anisotropy and inhomogeneity. We find that all the anisotropy measures are less than 10-4 in the case of those not affected by the expansion rate H and less than 10-6 Mpc-1 in the case of those which are. These results demonstrate quantitatively that the observable universe is indeed close to Friedmann-Lemaitre-Robertson-Walker (FLRW) on the largest scales and can be modeled adequately by an almost-FLRW model—that is, the anisotropies and inhomogeneities characterizing the observable universe on the largest scales are not too large to be considered perturbations to FLRW.

Onset of inflation in inhomogeneous cosmology

Abstract: We study how the initial inhomogeneities of the universe affect the onset of inflation in the closed universe. We consider the model of a chaotic inflation which is driven by a massive scalar field. In order to construct an inhomogeneous universe model, we use the long wavelength approximation (the gradient expansion method). We show the condition of the inhomogeneities for the universe to enter the inflationary phase.

Early Universe in Self-Creation Cosmology

Abstract: Spatially homogeneous and isotropic Robertson-Walker model of the universe is studied in Barber's second self-creation theory of gravitation in the presence of perfect fluid by using ‘gamma-law’ equation of state p =(γ-1)ρ. The parameter gamma varies continuously with cosmological time. Exact solutions of the field equations are obtained for inflationary period and radiation-dominated era by using the power law relation φ Rn-3 = B. Some physical properties of the models are also discussed.

Cosmology in Conformally Flat Spacetime

Abstract: A possible solution to cosmological age and redshift-distance difficulties has recently been proposed by applying the appropriate conformally flat spacetime (CFS) coordinates to the standard solution of the field equations in a standard dust model closed universe. Here it is shown that CFS time correctly measures the true age of the universe, thus answering a major theoretical objection to the proposal. It is also shown that the CFS interpretation leads to a strong Copernican principle and is in all other respects wholly self-consistent. The deceleration parameter q0 is related to t0, the present age of the universe divided by L, the scale length of its curvature (an absolute constant). The values of q0 and L are approximately and 9.2 × 109 yr, respectively. It is shown that the universe started everywhere simultaneously, with no recession velocity until the effects of its closed topology became significant. Conclusions to the contrary in standard theory (the big bang) stem from a different definition of recession velocity. The theoretical present cosmological mass density is quantified as 4.4 × 10-27 kg m-3 approximately, thus greatly reducing, in a closed universe, the observational requirement to find hidden mass. It is also shown that the prediction of standard theory, for a closed universe, of collapse toward a big crunch termination, will not in fact take place.

Is the Universe Open or Closed? The Density of Matter in the Universe

Abstract: This controversial book examines one of the most fundamental questions of modern cosmology: how much matter is there in the Universe? This issue affects theories of the origin and evolution of the Universe as well as its geometrical structure and ultimate fate. The authors discuss all the relevant cosmological and astrophysical evidence and come to the conclusion that the balance of arguments presently lies with a density of around twenty per cent of the critical density required for the Universe to ultimately recollapse. Because the arguments presented constitute a 'state-of-the-art' analysis of the observational and theoretical arguments surrounding the existence of dark matter, primordial nucleosynthesis, large-scale structure formation and the cosmic microwave background radiation, this study provides the reader with an indispensable introduction to the most exciting recent developments in modern cosmology. Written by two eminent cosmologists, this topical and provocative book will be essential reading for all cosmologists and astrophysicists.

Contributions of the Plasmons to the Energy Density and Pressure in the Early Universe

Abstract: The contributions of the plasmons to the energy density and pressure of the cosmic fluid in the early universe are calculated. It is found that the transverse and longitudinal plasmons make contributions to the energy density and pressure which amount to ~0.1% of the contributions of the ordinary photons. The present finding is expected not to influence the primordial nucleosynthesis in a drastic way. Nevertheless, the present investigation consolidates the conventional treatment of the photons in the early universe that assumes the smallness of the plasma effects.

Inhomogeneity of spatial curvature for inflation

Abstract: We study how the initial inhomogeneities of the spatial curvature affect the onset of inflation in the closed universe. We consider a cosmological model which contains a radiation and a cosmological constant. In order to treat the inhomogeneities in the closed universe, we improve the long wavelength approximation such that the non-small spatial curvature is tractable in the lowest order. Using the improved scheme, we show how large inhomogeneities of the spatial curvature prevent the occurrence of inflation.

