Following the detection of a merger of 30 solar-mass black holes by LIGO, astronomers began to wonder how prevalent black holes this size exist and how frequently they merge. This research was carried out by astronomers from the University of California, Irvine under the direction of James Bullock, a professor of physics and astronomy at UCI.
The goal of this study was to compute and classify stellar-remnant black holes using a cosmic inventory. The results indicated that the likelihood of tens of millions of black holes existing in the Milky Way is very high. This figure is significantly greater than anticipated.
“We think we’ve shown that there are as many as 100 million black holes in our galaxy,” stated James Bullock, a co-author of the research report published in the Monthly Notices of the Royal Astronomical Society.
The Laser Interferometer Gravitational-Wave Observatory, or LIGO, had discovered ripples in the space-time continuum caused by the collision of two black holes, each the size of thirty suns, a little over a year and a half before UCI launched its cosmic census.
Bullock has also stated that scientists generally believe that the majority of star remnant black holes have a mass similar to that of the sun. Some scientists were surprised to see direct evidence of black holes of such enormous sizes merging in a catastrophic collision.
The work done by UCI looked into the peculiarities of the LIGO finding theoretically. Under the direction of doctorate candidate Oliver Elbert, the study aimed to build a framework for comprehending similar future occurrences and to interpret the gravitational wave detections through the prism of current knowledge about galaxy development.
Elbert also came to the conclusion that it is feasible to determine the number and timing of black hole development in each galaxy by using the knowledge about star formation in various kinds of galaxies. Ultimately, older stars and, hence, older black holes reside in large galaxies.
Additionally, the size of a galaxy will determine how many black holes of a particular mass there are in it, according to Manoj Kaplinghat, a physics and astronomy professor at UCI and co-author of the work featured in this article. This is because metal-rich stars are more common in larger galaxies, while massive stars with low metallicity predominate in smaller dwarf galaxies.
Black holes also differ in mass. When the time comes for stars of different sizes to become black holes, their masses will likewise differ since they have different masses. The amount of mass and metallicity a star sheds throughout its lifetime also contributes to the variation in black hole masses; stars with lower metallicities will generate larger black holes since they shed less of it over time. For those with a high metallicity, the opposite is true.
James Bullock added, “We can tell how many black holes should have formed with 100 solar masses versus 10 solar masses because we have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they’re born.” “We were able to calculate the expected number of massive black holes, and it turned out to be millions—much more than I had predicted.”
The goal of the UCI research is to ascertain how often black holes occur in pairs, particularly when they merge, and how long it takes for that to happen. Additionally, they questioned whether the thirty solar-mass black holes identified by LIGO were created during the previous 100 million years and underwent a rapid merger, or if they originated billions of years earlier and just required an exceptionally long period. This investigation has illuminated the following phenomena.
Regarding the findings, Kaplinghat stated, “We demonstrate that only 0.1 to 1 percent of the black holes formed must merge to explain what LIGO observed.” An outstanding problem is, of course, how the black holes will reach close enough to merge in a reasonable amount of time.
Elbert, the lead researcher, stated that he anticipates many gravitational wave observations that will help scientists determine whether or not black holes collide in massive galaxies most of the time. He stated that such a discovery would highlight a crucial aspect of the physics causing them to come together. In addition, Kaplinghat adds the following to the conversation: “If the current ideas about stellar evolution are correct, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years.” He also notes that this finding is relatively close.
