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Riddle of cannibal black hole pairs solved ... nearly: Astroboffins explain all to El Reg

Stargazers perform grav wave 'paleontology' in simulations

Artist's view of a binary black hole. Pic credit: NASA, ESA and G Bacon (STScI)
Artist's view of a binary black hole ... Pic credit: NASA, ESA and G Bacon (STScI)

Astrophysicists are one step closer to understanding how pairs of merging black holes form in the far reaches of the cosmos.

The dramatic melding of two black holes produced the first gravitational waves detected by the Earth-based Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). While these waves confirmed Einstein’s theory of general relativity, questions were raised about how the holes were formed in the first place: it's rather rare for two such terrifying voids to smash into each other.

Using computer simulations, a team of scientists from the University of Birmingham, UK, and the University of Amsterdam, in the Netherlands, has modeled the types of stars that eventually evolve to become pairs of merging black holes. Their results were published Wednesday in Nature Communications, and shed light on how massive stars wind up as cannibalistic black holes meandering through the universe.

These giant voids rarely encounter objects in their path. We're told that before two holes collide, they may start out as huge stars no further than a fifth of the distance between our Earth and the Sun apart – that's just 18.6 million miles or 29.9 million kilometres, and very close by astronomical standards.

However, black holes are typically formed from massive stars that grow to be much larger than this gap and thus are further from their neighbors than the magic distance. Therefore, the stars have to be squeezed tightly together, a scenario described as “isolated binary evolution via a common-envelope phase.”

“Getting massive stars so close together is one of the main challenges, since massive stars expand during their lives to become up to a thousand times the size of the sun. In our model, massive stars are brought close together through a process known as a common envelope, where at some point both stars orbit inside of the same envelope,” Simon Stevenson, first author of the paper and a PhD student at the University of Birmingham, explained to The Register.

According to the boffins, the stars start out at wide distances from each other, passing mass to one another within a bubble of gas and dust. As gas leaks from the system, the doomed stars lose energy, and their orbits shrink, thus bringing them closer together.

“They then spiral in towards each other, shedding this outer envelope. This leaves a much closer binary, in which the stars can eventually form black holes that merge and are observed by LIGO,” Stevenson continued.

These strict constraints make merger events rare. By scrutinizing the observations made by LIGO, the researchers can piece together the holes' histories. Professor Ilya Mandel, senior author of the paper and researcher at the University of Birmingham, compares it to “a kind of paleontology for gravitational waves.”

“A paleontologist, who has never seen a living dinosaur, can figure out how the dinosaur looked and lived from its skeletal remains. In a similar way, we can analyze the mergers of black holes, and use these observations to figure out how those stars interacted during their brief but intense lives.”

The simulations have also revealed the typical properties of stars that go on to become a merging black hole. For example, the team found that a collision of two black holes with unequal masses means that the stars were probably formed almost completely from hydrogen and helium.

The study is still in its early stages, and the team will have to observe more black hole mergers to understand how to retrace the individual steps in the lives of the massive stars. Engineers are working to increase the sensitivity of an upgraded version of LIGO, which will make the detection of black hole collisions easier. ®

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