International Journal of Modern Physics D

This paper reviews the field of gravitational wave detection. The characteristics of gravitational waves and the possible sources of detectable waves are discussed. This includes a discussion of a new source, the stochastic background of cosmological gravitational waves from supernovae during the epoch of galaxy formation. Methods of using both resonant mass antennas and laser interferometer detectors are reviewed. Noise sources that limit detector sensitivity are summarised, while new methods, and new materials which can simplify and improve detector performance are described.

Besides MACHOs (Massive Astrophysical Compact Halo Objects) discovered by microlensing, cold molecular clouds (mainly of H₂) may well contribute substantially to the galactic halo dark matter. We describe a model for the formation and evolution of proto globular clusters towards either globular clusters or dark clusters of MACHOs and molecular clouds, depending on the distance from the galactic centre. Moreover, we discuss various methods to test this scenario, which rely upon observations in several bands of the electromagnetic spectrum. In particular, we estimate the γ-ray flux arising from halo molecular clouds through the interaction with high-energy cosmic-ray protons. Molecular clouds can also be detected via the absorption lines they would produce in the spectrum of stars located in the Large Magellanic Cloud and via the anisotropy they would introduce in the Cosmic Background Radiation when looking at the halo of M31 galaxy. Finally, we address the possibility of discovering MACHOs by infrared searches.

Self-collision of a nonrelativistic classical point-like body, or particle, in the spacetime containing closed time-like curves (time-machine spacetime) is considered. A point-like body (particle) is an idealization of a small ideal elastic billiard ball. The known model of a time machine is used containing a wormhole leading to the past. If the body enters one of the mouths of the wormhole, it emerges from another mouth in an earlier time so that both the particle and its “incarnation” coexist during some time and may collide. Such self-collisions are considered in the case when the size of the body is much less than the radius of the mouth, and the latter is much less than the distance between the mouths. Three-dimensional configurations of trajectories with a self-collision are presented. Their dynamics is investigated in detail. Configurations corresponding to multiple wormhole traversals are discussed. It is shown that, for each world line describing self-collision of a particle, dynamically equivalent configurations exist in which the particle collides not with itself but with an identical particle having a closed trajectory (Jinnee of Time Machine).

The mini-superspace approximation in quantum cosmology, whereby the space—time is restricted to the Friedmann form ds²=dt²−a²(t)dx², requires the integrated three-space ∫d³x to be finite, in order that the operator replacements p→−iħ∂/∂q are well defined for the canonically conjugate momenta and coordinates (p, q). We have previously agued that this procedure can be made exact by cutting off the physical space at the causal horizon, so that ∫d³x=1 and a(t₀)=(4π/3)^1/3ξ−1(t₀), where ξ≡d(ln a)/dt is the Hubble parameter and t₀ is the present time, assuming dx² to be flat. A corollary of this horizon hypothesis is that all quantum field theoretical integrals are similarly cut off. It is pointed out that the analysis by Jing and Fang of the two-year results for the COBE DMR observations of the quadrupole anisotropy of the cosmic microwave background radiation substantiates this idea (although the alternative explanation, that there are smaller density fluctuations at larger scales, is not ruled out). Also, the cosmological Schrödinger equation (Wheeler-DeWitt equation) for the wave function of the Universe Ψ, obtained from the heterotic superstring theory of Gross et al., is shown to be separable only when the physical space is isotropic and conformally flat, and in the approximation that quartic and higher-order terms in ξ are ignorable, and the solution is then expressed in terms of parabolic cylinder functions (Weber-Hermite functions). Finally, the occurrence of large density fluctuations via the indeterminacy relation Δπ_ξΔξ~ħ is further discussed.

The cosmic no-hair theorem is generalized for homogeneous and anisotropic spacetimes in which a scalar field is nonminimally coupled to the geometry. The result is that the Wald proof also holds here for all Bianchi-type universes except Bianchi type-IX. However, considering the asymptotic behaviour of parameters, it is possible to find a class of models where the conjecture holds for Bianchi-IX too.