PBS Space Time

To progress to the next level in understanding reality, we need to combine quantum mechanics and Einstein’s general relativity. And to do that, most physicists believe we need a theory of quantum gravity .. which means we need gravitons. But it also seems like the laws of physics make it impossible to ever detect this quantum particle of gravity. Almost like the universe is set up to keep the final answer forever out of our reach. So, can we outsmart the universe, catch a graviton, and finally solve physics?

The universe thrums with quantum fields; and the particles of matter and force emerge as vibrational manifestations of the deep symmetries of these fields. The layers and reflections of those symmetries give us the wonderful richness of what we call the standard model of particle physics. Except there seems to be something missing: the sterile neutrino.

Physicists have long believed that detecting the particle of gravity—the graviton—was fundamentally impossible, with the universe itself seeming to block every direct attempt. This episode explores a new generation of clever experiments that may finally let us detect gravity’s particle, and why even succeeding wouldn’t quite mean what we think it does.

We've known that the universe is expanding since 1929, and that its expansion is accelerating since 1998. The culprit behind the acceleration is unknown, so we live with a stand-in term "dark energy". Our modern cosmological model assumes that dark energy has a constant density--always the same amount of the outward-shoving stuff per volume. But there's recent evidence to the contrary--which may be why our primary efforts to measure the expansion rate of the universe disagree with each other.