In astrophysics, there is a saying that “black holes have no hair”.
This means that in general relativity, black holes are exceptionally simplistic objects. All you need to describe a black hole is its mass, electrical charge, and spin rate. With those three numbers alone, you have everything there is to know about black holes. In other words, they’re bald, and they don’t have any extra information.
This aspect of black holes is very frustrating for astrophysicists, who desperately want to understand how these cosmic giants operate. But since black holes have no “hair”, there is no way to learn more about them and what causes them to move. Unfortunately, black holes remain some of the most intriguing and mysterious objects in the universe.
But the concept of “hairless” black holes builds on our current understanding of general relativity, as originally formulated by Albert Einstein. This relativistic image focuses on the curvature of space-time. Any entity that has mass or energy will curve space-time around it, and that curvature tells those entities how to move.
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But this is not the only way to build the theory of relativity. A completely different approach would focus instead on “warp” rather than on the curvature of space-time. In this picture, any entity that has mass or energy has spacetime wrapped around it, and this twist tells other objects how to move.
The two approaches, one based on bending and the other based on torsion, are mathematically equivalent. But because it was Einstein who developed curvature-based language first, it has been used on a much larger scale. The torsion approach, known as “parallel” gravity for its mathematical use of parallel lines, provides plenty of scope for interesting theoretical insights that are not evident in the curvature approach.
For example, a team of theoretical physicists recently discovered how parallel dimensional gravity might deal with the problem of black hole hair. They detailed their work in a paper published in preprint database arXiv in July. (The research has not yet been peer-reviewed.)
The team investigated possible extensions of general relativity using what’s called a scalar domain, a quantum object that inhabits all of space and time. A famous example of a scalar domain is the Higgs boson, which is responsible for giving many particles their mass. There may be additional scalar fields populating the universe and subtly altering how gravity works. Physicists have long used these scalar fields in their attempts to explain the nature of cosmic mysteries such as dark matter and dark energy.
In general relativity based on uniform curvature, there are only a few ways to add scalar fields. But in parallel faraway gravity, there are many more options. The research team has discovered a way to add scalar fields to general relativity using the parallelism framework. Next, they used this approach to investigate whether these scalar fields, which might otherwise be invisible, might appear near black holes.
The end result: scalar fields added to general relativity, when explored through the parallel lens, have given black holes some hair.
The “hair” in this case is the presence of a strong scalar field near the black hole’s event horizon. More importantly, this scalar field carries information about the black hole within it, allowing scientists to understand more about black holes without having to dive into them.
Now that the researchers have determined how to give black holes some hair, they next need to work on the observational consequences of these findings. For example, future gravitational-wave observations may reveal subtle signatures of these scalar fields in the collisions of black holes.