On the Structure of the Early Universe

Abstract: We have postulated that the mass of the early universe possesses a (mass) fractal dimension 'd' during the early stages of its expansion and so we have come across many interesting consequences of the early universe lying in between matter and radiation dominated limits of it.

Einstein-like field equations with conserved source and decreasing ∧ term; Their cosmological consequences

Abstract: The consequences of a cosmological ∧ term varying asS−2 in a spatially isotropic universe with scale factorS and conserved matter tensor are investigated. One finds a perpetually expanding universe with positive ∧ and gravitational ‘constant’G that increases with time. The ‘hard’ equation of state 3P>U (U mass-energy density,P scalar pressure) applied to the early universe leads to the expansion lawS∝t (t cosmic time) which solves the horizon problem with no need of inflation. Also the flatness problem is resolved without inflation. The model does not affect the well known predictions on the cosmic light elements abundance which come from standard big bang cosmology.

Chaos in the Galactic Dynamics

Abstract: An elementary survey of cosmic chaos, its origins, and its physical impact in an open and multiply connected universe is given. A new type of cosmic evolution by global metrical deformations, unpredicted by Einstein's equations, is pointed out. The uniformity and the inhomogeneities of the galactic background are discussed in this context. There is a finite region in the open 3-space in which the galactic world-lines are chaotic, and the mixing taking place in this chaotic nucleus of the universe provides a mechanism to create the galactic equidistribution. We review in a completely untechnical way how global metrical deformations of space-time cause particle creation in quantum fields, and induce angular fluctuations in the temperature of the cosmic microwave background radiation. They can generate topology changes without distorting the constant curvature of the 3-space. The micro-topology causes parity violation (i.e., a violation of the space-reflection symmetry) by self-interference.

Quantum Mixtures in the Early Universe

Abstract: In Relativistic Schrodinger Theory for spinning matter, there exist mixtures which have vanishing spin density (Sμυ ≡ 0). Such a fermionic, but spinless quantum fluid is studied in detail. Its energy-momentum density can develop negative pressure which is able to accelerate the expansion of the universe and to create matter energy through continuous bouncing at minimal radius (“cosmic pumping”).

A system approach to the Einstein-de Sitter model of expansion of the universe

Abstract: It is shown that seeing the universe as a system is compatible not only with the standard hot big bang model for a flat universe but also with the classical exponential models (inflationary, steady-state expansion). One just has to assign the appropriate value to two parameters in agreement with the equation of state in force. An example of computation of the expansion up to now is given. It combines an inflationary stage with the radiation-dominated era followed by the matter-dominated era. The approach offers a means to conciliate the standard model with high values of the Hubble constant if such values were to be definitively confirmed.

Growth of density perturbations with a cosmological constant

Abstract: We investigate the influence both of the cosmological constant and of the physical processes that take place during the evolution of the Universe on the primordial cloud collapse. We study the evolution of a cloud with a density perturbation, b, from the recombination era to the present. As an example, we study an initial perturbation bj = 10-6 for a cloud of mass 10\ 105 and 106 M0 • In particular, we study the influence of the physical processes on the evolution of the peculiar velocity factor, f=d In bid Ina, where 'a' is the scalefactor ofthe Universe. We compare our results with the analytic expression for f obtained by Lahav et al. Our results show that the physical mechanisms (photon drag, photon cooling, recombination, etc.) amplify perturbations, and the cosmological constant is particularly important in a Universe dominated by it with a small matter density.

The large scale structure of the universe

Abstract: Recent developments concerning the large scale structure of the universe are discussed. All seem to indicate inhomogeneities on much larger scales than previously thought. Specific observations from above the atmosphere can aid in understanding several outstanding questions, like the spatial distribution of various absorbers, the global geometry of the universe, or the nature of large scale velocities over scales considerably larger than currently available.

Nonsingularity in the no- boundary Universe

Abstract: In the no-boundary Universe of Hartle and Hawking, the path integral for the quantum state of the Universe must be summed only over nonsingular histories. If the quantum corrections to the Hamilton-lacobi equation in the interpretation of the wave packet is taken into account, then all classical trajectories should be nonsingular. The quantum behaviour of the classical singularity in theS1×Sm model (m⩾2) is also clarified. It is argued that the Universe should evolve from the zero momentum state, instead from a zero volume state, to a 3-geometry state.

