Colliding black holes may tell us the universe’s expansion rate

This new technique has been called the “spectral siren,” and it may just be able to tell us about the early misunderstood “teenage” years of the Universe.

When two black holes collide with each other, it generates an event so powerful that it literally creates a ripple in spacetime that travels across the Universe. On Earth, the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Italian observatory Virgo can pick up those ripples for astronomers to analyze.

Over the past few years, these instruments have collected the readings from almost 100 pairs of black holes colliding.

These readings contain crucial information about how massive the black holes were before their collision. However, the data of the signals has been traveling across space and has thus expanded, which in turn changes its properties.

“For example, if you took a black hole and put it earlier in the universe, the signal would change, and it would look like a bigger black hole than it really is,” explained UChicago astrophysicist Daniel Holz, one of the two authors on the paper.

The astrophysicists figured out that if they could measure how that signal changed, then they could calculate the expansion rate of the Universe. But how could they possibly know how much it changed from the original?

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