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

Black holes are among some of the most mysterious objects in the Universe. They are the remnants of massive stars that have reached the end of their life cycles and collapsed into a region of spacetime that is incredibly dense. Their gravitational force is so strong that nothing can escape their surface.

Colliding black holes

As such, they have thus far not been considered a great source of information for physicists.

Now, two University of Chicago astrophysicists have figured out a method for how to use pairs of colliding black holes to measure how fast our Universe is expanding, according to a press release by the institution. From this data, they plan to deduce how the Universe evolved, what it is made out of, and where it’s going.

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.

Deducing the expansion rate

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?

In their new paper, Holz and first author Jose María Ezquiaga attempt to use knowledge about the whole population of black holes as a calibration tool. “So we measure the masses of the nearby black holes and understand their features, and then we look further away and see how much those further ones appear to have shifted,” said Ezquiaga, a NASA Einstein Postdoctoral Fellow and Kavli Institute for Cosmological Physics Fellow working with Holz at UChicago. “And this gives you a measure of the expansion of the universe.”

The new method is particularly appealing because it results in fewer uncertainties created by gaps in our scientific knowledge. “By using the entire population of black holes, the method can calibrate itself, directly identifying and correcting for errors,” Holz said. Could this method finally provide the answers we have been looking for?