In astrophysics, it is said that “black holes have no hair”. This peculiar expression is based on the theory of general relativity because black holes are very simple objects. All that is needed to describe them are their mass, electric charge, and rotation speed. With just these three pieces of information, one can describe a black holes. So, in other words, they do not possess additional information.

This aspect is extremely frustrating for astrophysicists. For decades, they have been desperately trying to understand how these cosmic giants work. But since black holes lack additional information, there is no way to know more about them and what makes them function. For this reason, they remain some of the most enigmatic and mysterious objects in the universe.

Twisting of space-time

The concept of “hairless” black holes is based on our current understanding of general relativity, as originally formulated by Albert Einstein. Relativity focuses on the curvature of space-time. Any entity with mass or energy will bend space-time around itself, and that bending determines the motion of these entities.

However, this is not the only approach to the theory of relativity. A completely different approach focuses on “twisting” rather than curvature of space-time. In this view, any entity with mass or energy twists space-time around itself, and this twist instructs other objects on how to move.

The two approaches, one based on curvature and the other on twisting, are mathematically equivalent. But since Einstein first developed the language based on curvature, it is much more widely used. The twisty approach, known as “teleparallel gravity” for its mathematical use of parallel lines, offers a lot of room for intriguing theoretical insights that are not obvious in the curvature approach.

Teleparallel gravity

For example, a team of theoretical physicists recently explored how teleparallel gravity could address the issue of black holes. Their work was posted on the preprint database arXiv in July (ref.). The physicists examined potential extensions of general relativity using what is called a scalar field. A famous example of a scalar field is the Higgs boson, which is responsible for giving mass to many particles.

There could be additional scalar fields populating the universe and altering the functioning of gravity. Physicists have long used scalar fields in an attempt to explain the nature of cosmic mysteries like dark matter and dark energy.

In regular curvature-based general relativity, there are only a certain number of ways to add scalar fields. But in teleparallel gravity, there are many more options. The research group found a way to add scalar fields to general relativity using the teleparallel structure. They then used this approach to investigate whether these scalar fields, which would otherwise be invisible, could appear near black holes.

If and how astrophysics will change

The result of the study is that scalar fields, when added to general relativity and explored through the teleparallel lens, provided new information about black holes. The “hair” is the presence of a strong scalar field near the event horizon. Essentially, this scalar field carries information inside the black holes. This would allow scientists to understand more about these cosmic objects without having to dive into them.

Now that researchers have identified this possibility, they must work on the observational consequences of these findings. For example, future observations of gravitational waves could reveal subtle traces of these scalar fields in black holes collisions.

Stefano Gallotta
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