Instability of the one-texture universe

Abstract: The one-texture universe, introduced by Davis in 1987, is a homogeneous mapping of a scalar field with an S^3 vacuum into a closed universe. It has long been known to mathematicians that such solutions, although static, are unstable. We show by explicit construction that there are four degenerate lowest modes which are unstable, corresponding to collapse of the texture towards a single point, in the case where gravitational back reaction is neglected. We discuss the instability time scale in both static and expanding space-times; in the latter case it is of order of the present age of the universe, suggesting that, though unstable, the one-texture universe could survive to the present. The cosmic microwave background constrains the initial magnitude of this unstable perturbation to be less than ∼10^-3.

Tracing the universe with clusters of galaxies

Abstract: Clusters of galaxies, the most massive virialized systems known, provide a powerful tool for studying the structure, the mass density, and the cosmology of our universe Clusters furnish one of the best estimates of the dynamical mass density parameter on 1 Mpc scale, $\Omega_{dyn}$; the best measure of the baryon density fraction in the universe; an excellent tracer of the large-scale structure of the universe; and, most recently, a powerful tracer of the evolution of structure in the universe and its unique cosmological implications I review the above measures and show that they portray a consistent picture of the universe and place stringent constraints on cosmology Each of these independent measures suggests a low-density universe, $\Omega_{m}$ $\simeq $ 03 $\pm $ 01, with mass approximately tracing light on large scales

Scale-invariant Perturbation of the Friedmann Universe

Abstract: We use de Vaucouleurs' power-law density-distance relation, to study a hierarchical perturbation of the Friedmann universe. We solve the Einstein equation and obtain the density contrast and the amplification factor for the perturbation. It is shown that, scale-invariant inhomogeneities decay in Einstein-de-Sitter universe. On the contrary, in an open universe, the inhomogeneities grow. For low values of $\Omega$, amplification peaks sharply and the fluctuations can grow by up to a factor of $10^{13}$ from the recombination to the present time. Our analysis of the closed universe further confirms that, unlike the common belief that perturbations grow faster with increasing $\Omega$, scale-invariant perturbations amplify with decreasing $\Omega$. This result is consistent with the very low density expected for a hierarchical universe.

MBR as the relic of big bang

Abstract: In this talk I outline some of the arguments in support of a cosmological and primordial origin of the observed microwave background radiation (MBR) in the early hot phase of the universe. This interpretation of the MBR is at the heart of the hot Big Bang model (HBBM) of the universe. The observed Planckian energy distribution of the microwave photons reflects the thermal equilibrium that can be set up naturally within HBBM in the dense early universe. Alternate interpretations face the challenge of extremely tight constraints on deviations from a Planckian distribution. Within HBBM, the formation of large scale structure is linked to tiny anisotropies in the angular distribution of the MBR photons. Recent measurements of these anisotropies seem to be broadly consistent with the predictions of the current scenarios of structure formation in the universe. Since these predictions are based on HBBM, the concurrence of data with theory provides additional support in favour of viewing the MBR as the relic of Big Bang.

Spherically symmetric cosmological perturbation in a universe with background field

Abstract: In a previous paper the equations of small cosmological perturbations of a theory of gravitation in flat space-time are derived They are applied to a homogeneous, isotropic, nonsingular cosmological model with radiation, matter and background field At the beginning of the universe small spherically symmetric inhomogeneities on almost all scales can arise by instability Later on the density contrast of dust grows exponentially during a short time epoch The solution during this time period is given analytically

On the self-shielding universe.

Abstract: Summary. — Because the mass of the proton is much larger than the mass of theelectron, and the non-baryonic dark matter does not affect the optical depth, theobservable universe is not self-shielded, i.e. does not possess an optical depth of unity.PACS 98.80.Dr – Theoretical cosmology. It was recently suggested [1] that the observable universe could be self-shielded forelectromagnetic radiation. This assertion was based on the remark that by postulatinga self-shielded, flat universe, containing only one type of particle of charge equal to theelementary charge e , a characteristic mass m 0 4( He 4 / Gc 3 ) 1/3 of the order of theelectron mass emerges, where H is the Hubble constant, G is the gravitationalconstant, and c is the speed of light.The objective of this note is to demonstrate that the universe is not and was notself-shielded for most of its existence. The quantities of interest are: the optical depth t , the critical density r c , the density parameterV, the number density of particles