Pulsars are dead stars spinning with flawless rhythm. Their radio blips are more precise than atomic clocks. However, gravitational waves — ripples in the fabric of spacetime, predicted by Einstein — disrupt that rhythm.

This hum could be an echo of the Big Bang — more precisely, from the era of inflation, when the universe expanded faster than light in a fraction of a second. The idea of such a stage was proposed by Alan Guth. If he's right, we're hearing gravitational waves for the first time not from black hole collisions but from the birth of the cosmos.

For this signal to be explained by inflation, the universe after expansion had to heat up to only a few billion degrees. For the event that gave birth to everything, that's extremely cold and poses a challenge to theorists.