  The direct detection of gravitational wave by Laser Interferometer
Gravitational-Wave Observatory indicates the coming of the era of
gravitational-wave astronomy and gravitational-wave cosmology. It is expected
that more and more gravitational-wave events will be detected by currently
existing and planned gravitational-wave detectors. The gravitational waves open
a new window to explore the Universe and various mysteries will be disclosed
through the gravitational-wave detection, combined with other cosmological
probes. The gravitational-wave physics is not only related to gravitation
theory, but also is closely tied to fundamental physics, cosmology and
astrophysics. In this review article, three kinds of sources of gravitational
waves and relevant physics will be discussed, namely gravitational waves
produced during the inflation and preheating phases of the Universe, the
gravitational waves produced during the first-order phase transition as the
Universe cools down and the gravitational waves from the three phases:
inspiral, merger and ringdown of a compact binary system, respectively. We will
also discuss the gravitational waves as a standard siren to explore the
evolution of the Universe.
