Gravitational waves are predicted by the Einstein's theory of General Relativity. The direct detection of gravitational waves is one of the most challenging tasks in modern science and engineering due to the 'weak' nature of gravity. Recent development of the laser interferometer technology, however, makes it possible to build a detector on Earth that is sensitive up to 100-1000 Mpc for strong sources. It implies an expected detection rate of neutron star mergers, which are one of the most important targets for ground-based detectors, ranges between a few to a few hundred per year. Therefore, we expect that the gravitational-wave observation will be routine within several years. Strongest gravitational-wave sources include tight binaries composed of compact objects, supernova explosions, gamma-ray bursts, mergers of supermassive black holes, etc. Together with the electromagnetic waves, the gravitational wave observation will allow us to explore the most exotic nature of astrophysical objects as well as the very early evolution of the universe. This review provides a comprehensive overview of the theory of gravitational waves, principles of detections, gravitational-wave detectors, astrophysical sources of gravitational waves, and future prospects.

On the framework of a linearly perturbed Friedmann-Robertson-Walker spacetime, we derive an expression for the cosmological angular diameter distance affected by scalar and tensor perturbations. Our expression is applicable in linear order to distances in general FRW models. We study the effect of a stocastic gravitaional wave background on the two-point correlation function of the angular diameter distance fluctuations and, on the basis of this we also derive an expression for the power spectrum of the angular diameter distance fluctuations.

In the framework of linear perturbation theory and linear approximation of spacetime anisotropy, we investigated the formulae for the CMBR temperature anisotropy and fluctuation spectrum which have their origin in the primordial tensor perturbations of the perturbed Bianchi type I universe model. The resulting formulae were compared with those of the flat Friedmann model.

The world wide efforts for detecting gravitational waves, the detectors in vogue and the expected astrophysical sources of gravitational waves will be discussed. Ground based detectors especially, the resonant bar detectors and laser interferometers will be described with a brief mention of the space based detector (the LISA project). Astrophysical sources of gravitational waves such as coalescing binaries, supernovae, pulsars/ rotating neutron stars, stochastic background will be discussed in the context of detection.

In contrast to conventional belief that extended inflation ends when the Universe percolates, we find inflation may continue at least many Hubble times even after the Universe percolates. What is observed is that inflation will not stop unless the global equation of state changes from inflationary one into radiation one. Thus the energy density of shorter wavelength gravitational waves induced by bubble collision at near the end of inflation should be at least Order (102)~O(103) times greater than previous estimation of Turner and Wilcek(TW).