  While many aspects of general relativity have been tested, and general
principles of quantum dynamics demand its quantization, there is no direct
evidence for that. It has been argued that development of detectors sensitive
to individual gravitons is unlikely, and perhaps impossible. We argue here,
however, that measurement of polarization of the Cosmic Microwave Background
due to a long wavelength stochastic background of gravitational waves from
Inflation in the Early Universe would firmly establish the quantization of
gravity.
