Physicists consider black holes to be one of the most mysterious objects in existence. Ironically, they are also considered among the simplest. Physicists like me have been trying for years to prove that black holes are more complex than they seem. And a recently approved European space mission called LISA will help us in this hunt.
Research from the 1970s shows that you can comprehensively describe a black hole using just three physical characteristics: their mass, charge and spin. All other properties of these massive dying stars, such as their detailed composition, density and temperature profiles, disappear as they turn into a black hole. They are that simple.
The idea that black holes have only three properties is called the ‘no-hair’ theorem, which implies that they have no ‘hairy’ details to complicate them.
Hairy black holes?
For decades, researchers in the astrophysics community have exploited loopholes or workarounds within the assumptions of the no-hair theorem to dream up potential hairy black hole scenarios. A hairy black hole has a physical property that scientists can in principle measure that goes beyond its mass, charge or spin. This feature must be a permanent part of the structure.
About a decade ago, Stefanos Aretakis, a physicist currently at the University of Toronto, showed mathematically that a black hole containing the maximum charge it can hold – called an extremely charged black hole – would develop “hair” on its horizon . A black hole’s horizon is the boundary where anything that crosses it, even light, cannot escape.
Aretakis’ analysis was more of a thought experiment using a highly simplified physics scenario, so it’s not something scientists expect to observe astrophysically. But supercharged black holes may not be the only kind that can have hair.
Since astrophysical objects such as stars and planets are known to spin, scientists expect black holes to spin as well, depending on how they form. Astronomical evidence has shown that black holes do indeed 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 spinning at maximum speed. This hair has to do with the rate of change, or gradient, of the curvature of space-time at the horizon. We also discovered that a black hole doesn’t really have to be spinning at maximum speed to have it, which is important because these maximum spinning black holes probably don’t arise in nature.
Detect and measure hair
My team wanted to develop a way to potentially measure this hair – a new solid property that could characterize a black hole beyond its mass, spin and charge. We set out to investigate how such a new property could leave a signature on a gravitational wave emitted by a rapidly spinning black hole.
A gravitational wave is a small disturbance in space-time usually caused by violent astrophysical events in the universe. The collisions of compact astrophysical objects such as black holes and neutron stars emit strong gravitational waves. An international network of gravity observatories, including the Laser Interferometer Gravitational-wave Observatory in the United States, routinely detects these waves.
Our recent studies suggest you can measure these hairy properties from gravitational wave data for rapidly spinning black holes. Looking at the gravitational wave data offers the possibility of some kind of signature that could indicate whether the black hole has this kind of hair.
Our ongoing research and the recent progress made by Som Bishoyi, a student on the team, are based on a mix of theoretical and computational models of rapidly spinning black holes. Our findings have not yet been field tested or observed in real black holes in space. But we hope that will change soon.
LISA gets the green light
In January 2024, the European Space Agency formally adopted the space-based Laser Interferometer Space Antenna (LISA) mission. LISA will look for gravitational waves, and the mission’s data can help my team with our tough questions about black holes.
Formal approval means the project gets the green light to move into 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. The spacecraft will each be 1.5 million miles apart, and they will exchange laser beams to measure the distance between each other to within about a billionth of an inch.
LISA will detect gravitational waves from supermassive black holes that are millions or even billions of times more massive than our Sun. It will create a map of space-time around rotating black holes, allowing physicists to understand with an unprecedented level of accuracy how gravity works in the immediate vicinity of black holes. Physicists hope that LISA can also measure any hairy properties of black holes.
With LIGO making new observations every day and LISA providing a glimpse into the space-time around black holes, this is one of the most exciting times to be a black hole physicist.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It is written by: Gaurav 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 computations were performed at the Massachusetts Green High Performance Computing Center, utilizing the resources of the URI Center for Computational Research. The author also acknowledges the support of the UMass-URI Gravity Research Consortium (U2GRC).