Black holes remain a fascinating cosmic puzzle. Whenever we think we have figured them out something else pops up. New theoretical work suggests that black holes can exert pressure on their environments adding to their thermodynamic complexity.

Black holes as they were originally defined were just an object so dense that nothing, not even light can escape. The fact is already mindboggling enough but soon scientists realized that taking this object at face value was a problem for every single branch of physics, but in particular quantum mechanics and relativity.

A lot of theoretical work has gone into fixing the issues, seminal among them Hawking’s discovery in 1974 that black holes emit thermal radiation. Hawking radiation, as it is now known, allowed physicists to understand black holes through the laws of thermodynamics. They have a temperature, volume and surface area, entropy, angular momentum, etc.

Something that didn’t appear in the black hole thermodynamics but it is a staple of the regular version is pressure. At least there’s never been a need in the equations to include pressure. But in recent work, as the equations are trying to describe black holes in better and better detail, a term appeared that puzzled researchers.

As reported in Physical Review D, the term comes into the calculation of the entropy of a black hole. Their work shows that the formula that determines entropy needs small corrections and those corrections affect many properties of the black hole. And, more excitingly, the correction takes on the form of something that looks like pressure.

"Our finding that Schwarzschild black holes have a pressure, as well as a temperature, is even more exciting given that it was a total surprise. I'm delighted that the research that we are undertaking at the University of Sussex into quantum gravity has furthered the scientific communities' wider understanding of the nature of black holes,” lead author Professor Xavier Calmet, from the University of Sussex, said in a statement.

"Hawking's landmark intuition that black holes are not black but have a radiation spectrum that is very similar to that of a black body makes black holes an ideal laboratory to investigate the interplay between quantum mechanics, gravity and thermodynamics.”

This is the very focus of their work. One possible theory that bridges general relativity and quantum mechanics is called quantum gravity. Black holes are a crucial testing ground for this hypothetical framework.

"If you consider black holes within only general relativity, one can show that they have a singularity in their centers where the laws of physics as we know them must breakdown. It is hoped that when quantum field theory is incorporated into general relativity, we might be able to find a new description of black holes,” Calmet explained.

"Our work is a step in this direction, and although the pressure exerted by the black hole that we were studying is tiny, the fact that it is present opens up multiple new possibilities, spanning the study of astrophysics, particle physics and quantum physics."

The researchers saw the emergence of the corrective terms but it was only a conversation last Christmas that made them consider it could be a pressure. Further calculation showed that under quantum gravity, black holes can exert pressure.

"It is exciting to work on a discovery that furthers our understanding of black holes – especially as a research student,” added co-author Folkert Kuipers, also at Sussex.

"The pin-drop moment when we realized that the mystery result in our equations was telling us that the black hole we were studying had a pressure – after months of grappling with it – was exhilarating."