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Particle horizon
About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.
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TL;DR: In this article, it is shown that, when the dimensionality of the kinetic energy term is increased in Einstein's field equations for the Friedmann-Robertson-Walker metric, stable bound states of the universe are only found forn=2, 3 and 4.
Abstract: It is shown that, when the dimensionality of the kinetic-energy term is increased in Einstein’s field equations for the Friedmann-Robertson-Walker metric inn dimensions, in the absence of a pressure term, stable bound states of the Universe are only found forn=2, 3 and 4. With the further assumption that space is closed, together with a suitable application of Mach’s principle, the casesn=2, 4 can be excluded, so that one has a cosmological approach to deriving the three-dimensionality of space. A speculative way to resolve the «short age» problem of a Ω>1 Universe is also described.
18 citations
TL;DR: In this article, the Friedmann-Robertson-Walker model of the universe is considered, and it is shown that the ultimate phase of perturbation growth is the formation of shock waves.
Abstract: After an introduction on the relativistic vs the Newtonian approach, the Friedmann--Robertson--Walker picture of the Universe is considered: isotropic and homogeneous solution, group-theoretic definition of homogeneity, and Friedmann solution are taken up. The paradox of the infinite gravitational potential in Newtonian theory is disposed of. It has long been believed that gravitational instability is the ultimate cause of matter clustering in the Universe. First, the idealized case of a static universe is treated. Then the Newtonian treatment of small perturbation theory is given for an expanding universe. Recent work concerning the extension of the theory to perturbations of finite amplitude is treated. Simple arguments are given showing that the ultimate phase of perturbation growth is the formation of shock waves. Matter compressed by shock wave forms rather flat clumps. Most of the above is written as examples of formal solutions of hydrodynamic equations with gravitational interaction. Finally, the connection of the theory to observational evidence is established. (RWR)
17 citations
TL;DR: In this article, the authors derived tensor perturbation equations in a diagonal Bianchi type-I universe and derived exact solutions for an axisymmetric Kasner background, in which wave propagation is along the symmetry axis.
Abstract: Tensor perturbation equations in a diagonal Bianchi type-I universe are derived in a preliminary study of the propagation of gravitational waves in anisotropic universes. Exact solutions to these equations are obtained for an axisymmetric Kasner background, in which wave propagation is along the symmetry axis. Near the singularity the universe behaves like a generalized Kasner solution and at large time like it possesses a directed stream of radiation with energy density equal to pressure. It is shown to be equivalent to thee linear limit of the Einstein-Rosen or Gowdy T/sup 3/ solution. The back reaction of gravitational waves on a type-I universe with matter is also studied in the long- and short-wavelength limits. High-frequency waves with sufficient energy could reverse the contracting axis of the universe into expansion, while waves propagating in all directions could bring about isotropization of the background. Long-wavelength perturbations act like an effective anisotropy potential which induces ''small oscillations'' on the background. The universe with such long waves can be shown to be equivalent to the weak-field limit of a ''corner run'' solution in the type-IX and type-VII/sub 0/ universes.
17 citations
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TL;DR: In this article, it was shown that the sensitivity estimates of the SKA and the number counts of the expected HI galaxies are sufficiently high to compensate for the observational uncertainties of the measurements and hence allow a statistical detection of the frequency shift.
Abstract: In recent years cosmology has undergone a revolution, with precise measurements of the microwave background radiation, large galaxy redshift surveys, and the discovery of the recent accelerated expansion of the Universe using observations of distant supernovae. In this light, the SKA enables us to do an ultimate test in cosmology by measuring the expansion rate of the Universe in real time. This can be done by a rather simple experiment of observing the neutral hydrogen (HI) signal of galaxies at two different epochs. The signal will encounter a change in frequency imprinted as the Universe expands over time and thus monitoring the drift in frequencies will provide a real time measure of the cosmic acceleration. Over a period of 12 years one would expected a frequency shift of the order of 0.1 Hz assuming a standard Lambda-CDM cosmology. Based on the sensitivity estimates of the SKA and the number counts of the expected HI galaxies, it is shown that the number counts are sufficiently high to compensate for the observational uncertainties of the measurements and hence allow a statistical detection of the frequency shift. [abstract abridged]
17 citations
TL;DR: The quark theory of hadrons provides a valuable tool for interpreting conditions during early epochs of the Universe as discussed by the authors, and an asymptotically free theory of quarks seems to rule out the possibility that the present-day entropy of the universe could be due to dissipation of low level fluctuations.
Abstract: The quark theory of hadrons provides a valuable tool for interpreting conditions during early epochs of the Universe. In particular, an asymptotically free theory of quarks seems to rule out the possibility that the present-day entropy of the Universe could be due to dissipation of low level fluctuations. Apparently either the early Universe was inhomogeneous on small mass scales or anisotropic.
17 citations