Particle horizon

About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.

General conditions for scale-invariant perturbations in an expanding universe

Abstract: We investigate the general properties of expanding cosmological models which generate scale-invariant curvature perturbations in the presence of a variable speed of sound. We show that in an expanding universe, generation of a super-Hubble, nearly scale-invariant spectrum of perturbations over a range of wavelengths consistent with observation requires at least one of three conditions: (1) accelerating expansion, (2) a speed of sound faster than the speed of light, or (3) super-Planckian energy density.

Gravitational Pressure and the Accelerating Universe

Abstract: In the context of the Teleparallel Equivalent of General Relativity (TEGR) one can obtain an alternative insight into General Relativity, as has been shown in addressing properties as energy, momentum and angular momentum of the gravitational field. In this paper, we apply the definition, that arises from the field equation of the the TEGR, for the stress-energy-momentum tensor of the gravitational field, whose spatial components naturally lead to the definition of gravitational pressure, to compute the total space-time pressure, due to the gravitational and matter fields, over a spherical, space-like two-surface of a Friedman-Robertson-Walker (FRW) universe, for any curvature index. In particular, for a spatially flat FRW universe in the actual era (i.e., for "cold matter"), it resulted that the pressure (now due only to the gravitational field) is outwardly directed over any spherical, spatial two-surface. This surface can be, in particular, the apparent horizon of a spatially flat FRW universe (in this case, the apparent horizon coincides with the Hubble horizon). Assuming the validity of the first law of thermodynamics for matter and gravity, and taking into account the contribution of the gravitational field to both the energy and the pressure terms in the first law of the (gravitational) thermodynamics, as well as considering the thermal character of the apparent horizon of the spatially flat FRW universe, we have thus obtained a value of the gravitational pressure that is very close to the observed value. We interpret this result as a possibility that the accelerated expansion of the actual universe might be due to the effect of the pressure of the very gravitational field, instead of an totally unkown (dark) energy.

A Multidimensional Geometric Expansion of Spacetime

Abstract: This is the first of 4 papers describing a Unified Field Theory based on a Multidimensional Geometric Expansion of Spacetime. Assumptions and properties of model – 1. Spacetime is defined by field based or structural relationships of distance and time. 2. The expansion of Space does not stop at the boundary of galaxies, but incrementally within the atom itself, thereby yielding the probabilistic properties associated with Quantum Mechanics. 3. The volume of Spacetime, or object, S, when viewed from an Absolute or “Eye of God” perspective outside the expansion, varies to the square of the Cosmological measure of time. Double the Age of the Universe and the volume increases 4 times. 4. Since “absolute” density decreases over time, the effect of gravity diminishing over time. (Effect of gravity varies by T^(-4/3) where T = Historical Location/ Age of Universe). 5. A local observer within “Observable Space” would not perceive any change in local measures of distance and intervals of time since all local clocks and local rulers proportionally change at the same rate. 6. Since all clock rates were faster in the past, cumulative measures of time experienced locally would actually be greater than would be expected when compared to Cosmological measures of time. 7. Stars would evolve more quickly than presently assumed, potentially resolving the issue in which some stars in Globular Clusters are determined to be older than the Universe. (This issue is not as resolved as many are inclined to believe). Two geometrically related and independent measures of time are established, local and Absolute, (also called Historical or Cosmological). 8. Accelerative fields associated with charge and gravity are predicted field relationships, which unites the two forces based on a dynamic multidimensional geometry. The succeeding papers are based on an expansion of the proposed model, wherein Observable Space is expanding within a moving and similarly expanding Unobserved Space.

Bianchi type-v cosmological models in scalar tensor theory of gravitation with some observable quantities

Abstract: A new class of a spatially homogeneous and anisotropic Bianchi type-V cosmological models of the universe for perfect fluid distribution within the framework of scalar-tensor theory of gravitation proposed by Saez and Ballester is investigated by applying the law of variation for the generalized mean Hubble’s parameter that yields a constant value of deceleration parameter. The variation for Hubble’s parameter generates two types of solutions for the average scale factor one is of power-law type and other is of the exponential form. Using these two forms, Einstein’s field equations are solved separately that correspond to singular and nonsingular models of the universe respectively. It is observed that for positive value of deceleration parameter q of the universe decelerates whereas for negative value of q the universe accelerates. Expressions for look-back timeredshift, neoclassical tests (proper distance , )) (z d luminosity distance redshift and event horizon are derived and their significance are described in detail. Some physical and geometrical properties of the models are also discussed.

String cosmology and the beginning-of-time myth

Abstract: The myth according to which the Universe –and time itself– started with a big bang will be challenged. Classical and quantum gravitational instabilities, can, respectively, inflate and warm up a primordial pre-bangian Universe satisfying a superstring-motivated principle of "Asymptotic Past Triviality". Properties of the pre-bangian Universe can be probed through its observable relics, which include: (i) a (possibly observable) stochastic gravitational-wave background; (ii) a (possible) new mechanism for seeding the galactic magnetic fields; (iii) a (possible) new source of large-scale structure and CMB anisotropy.