Showing papers on "Particle horizon published in 1974"

Black Holes in the Early Universe

Abstract: The existence of galaxies indicates that the early universe must have been inhomogeneous and might have been highly chaotic. This could have lead to regions of the size of the particle horizon undergoing gravitational collapse to produce black holes with initial masses from 10-5 g upwards. Radiation pressure in the early Universe would cause these black holes to grow by accretion. However, despite previous expectations, this accretion would not be very much unless the initial conditions of the Universe were arranged in a special and a causal manner. Observations indicate that, at the most, only a small fraction of the matter in the early Universe can have undergone gravitational collapse.

Was the big bang a whimper

Abstract: In many cases the spatially homogeneous cosmological models of General Relativity begin or end at a “big bang” where the density and temperature of the matter in the universe diverge. However in certain cases the spatially homogeneous development of these universes terminates at a singularity where all physical quantities are well—behaved (a “whimper”) and an associated Cauchy horizon. We examine the existence and nature of these singularities, and the possible fate of matter which crosses the Cauchy horizon in such a universe. The nature of both kinds of singularity is illustrated by simple models based on two-dimensional Minkowski space-time; and the possibility of other types of singularity occuring is considered.

Why is the universe homogeneous

Abstract: The very early universe must have been extremely homogeneous, even on scales far exceeding the particle horizon. Within the framework of the standard Friedmann cosmology, homogenization can only be achieved by quantum effects which violate classical causality. This could happen when the particle horizon was smaller than the Compton wavelength of the pion. The assumption that statistical departures from equilibrium started to grow after this epoch leads to a prediction of the density fluctuations at recombination. The amplitude ν of the fluctuations should have a maximum of about 0.007 on scales of 81017M. For smaller scales, ν ∝M +1/6, and for larger scales, ν ∝M −1/2. This suggests that superclusters condense out at a red shift of about 11, and clusters and then galaxies form shortly after by fragmentation.

Early Stages of the Universe

Abstract: When I was young theoretical cosmology was more a series of exercises in geometry than a branch of astrophysics. Moreover these exercises were based on little more than the Hubble law of red shifts and a general impression that on a large scale the Universe was roughly homogeneous and isotropic. Now all that has changed. While geometrical considerations retain their importance through the dominant role played by general relativity, cosmology has also become highly astrophysical. As a result we can now say a great deal about the early stages of the Universe. My aim is to give a brief account of our present understanding of this fascinating subject, at the same time highlighting the most important problems that remain.

The cosmological effects of generalized field equations in relativity

Abstract: An autonomous system of equations, describing uniform cosmological models, is formulated by using the perfect fluid approximation of Einstein’s equations These equations contain an arbitrary function related to the matter content of the universe, which may include negative energy fields This function, designated α , is assumed to depend on the density and expansion rate of the universe only Geometrical methods of analysis are used to study the behaviour of all models described by this system The analysis shows that there are only three possible modes of behaviour that can be exhibited by a uniform universe Examples of the first two classes are well known in the ‘big-bang’ and ‘steady-state’ theories However, it is shown that the familiar theories are not unique, but an infinite number of both such types of model exist for various α It is also shown that all steady-state models in an expanding universe are stable The third class of model, associated with periodic behaviour, is of two types The first is demonstrated by a universe which oscillates between expansion and contraction but never achieves infinite density The second consists of ever-expanding (or contracting) models in which the density and expansion rate oscillate between finite values These latter models possess evolutionary characteristics on ‘short’ time-scales, while satisfying the ‘perfect cosmological principle’ in the large, and only arise in the presence of gross non-linearities introduced by the function α Both the periodic and steady-state classes occur only in the case of negative energy fields

Dirac's cosmological coincidence: An explanation

Abstract: In a previous paper, the author suggested that the universe could have been homogenized when the particle horizon was smaller than the Compton wavelength of the pion. This hypothesis also explains in a very natural manner the curious coincidence, noticed by Dirac in 1937, between atomic and cosmological constants.