Particle horizon

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

Validity of the generalized second law of thermodynamics of the universe bounded by the event horizon in brane scenario

Abstract: In this paper, we examine the validity of the generalized second law of thermodynamics of the universe bounded by the event horizon in brane-world gravity. Here we consider a homogeneous and isotropic model of the universe in the one case where it is filled with a perfect fluid and in another case where a holographic dark energy model of the universe has been considered.

The Whirl Theory of the Origin of Structure in the Universe

Abstract: There are at least two reasons for examining different theories of galaxy formation: (i) We do not know the initial conditions in the early Universe; (ii) We do not know which forces were most important for the origin and evolution of initial perturbations. The first reason forces us to deal with perturbations of different types, i.e. adiabatic, turbulent and entropy perturbations. The second makes it necessary to examine the influence of non-gravitational forces — for instance, local vortices. The whirl theory of the formation of stroduces — by means of initial conditions — non-potential vortex perturbations.

Long-wavelength perturbations of a Friedmann universe, and anisotropy of the microwave background radiation

Abstract: Observational evidence, which is necessarily confined to a region of the universe limited in space (within the observer's horizon), implies a high degree of homogeneity and isotropy for the large-scale structure of the universe. In principle, substantial deviations of the properties of the real universe from the parameters of an idealized Friedmann cosmological model could have prevailed on scales exceeding that of the horizon. Constraints on the amplitude of perturbations with such long wavelengths are imposed by the virtual isotropy (deltaT/T<10/sup -4/) of the observed background radiation. This information on deltaT/T together with the natural hypothesis that the perturbations are statistically independent implies that on spatial scales exceeding the horizon there exist no significant (with amplitude of order greater than deltaT/T) perturbations in density. For certain types of perturbations in the metric (in the gravitational field), the amplitude could be appreciable without contradicting the empirical limits on deltaT/T.

A weak acceleration effect due to residual gravity in a multiply connected universe

Abstract: Context. Understanding dark energy and measuring the topology of the Universe are two of the biggest open questions in physical cosmology. It was previously shown that multiple connectedness, via the twin paradox of special relativity, provides a novel physical justification for an assumption of the standard FLRW model: it implies a favoured space-time splitting (comoving coordinates). Aims. Could cosmic topology also imply dark energy? Methods. We use a weak field (Newtonian) approximation of gravity and consider the gravitational effect from distant, multiple copies of a large, collapsed (virialised) object today (i.e. a massive galaxy cluster), taking into account the finite propagation speed of gravity, in a flat, multiply connected universe, and assume that due to a prior epoch of fast expansion (e.g. inflation), the gravitational effect of the distant copies is felt locally, from beyond the naively calculated horizon. Results. We find that for a universe with a T' x R 2 spatial section, the residual Newtonian gravitational force (to first order) provides an anisotropic effect that repels test particles from the cluster in the compact direction, in a way algebraically similar to that of dark energy. For a typical test object at comoving distance X from the nearest dense nodes of the cosmic web of density perturbations, the pressure-to-density ratio w of the equation of state in an FLRW universe, is ω ∼ -(X/L) 3 , where L is the size of the fundamental domain, i.e. of the Universe. Clearly, |ω| « 1. For a T 3 spatial section of exactly equal fundamental lengths, the effect cancels to zero. For a T3 spatial section of unequal fundamental lengths, the acceleration effect is anisotropic in the sense that it will tend to equalise the three fundamental lengths. Conclusions. Provided that at least a modest amount of inflation occurred in the early Universe, and given some other conditions, multiple connectedness does generate an effect similar to that of dark energy, but the amplitude of the effect at the present epoch is too small to explain the observed dark energy density and its anisotropy makes it an unrealistic candidate for the observed dark energy.