Near the center of the Milky Way galaxy is a massive object astronomers call Sagittarius A*. This “supermassive” black hole may have grown along with our galaxy, and it’s not alone. Scientists believe that a similar giant lies at the heart of almost all the 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 dedicated his career to studying the behavior of these difficult-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 tried to determine what you might find if you could place each of these black holes one after the other on a gigantic scale. Their 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, shedding light on the forces that shaped objects like Sagittarius A* as they grew from tiny black holes to 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 very early on,” Simon said. He published his findings 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 investigating a phenomenon known as “gravitational wave background.” The concept refers to the constant stream of gravitational waves, or giant waves in space and time, that propagate through the universe almost constantly. This cosmic drive also originates from supermassive black holes. Simon explained that if two galaxies collide with each other in space, the central black holes could also collide and even merge. They spin 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 to gravitational waves, scientists first need to know how massive supermassive black holes in the universe really are. Bigger saucers create a bigger explosion and produce many more gravitational waves, Simon said.
There is only one problem. “We already have good measurements of the masses of supermassive black holes in our galaxy and nearby galaxies,” he said. “We don’t have the same kind of measurements for distant galaxies. We just have to guess.”
In his new investigation, Simon decided to guess in a whole 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 look at distant galaxies, they’re looking back in time.) Simon used this information to calculate the approximate masses of the black holes in the largest galaxies in the universe. He then used computer models to simulate the background gravitational waves that these galaxies would create and that currently wash over 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 many more large galaxies scattered throughout the universe billions of years ago than some previous studies had predicted. It did not 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, their research adds to a growing body of evidence that suggests they may not need as long as astrophysicists believed. For example, the NANOGrav team saw similar signs of giant black holes lurking in the universe billions of years ago. For now, Simon hopes to explore the full range of black holes that extend even further back in time, revealing clues to how the Milky Way and eventually our solar system came to be.