  Inflationary cosmology is successful in explaining a number of outstanding
cosmological issues including the flatness, the horizon and the relic issues.
More spectacular is the experimental confirmation of the structure as arose
from the inflationary quantum fluctuations. However, the physics in the
inflationary era is unclear. Polarization observations of Cosmic Microwave
Background (CMB) missions may detect the tensor mode effects of inflationary
gravitational waves (GWs) and give an energy scale of inflation. To probe the
inflationary physics, direct observation of gravitational waves generated in
the inflationary era is needed. In this essay, we advocate that the direct
observation of these GWs with sensitivity Omega-gw down to 10**(-23) is
possible using present projected technology development if foreground could be
separated.
