Massive, Colliding Black Holes May Expand Along With the Universe

Scientists may have solved the mystery of how most massive black holes are formed.

Researchers from the University of Hawaiʻi at Mānoa, the University of Chicago, and the University of Michigan at Ann Arbor, propose that black holes with masses that were previously unexplainable could be growing hand-in-hand with the accelerating expansion of the Universe.

The phenomenon could be an example of what the team calls "cosmological coupling."

Since the first detection of gravitational waves from a black hole merger event in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), scientists have been surprised by how large some of the black holes involved in these events seem to be.

Until they began measuring gravitational waves, the tiny ripples in spacetime originally predicted by Albert Einstein in his 1915 theory of general relativity, researchers had believed that the black holes in merger events should be no larger than about 40 times that of the sun, or 40 solar masses.

This is because the merging black holes are created from collapsing stars in binary systems, where the stars form and evolve together and both eventually form a black hole. These binary stars shouldn't be able to hold themselves together at masses greater enough to leave behind a 40 solar mass stellar remnant when they undergo the complete gravitational collapse that births a black hole.

However, the LIGO observatory and its fellow gravitational wave detector VIRGO frequently observe mergers that involve black holes as great as 50 solar mass, or even 100 solar masses.

While a range of formation scenarios have been put forward to explain the mass of these merging black holes, none have successfully explained the diversity of black hole mergers observed so far.

This new research puts forward a single formation pathway that could lead to both small black holes masses and larger masses. The study, published in Astrophysical Journal Letters, links this formation pathway to the accelerating expansion of the Universe, rather than modeling black holes in a static, non-expanding Universe.

"It's an assumption that simplifies Einstein's equations because a universe that doesn't grow has much less to keep track of," Kevin S. Croker, professor at the UH Mānoa Department of Physics and Astronomy and the paper's first author, explained. "There is a trade-off though: predictions may only be reasonable for a limited amount of time."

Only considering a static Universe is something that makes sense in the context of gravitational-wave detections because the individual events picked up by LIGO and VIRGO last just seconds. But, the actual collision events occurring in the Universe take billions of years to occur.

This means that in the time it takes for these black holes to form and eventually merge, the Universe has expanded considerably. Working in some of the subtle elements of general relativity led the team to conclude that the masses of black holes could be increasing in lockstep with this universal expansion.

Croker and his fellow authors label this "cosmological coupling." And this isn't the only example of such a coupling.

Perhaps the most well-known example of cosmological coupling is the loss of energy from light as the Universe expands. This leads to a so-called "redshift" in light. As the Universe expands it lengthens the wavelength of light, pulling it away from the high-energy blue area of the electromagnetic spectrum to the less energetic red region.

This phenomenon increases the further away a galaxy is and thus the more extreme the redshift is. Not only has it become a key measuring stick for astronomers, but it was also key in Edwin Hubble's discovery that the Universe was expanding. It also led to the later discovery in the 1990s that this expansion is accelerating.

"We thought to consider the opposite effect," said the paper's co-author and UH Mānoa Physics and Astronomy Professor Duncan Farrah. "What would LIGO–Virgo observe if black holes were cosmologically coupled and gained energy without needing to consume other stars or gas?"

To investigate this possibility, the team created a simulation of the birth, life, and gravitational collapse of millions of large stars in binary systems. With pairings that saw both stars leave behind black holes, the team linked the size of these black holes to the size of the Universe at the time of their formation.

They discovered that as the Universe expanded in their model, the masses of the black holes also grew as they spiraled towards each other. What resulted from this was an increase in merger events and an increase in events that involved increasingly massive black holes.

The team then compared the results of their simulations to data collected by LIGO and VIRGO, finding a good agreement between the two. This came as a surprise to some of the paper's authors themselves.

Co-author and University of Michigan Professor Gregory Tarlé said: "I have to say I didn't know what to think at first. It was such a simple idea, I was surprised it worked so well."

One advantage that this pathway of black hole evolution has over other competing models used to explain how massive black holes form and merge is that it doesn't require any changes to our current understanding of the formation, life, and death of stars.

The team will further test this theory as gravitational wave detectors become increasingly sensitive and provide further data that could verify the connection between the expansion of the Universe and black hole masses.

The researchers say that despite these positive findings, the mystery of massive black holes in mergers detected by gravitational waves might not yet be solved.

"Many aspects of merging black holes are not known in detail, such as the dominant formation environments and the intricate physical processes that persist throughout their lives," said research co-author and NASA Hubble Fellow Michael Zevin. "While we used a simulated stellar population that reflects the data we currently have, there's a lot of wiggle room.

"We can see that cosmological coupling is a useful idea, but we can't yet measure the strength of this coupling."