by Physicists consider black holes to be one of the most mysterious things in existence. Ironically, they are also considered to be one of the simplest. For years, physicists like me have been trying to prove that black holes are more complicated than they seem. And a newly approved European space mission called Liza will help us in this hunt.
Research from the 1970s suggests that you can comprehensively describe a black hole using only three physical properties – mass, charge and spin. All other properties of these dying massive stars, such as their detailed composition, density and temperature profiles, disappear as they transform into a black hole. That’s how simple they are.
The idea that black holes have only three characteristics is called the “hairless” theorem, implying that they have no “hairy” details that complicate them.
Hairy black holes?
For decades, researchers in the astrophysics community have been exploiting loopholes or workarounds within the assumptions of the hairless theorem to find cases of hairy black holes. A supermassive black hole has a physical property that scientists can measure – in principle – beyond its mass, charge or spin. This property must be a permanent part of its structure.
About a decade ago, Stefanos Aretakis, now a physicist at the University of Toronto, showed mathematically that a black hole with the maximum possible charge – known as an extreme charge black hole – would develop a “hair” on its horizon. A black hole’s horizon is the boundary beyond which anything that crosses it, not even light, can escape.
Aretakis’ analysis was more of a thought experiment using a very simplified physical scenario, so it’s not something scientists expect to observe astronomically. But supercharged black holes may not be the only type that can have hair.
Since astrophysical objects such as stars and planets are known to spin, scientists expect black holes to spin as well, based on how they form. Astronomical evidence shows that black holes have spin, although researchers do not know what the typical spin value is for an astrophysical black hole.
Using computer simulations, my team recently discovered similar types of hair in black holes that are spinning at a maximum rate. This hair relates to the rate of change, or gradient, of the curvature of space-time at the horizon. We also found that a black hole doesn’t have to be spinning as much as possible to have hair, which is significant because the most spinning black holes in nature probably don’t form.
Hair detection and measurement
My team wanted to develop a way to measure this hair – a new constant property that could characterize a black hole beyond its mass, spin and charge. We began by looking at how such a new property might leave a signature on a gravitational wave emitted by a rapidly spinning black hole.
A gravitational wave is a tiny disturbance in space-time usually caused by violent astrophysical events in the universe. Collisions of dense astrophysical objects such as black holes and neutron stars release strong gravitational waves. These waves are routinely detected by an international network of gravitational observatories, including the Laser Interferometer Gravitational-Wave Observatory in the United States.
Our recent studies suggest that these hairy characteristics can be measured from the gravitational data of rapidly spinning black holes. Looking at the gravitational wave data gives you the kind of signature that could indicate whether the black hole has this type of hair.
Our ongoing studies and recent advances by Som Bishoyi, a student on the team, are based on a blend of theoretical and computational models of rapidly spinning black holes. Our results have not yet been tested in the field or observed in real black holes out in space. But we hope that will change soon.
LISA gets permission to proceed
In January 2024, the European Space Agency formally accepted the mission based on the Space Laser Interferometer, or LISA. LISA will look for gravitational waves, and the data from the mission could help my team with our piercing black hair questions.
Formal acceptance means the project is allowed to move forward to the construction phase, with a planned launch in 2035. LISA consists of three spacecraft configured in a perfect equilateral triangle that will track behind the Earth around the Sun. Each spacecraft will be 1.6 million miles (2.5 million kilometers) apart, and will exchange laser beams to measure the distance between each other down to about a billionth of an inch.
LISA will detect gravitational waves from supermassive black holes that are millions or even billions of times larger than our Sun. It will map the spacetime around rotating black holes, helping physicists understand how gravity works in the immediate vicinity of black holes to an unprecedented level of accuracy. Physicists hope that LISA will also be able to measure any hairy characteristics that black holes may have.
With LIGO making new observations every day and LISA providing a glimpse into the space-time around black holes, it’s now one of the most exciting times to be a black hole physicist.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Gauav Khanna, University of Rhode Island
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The article presents work done in collaboration with Stefanos Aretakis, Kevin Gonzalez-Quesada, Lior Burko, Subir Sabharwal and Som Bishoyi. This research was supported by the US National Science Foundation. All calculations were performed at the Massachusetts Green High Performance Computing Center leveraging the resources of the URI Center for Computational Research. The author also acknowledges support from the UMass-URI Gravity Research Consortium (U2GRC).
