  Recent measurements of temperature fluctuations in the cosmic microwave
background (CMB) indicate that the Universe is flat and that large-scale
structure grew via gravitational infall from primordial adiabatic
perturbations. Both of these observations seem to indicate that we are on the
right track with inflation. But what is the new physics responsible for
inflation? This question can be answered with observations of the polarization
of the CMB. Inflation predicts robustly the existence of a stochastic
background of cosmological gravitational waves with an amplitude proportional
to the square of the energy scale of inflation. This gravitational-wave
background induces a unique signature in the polarization of the CMB. If
inflation took place at an energy scale much smaller than that of grand
unification, then the signal will be too small to be detectable. However, if
inflation had something to do with grand unification or Planck-scale physics,
then the signal is conceivably detectable in the optimistic case by the Planck
satellite, or if not, then by a dedicated post-Planck CMB polarization
experiment. Realistic developments in detector technology as well as a proper
scan strategy could produce such a post-Planck experiment that would improve on
Planck's sensitivity to the gravitational-wave background by several orders of
magnitude in a decade timescale.
