Unraveling the Mystery of 'Hairless' Black Holes in Astrophysics

 The phrase "a black hole is hairless" is a whimsical expression in astrophysics. It refers to the concept in relativity theory that black holes are entities whose characteristics can be fully defined by their electric charge, mass, and rotation speed. This alone could spark a fascinating discourse about these celestial objects.

In other words, they are bare, devoid of any extra information. This feature of black holes is quite perplexing for astrophysicists striving to comprehend the functioning of these cosmic giants. However, the hairless nature of black holes leaves them enigmatic to experts, making their operation unclear. 

Black hole: An enigma in space 

Regrettably, black holes are still considered one of the most perplexing and enigmatic entities in the universe. The notion of 'bald' black holes is rooted in our current comprehension of general relativity, originally conceived by Albert Einstein. This relativity perspective emphasizes space-time curvature. Any object with mass or energy will cause the space-time around it to curve, and this curvature instructs these objects on how to move.

Manipulation of Space-Time

However, there's another way to construct a relativity theory. As Live Science states, there's a completely different approach that centers on 'twisting' space-time rather than curving it.

In this theory, any object with mass or energy causes space-time to twist around it, and this twist dictates how other objects should move. Both methods, one based on curvature and the other on torsion, are mathematically identical. But since Einstein first introduced the curvature-based language, it has been more widely adopted.

The torsion approach, dubbed "teleparallel" gravity due to its mathematical application of parallel lines, offers ample room for fascinating theoretical insights that are not apparent in the curvature approach.

For instance, a group of theoretical physicists recently probed how teleparallel gravity could address the issue of black hole 'hairiness.' They documented their work in a paper published in July in the arXiv preprint database (the research is still being peer-reviewed).

The group examined potential extensions of general relativity using a scalar field, a quantum entity that exists throughout space and time. A well-known example of a scalar field is the Higgs boson, which imparts mass to numerous particles. There could be other scalar fields present in the universe subtly modifying gravity's behavior, and these scalar fields have been used by physicists for years to account for cosmic enigmas like dark matter and "dark energy."

Regarding standard curvature

In standard curvature-based general relativity, there are limited ways to include scalar fields. However, in teleparallel gravity, there are much more options. This research team discovered a method to incorporate scalar fields into general relativity using the teleparallel framework. They then employed this method to investigate if these typically invisible scalar fields could appear near black holes.

The findings

The outcome: when scalar fields were included into general relativity and examined through the teleparallel perspective, black holes developed hair. The 'hair' in this context is a strong scalar field present near a black hole's event horizon. Essentially, this scalar field carries information about the black hole itself, allowing scientists to learn more about black holes without needing to venture inside them.

Now that scientists have discovered how to give black holes hair, they need to explore the observational implications of these findings. For instance, future gravitational wave observations might reveal subtle evidence of these scalar fields during black hole collisions. Could it be possible, at least theoretically, to escape these space behemoths one day?