Showing papers on "Dipole anisotropy published in 1995"
TL;DR: The Grishchuk–Zel’dovich effect is analyzed in open universes, corresponding to density parameter 0 < 1 with no cosmological constant, and the recently discussed super-curvature modes are concentrated on.
Abstract: The Grishchuk–Zel’dovich effect is the contribution to the microwave background anisotropy from an extremely large scale adiabatic density perturbation, on the standard hypothesis that this perturbation is a typical realization of a homogeneous Gaussian random field. We analyze this effect in open universes, corresponding to density parameter 0 < 1 with no cosmological constant, and concentrate on the recently discussed super-curvature modes. The effect is present in all of the low multipoles of the anisotropy, in contrast with the 0 = 1 case where only the quadrupole receives a contribution. However, for no value of 0 can a very large scale perturbation generate a spectrum capable of matching observations across a wide range of multipoles. We evaluate the magnitude of the effect coming from a given wavenumber as a function of the magnitude of the density perturbation, conveniently specified by the mean-square curvature perturbation. From the absence of the effect at the observed level, we find that for 0.25 ≤ 0 ≤ 0.8, a curvature perturbation of order unity is permitted only for inverse wavenumbers more than one thousand times the size of the observable universe. As 0 tends to one, the constraint weakens to the flat space result that the inverse wavenumber be more than a hundred times the size of the observable universe, whereas for 0 < 0.25 it becomes stronger. We explain the physical meaning of these results, by relating them to the correlation length of the perturbation. Finally, in an Appendix we consider the dipole anisotropy and show that it always leads to weaker constraints.
20 citations
TL;DR: The crossover phenomenon is a result of borehole stress concentrations, and can be employed in a new technique for distinguishing stress-induced dipole anisotropy from intrinsic anisotropic.
Abstract: Tectonic stress-induced dipole anisotropy exhibits a crossover n the two principal flexural wave slowness dispersions oriented parallel and normal to the far-field uniaxial compressive stress direction. This crossover phenomenon is a result of borehole stress concentrations, and can be employed in a new technique for distinguishing stress-induced anisotropy from intrinsic anisotropy. Theoretical modeling and laboratory measurements have been made on a large block of dry Berea sandstone subject to a uniaxial stress of up to 5 MPa. The two flexural dispersions are obtained by Prony’s processing of an array of waveforms for dipole orientations parallel and normal to the stress direction. The theoretical dispersions in the presence of biasing stresses are obtained from the solution of equations of motion for small dynamic fields superposed on a static bias. Good agreement has been obtained between the theoretical predictions and measured dispersions including the crossover phenomenon a feature that is exclusively due to the stress-induced anisotropy.
13 citations
TL;DR: In this article, the distance-redshift relation in the simple void model was investigated, where the position of the observer is off center of the void, and the peculiar velocity correction was measured by the observed dipole anisotropy of cosmic microwave background.
Abstract: We investigate the distance-redshift relation in the simple void model. As discussed by Moffat and Tatarski, if the observer stays at the center of the void, the observed Hubble parameter is not so different from the background Hubble parameter. However, if the position of observer is off center of the void, we must consider the peculiar velocity correction which is measured by the observed dipole anisotropy of cosmic microwave background. This peculiar velocity correction for the redshift is crucial to determine the Hubble parameter and we shall discuss this effect. Further the results of Turner et al by the N-body simulation will be also considered.
9 citations
Journal Article•
2 citations
01 Jan 1995
TL;DR: The role of gravity and General Relativity in cosmology including the geometry, topology, and dynamics of the universe is examined in this article, where the COBE Differential Microwave Radiometers (DMR) have mapped the cosmic microwave background anisotropy and measured the primordial density distribution on super-horizon scales.
Abstract: Observations of the Cosmic Microwave Background (CMB) Radiation have put the standard model of cosmology, the Big Bang, on firm footing and provide tests of various ideas of large scale structure formation. CMB observations now let us test the role of gravity and General Relativity in cosmology including the geometry, topology, and dynamics of the Universe. Foreground galactic emissions, dust thermal emission and emission from energetic electrons, provide a serious limit to observations. Nevertheless, observations may determine if the evolution of the Universe can be understood from fundamental physical principles. The COBE Differential Microwave Radiometers (DMR) has mapped the cosmic microwave background anisotropy and measured the primordial density distribution on super-horizon scales.