Black hole collisions could help us measure how fast the universe is expanding
In a new study, two University of Chicago astrophysicists laid out a method for how to use pairs of colliding black holes (shown as an artist’s rendition above) to measure how fast our universe is expanding. Credit: Simulating eXtreme Spacetimes (SXS) Project

Scientists may have found a way to tell us about the history of the universe by using a black hole's final moments.

In a new study published in Physical Review Letters, two University of Chicago astrophysicists show how to use pairs of colliding black holes to measure how fast our universe is expanding.

The scientists think the new technique may be able to tell them about the teenage years of the universe.

There is a ruler in the universe.

The Hubble constant is how fast the universe is expanding. Scientists are eager to find alternate ways to measure this rate because of the different methods that have been used so far. Our understanding of fundamental questions like the age, history and makeup of the universe are affected by the number's accuracy.

This calculation can be made using special detectors that pick up the echoes of black holes.

Twice a year, two black holes will slam into each other, creating ripples in space-time that travel across the universe. On Earth, the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Italian observatory Virgo are able to pick up those ripples.

The readings from almost 100 pairs of black holes have been collected over the past few years.

Information about the size of the black holes is contained in the signal from each collision. The signal has traveled across space and the universe has expanded, which affects the signal's properties. If you put a black hole earlier in the universe, the signal would change and it would look like a bigger black hole than it actually is.

The expansion rate of the universe can be calculated if scientists can figure out how that signal changed. They don't know how much has changed from the original.

In their new paper, the authors suggest that we can use our knowledge of the whole population of black holes as a calibration tool. Evidence shows that most of the black holes have between five and 40 times the mass of the sun. "So we measure the mass 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." This shows you the expansion of the universe.

The "spectral sirens" method is a new approach to the "standard sirens" method which has been pioneered. The'standard candle' methods are used in astronomy.

As LIGO's capabilities expand, the method may provide a unique window into the "teenage" years of the universe, that are hard to study with other methods.

The microwave background of the universe can be used to look at the earliest moments of the universe, and the universe's more recent history can also be studied. The in-between period is of special scientific interest.

We are interested in studying the transition from dark matter being the main force in the universe to dark energy being the main force in the universe.

The authors said that there are less uncertainties created by gaps in our scientific knowledge. The method can be used to identify and correct errors by using the entire black hole population. The Hubble constant is calculated using methods that involve a lot of complicated physics and astrophysics. If there is something we don't know, the measurement might be thrown off.

The new black hole method is based on Einstein's theory of gravity, which has stood up against all the other ways scientists have tried to test it.

Calibration will be more accurate if they have more readings from black holes. In a few years, we should have thousands of these signals, and more in the next decade or two. It would be a great way to learn about the universe.

More information: Jose María Ezquiaga et al, Spectral Sirens: Cosmology from the Full Mass Distribution of Compact Binaries, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.061102 Journal information: Physical Review Letters