Since the first observation of black hole mergers by the LIGO Laser Interferometer’s Gravitational Wave Observatory in 2015, astronomers have been repeatedly surprised by their large masses. Although they do not emit light, black hole mergers are observed through their emission of gravitational waves – ripples in the fabric of space-time that were predicted by Einstein’s theory of general relativity. Physicists originally expected black holes to have masses less than about 40 times that of the Sun, because fused black holes arise from massive stars, which may ‘not stick together if they get too big.

The LIGO and Virgo observatories, however, have found many black holes with masses greater than 50 suns, some as massive as 100 suns. Numerous formation scenarios have been proposed to produce such large black holes, but no single scenario has been proposed that has been able to explain the diversity of black hole mergers observed so far, and there is no agreement on which combination of formation scenarios is physically feasible. . This new study, published in Astrophysical Journal Letters, is the first to show that the masses of large and small black holes can result from a single path, in which black holes gain mass from the expansion of the universe itself.

Astronomers often model black holes within a universe that cannot expand. It is an assumption that it simplifies Einstein’s equations because a universe that does not grow has much less to follow. However, there is a trade-off: predictions can only be reasonable for a limited period of time.

Because the individual events detectable by LIGO – Virgo only last a few seconds, when analyzing an individual event, this simplification is sensible. But these same mergers are potentially billions of years in the making. During the time between the formation of a pair of black holes and their eventual merger, the universe grows deeply. If the more subtle aspects of Einstein’s theory are carefully considered, then a surprising possibility arises: the masses of black holes could grow in unison with the universe, a phenomenon that Croker and his team call cosmological coupling.

To investigate this hypothesis, the researchers simulated the birth, life, and death of millions of pairs of large stars. Any pair in which both stars died to form black holes was related to the size of the universe, beginning at the time of their death. As the universe continued to grow, the masses of these black holes grew as they spiraled around each other. The result was not only more massive black holes when they merged, but also many more mergers. When the researchers compared the LIGO – Virgo data with their predictions, they matched reasonably well. “I have to say, I didn’t know what to think at first,” said research co-author and University of Michigan professor Gregory Tarle. “It was such a simple idea, I was surprised that it worked so well. ”

According to the researchers, this new model is important because it does not require any change in our current understanding of stellar formation, evolution, or death. The agreement between the new model and our current data comes simply from recognizing that realistic black holes do not exist in a static universe. However, the researchers were careful to emphasize that the mystery of LIGO, Virgo’s huge black holes, is far from solved.

Kevin S. Croker, Michael Zevin, Duncan Farrah, Kurtis A. Nishimura, Gregory Tarl & 2013265929;. Cosmologically Coupled Compact Objects: A Single Parameter Model for LIGO-Virgo Redshift and Mass Distributions. The Astrophysical Journal Letters, 2021; 921 2: L22 DOI: 10.3847 / 2041-8213