Particle horizon

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

Cosmic concordance and the fine structure constant

Abstract: Recent measurements of a peak in the angular power spectrum of the cosmic microwave background suggest that the geometry of the universe is close to flat. But if other accepted indicators of cosmological parameters are also correct then the best fit model is marginally closed, with the peak in the spectrum at slightly larger scales than in a flat universe. If these observations persevere, one way they might be reconciled with a flat universe is if the fine structure constant had a lower value at earlier times, which would delay the recombination of electrons and protons and also act to suppress secondary oscillations as observed. We discuss evidence for a few percent increase in the fine structure constant between the time of recombination and the present.

Can the Universe escape eternal acceleration

Abstract: Recent astronomical observations of distant supernovae light-curves suggest that the expansion of the universe has recently begun to accelerate. Acceleration is created by an anti-gravitational repulsive stress, like that produced by a positive cosmological constant, or universal vacuum energy. It creates a rather bleak eschatological picture. An ever-expanding universe's future appears to be increasingly dominated by its constant vacuum energy. A universe doomed to accelerate forever will produce a state of growing uniformity and cosmic loneliness. Structures participating in the cosmological expansion will ultimately leave each others' horizons and information-processing must eventually die out. Here, we examine whether this picture is the only interpretation of the observations. We find that in many well-motivated scenarios the observed spell of vacuum domination is only a transient phenomenon. Soon after acceleration starts, the vacuum energy's anti-gravitational properties are reversed, and a matter-dominated decelerating cosmic expansion resumes. Thus, contrary to general expectations, once an accelerating universe does not mean always an accelerating universe.

Description of our cosmological spacetime as a perturbed conformal Newtonian metric and implications for the backreaction proposal for the accelerating universe

Abstract: It has been argued that the spacetime of our universe can be accurately described by a perturbed conformal Newtonian metric, and hence even large density inhomogeneities in a dust universe cannot change the observables predicted by the homogeneous dust model. In this paper we study a spherically symmetric dust model and illustrate conditions under which large spatial variations in the expansion rate can invalidate the argument.

Repulsive gravity in the very early Universe

Abstract: I present two examples in which the curvature singularity of a radiation-dominated Universe is regularized by (a) the repulsive effects of spin interactions, and (b) the repulsive effects arising from a breaking of the local gravitational gauge symmetry. In both cases the collapse of an initial, asymptotically flat state is stopped, and the Universe bounces towards a state of decelerated expansion. The emerging picture is typical of the pre-big bang scenario, with the main difference that the string cosmology dilaton is replaced by a classical radiation fluid, and the solutions are not duality-invariant.

Chaotic inflating universe

Abstract: It is shown that inflation is a natural result of chaotic initial conditions in the early universe These initial conditions are found in a wide class of elementary particle theories