Event horizons around black holes do exist, say astroboffins

Einstein's Theory of General Relativity lasts another day

Physicists in the US have found evidence that event horizons around black holes do exist, reinforcing Einstein’s theory of General Relativity.

Black holes are strange objects that can grow to monstrous sizes. The gravitational field around them is believed to be so strong that even light cannot escape. Event horizons outline a boundary, where objects falling into the black hole are doomed, as they will eventually be sucked into the void.

But there is no direct proof that event horizons exist. After a little mathematical fudging of Einstein’s theory of General Relativity, some physicists have theorized that black holes might actually be a different class of supermassive objects, where local material is not swallowed but instead crashes onto a hard surface.

A team of physicists from the University of Texas at Austin and Harvard University in Massachusetts has put that idea to the test. The results have been published in the Monthly Notices of the Royal Astronomical Society [arXiv].

Pawan Kumar, co-author of the paper and professor of astrophysics at UT Austin, said on Tuesday: “Our motive is not so much to establish that there is a hard surface, but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes.”

The team worked backwards to figure out what would be observed if a star did indeed hit another more massive object instead of a black hole. They predicted that the star’s gas would surround the object, and keep shining for months or even years before it was destroyed by the impact.

Since nearly every galaxy harbors a supermassive black hole, the physicists calculated how many of these events should be observable. They only considered black holes weighing more than 100 million times the mass of the Sun, allowing them to narrow down to about a million of them within a few billion light-years of Earth.

Next, they searched for any “transients” – objects that flicker for a while before burning out – a sign that stars are falling and crashing into a hard surface. But the archive of telescope observations made using Pan-STARRS (the Panoramic Survey Telescope and Rapid Response System), a telescope in Hawaii, showed no such events.

“Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true,” Wenbin Lu, first author of the paper and graduate student at UT Austin, said.

“Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades,” added Ramesh Narayan, co-author of the paper and professor of natural sciences at Harvard.

“General Relativity has passed another critical test.” ®

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