Black Hole Enigma –Does the Event Horizon Exist?

Supermassive black hole


Could there be as yet unknown supermassive objects lurking at the hearts of galaxies other than black holes? An object so strange that it has managed to avoid gravitational collapse to form a singularity, the smallest object in the universe in an infinitely Planck-scale space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate?

In 2017, astronomers at The University of Texas at Austin and Harvard University put the basic principle of black holes–cosmic entities of such great gravity that nothing can escape their grip, are surrounded by a so-called event horizon– to the test, showing that matter completely vanishes when pulled in. Their results constitute another successful test for Albert Einstein’s General Theory of Relativity.

Though widely accepted, the existence of event horizons, where the escape velocity for an object would have to exceed the speed of light, has not been proved.

Do Event Horizons Exist?

“Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” said Pawan Kumar, a professor of astrophysics at The University of Texas at Austin. The event horizon, it is a horizon beyond which we cannot see, hides the singularity at the center of the black hole where the known laws of physics break down. The cosmic censorship conjecture states that all singularities are hidden by an event horizon and this failure of the laws of physics is not observable.

While a singularity has no surface area, the team suggests, “the non-collapsed object would have a hard surface. So material being pulled closer—a star, for instance—would not actually fall into a black hole, but hit this hard surface and be destroyed.”

Kumar, Wenbin Lu, a theoretical astrophysicist currently at Caltech, and Ramesh Narayan, a theorist from the Harvard-Smithsonian Center for Astrophysics, developed a test to determine which idea is valid. “Our motive is not so much to establish that there is a hard surface,” Kumar said, “but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes.”

The team figured out what a telescope would see when a star hit the hard surface of a supermassive object at the center of a nearby galaxy: The star’s gas would envelope the object, shining for months, perhaps even years.

The Search for the Hard Surfaced Objects

“We estimated the rate of stars falling onto supermassive black holes,” Lu said. “Nearly every galaxy has one. We only considered the most massive ones, which weigh about 100 million solar masses or more. There are about a million of them within a few billion light-years of Earth.”

They then searched a recent archive of telescope observations. Pan-STARRS, a 1.8-meter telescope in Hawaii, recently completed a project to survey half of the northern hemisphere sky. The telescope scanned the area repeatedly during a period of 3.5 years, looking for “transients”—things that glow for a while and then fade. Their goal was to find transients with the expected light signature of a star falling toward a supermassive object and hitting a hard surface.

“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,” Lu said.

But not one hard-surfaced object has as yet been detected.

Event Horizons Validated (so far)

“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,” Narayan said. “General Relativity has passed another critical test.”

Now the team is proposing to improve the test with an even larger telescope: the 8.4-meter Large Synoptic Survey Telescope (LSST, now under construction in Chile). Like Pan-STARRS, LSST will make repeated surveys of the sky over time, revealing transients—but with much greater sensitivity.

This research was published in the journal Monthly Notices of the Royal Astronomical Society.

Editorial Note: I think it is too early to tell whether there is a contradiction between the new result reported in today’s post and Netta Engelhardt’s calculation that corrects Hawking’s 1974 formula indicating that information does, in fact, escape black holes via their radiation. The new result should first be confirmed by other teams, ideally using different (and larger) data sets, and it needs to be compared to a quantitative observational signature of the escaping information Netta predicts. 

I’m sure this will be an area of increased active research given the recent developments and the many sky surveys coming online and producing data, like the ongoing Pan-STARRS and the future LSST.

Avi Shporer, with the MIT Kavli Institute for Astrophysics and Space Research via University of Texas at Austin

Image credit: With thanks to Pixabay