The Universe is Humming!

by Daniel Idfresne

The prevailing idea of space is that it is, well—it’s space. Space is nothingness, matterless, and undoubtedly soundless. Ask the countless shows and movies that cut background music or sound effects out when their characters float among the stars. Sound cannot travel through space because almost no atoms exist in a vacuum.

Yet, the universe can hum! In fact, it’s humming as you read this article. Phil Plait, the author of the Bad Astronomy Newsletter, writes that this hum is “a cacophonous tune, wildly varying in pitch” and could be classified as “baso molto profundissimo: Such a low pitch that it would take years to hear a single note.”

This hum is the result of the cosmic gravitational wave background. Gravitational waves are the expansion and contraction of space created by moving objects with mass. Plait helps readers visualize this concept using rocks and a lake as an analogy. Gravitational waves behave the way the surface of a pond does when someone drops a pebble. The universe is vast, so only objects with a large mass accelerated at an exceptionally high rate can create detectable gravitational waves. Those objects tend to be black holes.

Plait explains that if two black holes were in close orbit, they would “emit decently strong gravitational waves.” These waves “steal energy from their orbit,” so they orbit closer and faster, which emits more gravitational waves. This positive feedback loop results in one even more gigantic black hole.

Still, these waves wouldn’t be grand enough for scientists to detect them. Supermassive black holes are at the center of every giant galaxy. A low-pitched hum—constructed from combined gravitational waves of different pitches—rings throughout the universe when millions of these supermassive black hole pairs emit gravitational waves.

However, since no one has detected any close supermassive black hole pair, cosmic gravitational wave background is theory—until now.

The 90 scientists at The North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, recently found evidence that there is an extraordinarily high chance—in fact, greater than a 99% chance—that cosmic gravitational wave background is a real thing.

To figure out how they did so and what this means for our understanding of black holes and the universe writ large, check out Phil Plait’s article on the universe humming.