  We study the effects of various phenomena which may have happened in the
early universe on the spectrum of inflationary gravitational waves. The
phenomena include phase transitions, entropy productions from non-relativistic
matter, the production of dark radiation, and decoupling of dark
matter/radiation from thermal bath. These events can create several
characteristic signatures in the inflationary gravitational wave spectrum,
which may be direct probes of the history of the early universe and the nature
of high-energy physics.
