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"It could be defects in the structure of spacetime, colliding cosmic strings. I mean, I'm just spit-balling crazy ideas that people are coming up with because there's not a conventional explanation."

Such phase transitions could give rise to cosmic strings, hypothesized one-dimensional sinews of energy that can warp, snap and break as they undulate through the universe, producing gravitational waves.

Another possible explanation is that the hum is also coming from cosmic strings, which are remnants from the early universe when it cooled down quickly and left cracks that are floating around in space.

But other, stranger phenomena, such as cosmic strings or massively inflated quantum fluctuations from right after the big bang, could also be contributing to the gravitational-wave background.

But the gravitational-wave background could also be coming from something else, like hypothetical cracks in space-time known as cosmic strings.