Colliding black holes could help measure the universe's expansion rate

Astronomers at the University of Chicago have proposed a new method for how to use pairs of colliding black holes to measure the expansion rate of the universe. The new technique, dubbed spectral siren, may be able to tell us about the otherwise elusive "teenage" years of the universe, the scientists think.

Scientists have known for almost a century that the universe is expanding, but exactly how fast space is stretching, a value known as the Hubble constant, has remained stubbornly elusive. The new study offers a way to make this calculation, using special detectors that pick up the cosmic echoes of black hole collisions.

The collision of two black holes is so powerful event that it literally creates a ripple in space-time that travels across the universe. The signal from each collision contains information about how massive the black holes. However, the signal has been travelling across space, and during that time the universe has expanded, which changes the properties of the signal. If astronomers can figure out a way to measure how that signal changed, they can calculate the expansion rate of the universe.

In their new paper, UChicago astronomers Daniel Holz and Jose María Ezquiaga suggest that they can use the newfound knowledge about the whole population of black holes as a calibration tool.

While other methods used to calculate the Hubble constant rely on our current understanding of the physics of stars and galaxies, which involves a lot of complicated physics and astrophysics, this new black hole method relies almost purely on Einstein's well-studied theory of gravity, which has stood up against all the ways scientists have tried to test it so far.

"By using the entire population of black holes, the method can calibrate itself, directly identifying and correcting for errors," Holz said in a statement.