Detecting relics of a thermal gravitational wave background in the early
  Universe
  A thermal gravitational wave background can be produced in the early Universe
if a radiation dominated epoch precedes the usual inflationary stage. This
background provides a unique way to study the initial state of the Universe. We
discuss the imprint of this thermal spectra of gravitons on the cosmic
microwave background (CMB) power spectra, and its possible detection by CMB
observations. Assuming the inflationary stage is a pure de Sitter expansion we
find that, if the number of e-folds of inflation is smaller than 65, the signal
of this thermal spectrum can be detected by the observations of Planck and
PolarBear experiments, or the planned EPIC experiments. This bound can be even
looser if inflation-like stage is the sub-exponential.
