Leonid P. Grishchuk

Gravitational wave astronomy: In Anticipation of first sources to be detected

The first generation of long-baseline laser interferometric detectors of gravitational waves will start collecting data in 2001-2003. We carefully analyse their planned performance and compare it with the expected strengths of astrophysical sources. The scientific importance of the anticipated discovery of various gravitatinal wave signals and the reliability of theoretical predictions are taken into account in our analysis. We try to be conservative both in evaluating the theoretical uncertainties about a source and the prospects of its detection. After having considered many possible sources, we place our emphasis on (1) inspiraling binaries consisting of stellar mass black holes and (2) relic gravitational waves. We draw the conclusion that inspiraling binary black holes are likely to be detected first by the initial ground-based interferometers. We estimate that the initial interferometers will see 2-3 events per year from black hole binaries with component masses 10-15M_\odot, with a signal-to-noise ratio of around 2-3, in each of a network of detectors consisting of GEO, VIRGO and the two LIGOs. It appears that other possible sources, including coalescing neutron stars, are unlikely to be detected by the initial instruments. We also argue that relic gravitational waves may be discovered by the space-based interferometers in the frequency interval 2x10^{-3}-10^{-2} Hz, at the signal-to-noise ratio level around 3.

Discovering Relic Gravitational Waves in Cosmic Microwave Background Radiation

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 Wheeler’s 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.

GRAVITON CREATION IN THE EARLY UNIVERSE

Astronomers investigate two kinds of electromagnetic radiation: ( I ) that which was emitted by localized sources such as stars, galaxies, and nebulae in recent (from cosmological point of view) times, and ( 2 ) the 2.7K background blackbody radiation. The background radiation is isotropic and was created by the primeval plasma when the universe was much “hotter” than now. Gravitational radiation bathing the Earth should also be of two kinds: that which was emitted by localized sources, and an isotropic “relict” background. The only difference is that the gravitational-wave background may not be thermal. In Pact, the graviton creation mechanism, described below, predicts that i t musf be nonthermal. The electromagnetic background radiation had enough time in the past to thermalize. This is not the case for gravitational radiation. For this reason the form of the spectrum of gravitational radiation, which depends on the rate of expansion, could give us very important information about the earliest stages of the universe. On the other hand, the absence of gravitons in sufiicient amount can also permit us to place restrictions on the possible models of the early universe. Let us start from a short description of the physical mechanism of graviton creation i n an external gravitational field. (For a more detailed description see References I and 1 I . ) First, we assume that the classical Einstein equations are valid. Second, we assume that the Fricdman metric, describing the large-scale structure ofgravitational field i n the prcsent univcrse, can be extrapolated back in time up to the limit of applicability of the classical theory of gravitation. This second assumption is not a weak point of thc theory but rather a very strong’one, because this is the simplest assumption that we can make about the structure of the gravitational field in the early universe. This assumption implies that there is no creation of all other known particles, b u t it allows the creation of gravitons. This assumption is usual and plays a central role in the so called “standard cosmological model.” More complicated models will also allow graviton production, but the equations will become more complicated and messy. A weak gravitational wave in an external gravitational field

Relic gravitational waves and their detection

The range of expected amplitudes and spectral slopes of relic (squeezed) gravitational waves, predicted by theory and partially supported by observations, is within the reach of sensitive gravity-wave detectors. In the most favorable case, the detection of relic gravitational waves can be achieved by the cross-correlation of outputs of the initial laser interferometers in LIGO, VIRGO, GEO600. In the more realistic case, the sensitivity of advanced ground-based and space-based laser interferometers will be needed. The specific statistical signature of relic gravitational waves, associated with the phenomenon of squeezing, is a potential reserve for further improvement of the signal to noise ratio.

Complete Cosmological Theories

Modern cosmology successfully describes the main features of the Universe but uses for that some specific initial data for which we do not yet have a reasonable explanation. There is no theory capable of not only describing the observable world but also giving an answer to why and for what reasons the Universe has these or other properties. The most important unsolved issue is the nature of the cosmological singularity whose existence is predicted by the classical (not quantum) relativistic theory of gravity. For a thorough analysis of the singularity one needs a quantum theory of gravity which is yet to be constructed. Beacause of the lack of such a theory one has to start doing cosmology from classical stages by introducing different initial conditions and comparing theiur consequences with real observations.

On the electromagnetic detection of gravitational waves

The general principles of the electromagnetic detection of gravitational waves are discussed. A critical comment on a previous paper by Baierlein [1] devoted to the same problem is given.

Signatures of Quantum Gravity in the Large-Scale Universe

The quantum gravity processes that have taken place in the very early Universe are probably responsible for the observed large-scale cosmological perturbations. The comparison of the theory with the detected microwave background anisotropies favors the conclusion that the very early Universe was not driven by a scalar field with whichever scalar field potential. At the same time, the observations allow us to conclude that there is a good probability of a direct detection of the higher frequency relic gravitational waves with the help of the advanced laser interferometers.