The cosmic microwave background sky, here mapped by NASA's Wilkinson Microwave Anisotropy Probe, has a polarization, represented by white bars. Future experiments might measure the polarization with enough sensitivity to prove the existence of gravitons, the quanta of gravity. Can a quantum of gravity ever be detected? Two physicists suggest that it can — using the entire Universe as a detector. Researchers think that the gravitational force is transmitted by an elementary particle called the graviton, just as the electromagnetic force is carried by photons. But most of them despair about ever recording individual gravitons. That is because gravity is...