  The primordial spectrum of cosmological tensor perturbations is considered as
a possible probe of quantum gravity effects. Together with string theory, loop
quantum gravity is one of the most promising frameworks to study quantum
effects in the early universe. We show that the associated holonomy correction
should modify the potential seen by gravitational waves during the inflationary
amplification. The resulting power spectrum should exhibit a characteristic
tilt. This opens a new window for cosmological tests of quantum gravity.
