Near the center of the Milky Way galaxy is a massive object that astronomers call Sagittarius A*. This “supermassive” black hole may have grown along with our galaxy, and it is not alone. Scientists believe that a similar giant lies at the heart of almost all large galaxies in the universe.
Some of them can get really big, said Joseph Simon, a postdoctoral researcher in the Department of Astrophysics and Planetary Sciences at the University of Colorado Boulder.
“The black hole at the center of our galaxy is millions of times more massive than the Sun, but we also see others that we think are billions of times the mass of the Sun,” he said.
The astrophysicist has devoted his career to studying the behavior of these hard-to-observe objects. In a recent study, he used computer simulations, or “models,” to predict the masses of the largest supermassive black holes in the universe — a mathematical concept known as the black hole mass function.
In other words, Simon sought to determine what you might find if you could put each of these black holes one after the other on a gigantic scale.
His calculations suggest that billions of years ago, black holes may have been much larger on average than scientists previously suspected. The findings could help researchers solve a larger mystery, elucidating the forces that shaped objects like Sagittarius A* as they grew from tiny black holes into the giants they are today.
“We’re starting to see from a variety of different sources that there were very massive things in the universe from very early on,” Simon said.
He published his findings on May 30 in Astrophysical Journal Letters.
For Simon, these “very bulky things” are his bread and butter.
The astrophysicist is part of a second research effort called the North American Nanohertz Gravitational-Wave Observatory (NANOGrav). With the project, Simon and hundreds of other scientists in the United States and Canada have spent 15 years researching a phenomenon known as the “gravitational wave background.” The concept refers to the constant stream of gravitational waves, or giant ripples in space and time, that ripple through the universe on an almost constant basis.
This cosmic momentum also has its origins in supermassive black holes. Simon explained that if two galaxies collided with each other in space, the central black holes might also collide and even merge. They whirl around before crashing into each other like two cymbals in an orchestra – only this cymbal generates gravitational waves, literally warping the fabric of the universe.
To understand the background of gravitational waves, scientists first need to know how massive the supermassive black holes in the universe really are. Bigger cymbals create a bigger blast and produce much more gravitational waves, Simon said.
There is only one problem.
“We already have good measurements of the masses of the supermassive black holes of our galaxy and nearby galaxies,” he said. “We don’t have the same kinds of measurements for distant galaxies. We just have to guess.”
Black holes are on the rise
In his new research, Simon decided to guess in a completely new way.
First, he collected information on hundreds of thousands of galaxies, some billions of years old. (Light can only travel so fast, so when humans observe distant galaxies, they’re looking back in time.) Simon used this information to calculate approximate black hole masses for the largest galaxies in the universe. Then he used computer models to simulate the background gravitational waves these galaxies would create that currently wash Earth.
Simon’s findings reveal the full range of supermassive black hole masses in the universe dating back nearly 4 billion years. He also noticed something strange: there seemed to be a lot more large galaxies scattered throughout the universe billions of years ago than some previous studies had predicted. It didn’t make sense.
“There was an expectation that you would only see these really massive systems in the near universe,” Simon said. “It takes time for black holes to grow.”
However, his research adds to a growing body of evidence that suggests they may not need as much time as astrophysicists once believed. For example, the NANOGrav team saw similar signals of giant black holes lurking in the universe billions of years ago.
For now, Simon hopes to explore the full range of black holes stretching even further back in time, revealing clues about how the Milky Way, and eventually our solar system, came to be.
“Understanding the masses of black holes is critical to some of these foundational questions like the gravitational wave background, but also how galaxies grow and how our universe evolved,” Simon said.
Joseph Simon, Proxies Exploration of the Mass Function of a Black Hole’s Supermassive Mass: Implications for Pulsar Time Arrays, Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acd18e
“Infuriatingly humble alcohol fanatic. Unapologetic beer practitioner. Analyst.”