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Register a new userGravitational waves in a flat radiation-matter universe including anisotropic stress
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We present novel analytical solutions for linear-order gravitational waves or tensor perturbations in a flat Friedmann-Robertson-Walker universe containing two perfect fluids, radiation and pressureless dust, and allowing for neutrino anisotropic stress. One of the results is applicable to any sub-horizon gravitational wave in such a universe. Another result is applicable to gravitational waves of primordial origin, for example produced during inflation, and works both before and after they cross the horizon. These results improve on analytical approximations previously set out in the literature. Comparison with numerical solutions shows that both these approximations are accurate to within $1%$, or better, for a wide range of wave-numbers relevant for cosmology.
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The construction of exact linearized solutions to the Einstein equations within the Bondi-Sachs formalism is extended to the case of linearization about de Sitter spacetime. The gravitational wave field measured by distant observers is constructed, leading to a determination of the energy measured by such observers. It is found that gravitational wave energy conservation does not normally apply to inertial observers, but that it can be formulated for a class of accelerated observers, i.e. with worldlines that are timelike but not geodesic.
We study the tensorial modes of the two-fluid model, where one of this fluids has an equation of state $p = - rho/3$ (variable cosmological constant, cosmic string fluid, texture) or $p = - rho$ (cosmological constant), while the other fluid is an ordinary matter (radiation, stiff matter, incoherent matter). In the first case, it is possible to have a closed Universe whose dynamics can be that of an open Universe providing alternative solutions for the age and horizon problems. This study of the gravitational waves is extended for all values of the effective curvature $k_{eff}=k-frac{8pi G}{3}rho_{0s}$, that is, positive, negative or zero, $k$ being the curvature of the spacelike section. In the second case, we restrict ourselves to a flat spatial section. The behaviour of gravitational waves have, in each case, very particular features, that can be reflected in the anisotropy spectrum of Cosmic Microwave Background Radiation. We make also some considerations of these models as candidate to dark matter models.
Astronomical observations in the electromagnetic window - microwave, radio and optical - have revealed that most of the Universe is dark. The only reason we know that dark matter exists is because of its gravitational influence on luminous matter. It is plausible that a small fraction of that dark matter is clumped, and strongly gravitating. Such systems are potential sources of gravitational radiation that can be observed with a world-wide network of gravitational wave antennas. Electromagnetic astronomy has also revealed objects and phenomena - supernovae, neutron stars, black holes and the big bang - that are without doubt extremely strong emitters of the radiation targeted by the gravitational wave interferometric and resonant bar detectors. In this talk I will highlight why gravitational waves arise in Einsteins theory, how they interact with matter, what the chief astronomical sources of the radiation are, and in which way by observing them we can gain a better understanding of the dark and dense Universe.
We study a collapsing system attracted by a spherically symmetric gravitational source, with an increasing mass, that generates back-reaction effects that are the source of space-time waves. As an example, we consider an exponential collapse and the space-time waves emitted during this collapse due to the back-reaction effects, originated by geometrical deformation driven by the increment of the gravitational attracting mass during the collapse.
The authority of J. A. Wheeler in many areas of gravitational physics is immense, and there is a connection with the study of relic gravitational waves as well. I begin with a brief description of Wheelers influence on this study. One part of the paper is essentially a detailed justification of the very existence of relic gravitational waves, account of their properties related to the quantum-mechanical origin, derivation of the expected magnitude of their effects, and reasoning why they should be detectable in the relatively near future. This line of argument includes the comparison of relic gravitational waves with density perturbations of quantum-mechanical origin, and the severe criticism of methods and predictions of inflationary theory. Another part of the paper is devoted to active searches for relic gravitational waves in cosmic microwave background radiation (CMB). Here, the emphasis is on the temperature-polarization TE cross-correlation function of CMB. The expected numerical level of the correlation, its sign, statistics, and the most appropriate interval of angular scales are identified. Other correlation functions are also considered. The overall conclusion is such that the observational discovery of relic gravitational waves looks like the matter of a few coming years, rather than a few decades.
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