Particle horizon

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

Modulated decay in the multi-component Universe

Abstract: The early Universe after inflation may have oscillations, kinations (nonoscillatory evolution of a field), topological defects, relativistic and non-relativistic particles at the same time. The Universe whose energy density is a sum of those components can be called the multi-component Universe. The components, which may have distinguishable density scalings, may decay modulated. In this paper we study generation of the curvature perturbations caused by the modulated decay in the multi-component Universe.

FRW universe models in conformally flat-spacetime coordinates III: Universe models with positive spatial curvature

Abstract: We deduce general expressions for the line element of universe models with positive spatial curvature described by conformally flat-spacetime coordinates. Models with dust, radiation and vacuum energy are exhibited. Discussing the existence of particle horizons we show that there is continual annihilation of space, matter and energy in a dust- and radiation-dominated universe, and continual creation in a Lorentz Invariant Vacuum Energy (LIVE)-dominated universe when conformal time is used in Friedmann-Robertson-Walker (FRW) models with positive spatial curvature. A general procedure is given for finding coordinates to be used in Penrose diagrams. We also calculate the age and the redshift of some universe models using conformal time.

Local irregularities in a universe satisfying the cosmological principle: Thesis

Abstract: A general relativistic model of the universe is considered in which local irregularities ( ) in the space-time metric are superimposed upon the values for a completely homogeneous, isotropic universe. The mass tensor is taken to be that of a perfect fluid. The mass density y? is considered to fluctuate about a mean of z? , the density in the completely homogeneous model. The effect of pressure is included in the model. A system of simultaneous equations is derived which expresses the irregularities in the metric tensor in terms of the irregularities in the mass tensor. This system reduces to single equations for the diagonal components of the metric tensor under quite general conditions. Solutions are obtained for these components, and conditions affecting the validity of these solutions are examined. A formal solution for all the components of is obtained in terms of a Green's tensor. The solution is found explicitly only for the case of an "isolated system". The equations of motion and of continuity for the perfect fluid of the model are expressed in coordinates IV. defined by the mean motion of matter at each point in spacetime (cosmic coordinates). An equation of energy conservation for the irregularities in mass density is derived and written in integral form. This equation should be of significance to cosmogony. The relation of the local inertial coordinate system to the cosmic coordinate system is examined. It is shown that Newton’s Law of Gravitation is valid in the local inertial system. The equation of energy conservation in these coordinates is found to include the kinetic energy of expansion and the gravitational potential energy of the "substratum" . By comparing the equation of energy conservation in the completely homogeneous,isotropic model with that of the model including local irregularities,we are able to find expressions for the cosmic density, pressure, and internal energy in terms of the local fluctuations. The bearing of these results upon cosmogony is discussed. Various interpretations of the ideas of Mach are recalled and criticized. The relation of the results of this thesis to "Mach's Principle" is examined.

Accelerated cosmic expansion in a scalar-field universe

Abstract: We consider here a spherically symmetric but inhomogeneous universe filled with a massless scalar field. The model obeys two constraints. The first one is that the gradient of the scalar field is timelike everywhere. The second constraint is that the radial coordinate basis vector is a unit vector field in the comoving coordinate system. We find that the resultant dynamical solutions compose a one-parameter family of self-similar models which is known as the Roberts solution. The solutions are divided into three classes. The first class consists of solutions with only one spacelike singularity in the synchronous-comoving chart. The second class consists of solutions with two singularities which are null and spacelike, respectively. The third class consists of solutions with two spacelike singularities which correspond to the big bang and big crunch, respectively. We see that, in the first case, a comoving volume exponentially expands as in an inflationary period; the fluid elements are accelerated outwards from the symmetry center, even though the strong energy condition is satisfied. This behavior is very different from that observed in the homogeneous and isotropic universe in which the fluid elements would move outwards with deceleration, if the strong energy conditions are satisfied. We aremore » thus able to achieve the accelerated expansion of the universe for the models considered here, without a need to violate the energy conditions. The cosmological features of the models are examined in some detail.« less