A New Study Demonstrates Black Holes Grow as the Universe Expands

This image depicts two black holes on a collision course in which they merge to become one.

A group of researchers from the University of Hawaiʻi at Mānoa, the University of Chicago, and the University of Michigan at Ann Arbor published a paper on November 3 proposing black holes grow as the universe expands. They developed this hypothesis after observing black holes with unexpectedly large masses.

After the LIGO facility detected gravitational waves in 2015, astronomers worked to calculate the masses of the two black holes that had collided to produce the ripples in spacetime. The astronomers expected to find two stellar black holes, which form from the collapse of large stars and have a mass between five and 30 times that of our solar system’s sun. However, the black holes detected had masses of 65 and 85 suns.

The artwork above portrays a stellar-mass black hole called IGR J17091-3624 absorbing the gas from another star.

One explanation for why these black holes are much larger than expected is that they grew from absorbing matter, such as dust, gas, stars, or other black holes. The new study published in the Astrophysical Journal Letters posits black holes gain mass from the expansion of the Universe itself, a process the astronomers are calling cosmological coupling.

Like IGR J17091-3624, this stellar black hole swallows gas from the stars around it, thus causing it to grow.

Physicists typically model black holes in simulated universes, which do not expand. To test their hypothesis, researchers accounted for this expansion in the new study. They then demonstrated how black holes grow over time in an expanding universe by simulating millions of pairs of large stars’ births, lives, and deaths.

This new model does not require changes to the modern understanding of stellar evolution. For this reason, it is deemed a breakthrough in astrophysics and the modern understanding of the universe.