“If you are not completely confused by quantum mechanics, you do not understand it.” -John Wheeler
Now that LIGO has detected gravitational waves directly, it’s time to examine all the different sources that they come from, and what they can teach us about the Universe. While accelerating masses in strong gravitational fields — black holes, neutron stars and the like — will produce gravitational waves capable of being seen by direct detection experiments, there’s another class of gravitational waves that’s observable through its effects on the leftover light from the Big Bang: gravitational waves from inflation.
Unlike all the other sources of gravitational waves, however, the ones from inflation are purely quantum in origin: if gravitation is not a fundamentally quantum force, these fluctuations cannot exist! To discover this signature would teach us that Einstein’s relativity is not the final, fundamental theory of gravity, and that a truly quantum theory is needed to describe our Universe. Current and planned experiments will probe for this signal down to an improved factor of 100 over where we are now, giving us all the hope in the world that we’ll find it.