A new type of black holes: hairy and surrounded by rings of elementary particles ⚫

By Romain Gervalle, Associate Researcher in Gravitation Theories, University of Tours & Mikhail Volkov, Professor of Physics, University of Tours

In our study published on October 21, 2024, in the journal Physical Review Letters, we predict the existence of a new type of black hole that would be surrounded by rings similar in shape to those of Saturn, but made up of elementary particles.


Einstein's theory of General Relativity predicts the existence of black holes: regions in spacetime where gravity is so intense that nothing, not even light, can escape. Their hypothetical existence was formulated in 1916 when the solutions to the mathematical equations of General Relativity describing black holes were obtained.

However, their actual existence was subsequently debated by scientists for nearly a century. In 1965, the theoretical work of British mathematician Roger Penrose demonstrated that black holes are inevitably formed by the gravitational collapse of stars, and it wasn't until the 1990s that astronomical observations, led by two American astrophysicists Reinhard Genzel and Andrea Ghez, revealed what appears to be a gigantic black hole at the center of our galaxy. This discovery earned them, along with Roger Penrose, the Nobel Prize in Physics in 2020.

Today, thanks to observations by the international collaboration Event Horizon Telescope, we are able to discern the shadow of the black hole located at the center of the Milky Way.

The existence of black holes in the Universe has thus been firmly established, but the discoveries do not stop there. We now predict the existence of a new type of black hole, suggested by the Standard Model of fundamental forces.

The origin of black holes

It is now commonly accepted that black holes are omnipresent in our Universe. Generally, two scenarios explain their formation.

First, there are stellar black holes, formed by the gravitational collapse of ordinary stars at the end of their life, that is, when they eventually collapse under their own weight after consuming all their fuel. Their mass typically ranges from 2-3 solar masses to tens—or even hundreds—of solar masses. After their formation, these stellar black holes can grow by absorbing surrounding matter. They can also merge with each other, with a significant emission of gravitational waves, the first detection of which was recognized by the Nobel Prize in Physics in 2017.

It is also possible that some black holes, called primordial, were formed by the collapse of primary matter during the first second after the Big Bang. The mass of these primordial black holes can be gigantic, up to billions of solar masses for the supermassive black holes located at the center of most galaxies.

But it can also be small, on the order of that of planets or asteroids, concentrated in a radius of less than one centimeter! It is therefore possible that the Universe is dotted with these tiny primordial black holes, whose future detection is a major challenge for observational astrophysics. Even lighter primordial black holes would have evaporated very quickly according to the Hawking process and would not have survived until today.

Our results suggest that some of the small primordial black holes that still exist today could possess a new property: being "hairy."

Black holes "have no hair"

Stellar black holes retain no memory of the star that collapsed to lead to their formation, except that of its mass, electric (or magnetic) charge, and rotation speed. All other characteristics of their initial state (for example, the chemical composition of the star) are completely lost during the collapse, and all black holes of the same mass, same charge, and same rotation speed are absolutely identical.

American physicist John Wheeler illustrated this property with a famous phrase: "black holes have no hair", where by "hair" we mean any parameter other than mass, charge, and rotation speed.

This property of stellar black holes is confirmed by the uniqueness theorems, while for primordial black holes, it has been postulated as a conjecture, partially confirmed by a series of "no-hair theorems."

And yet... the beginnings of hairy black holes

Among the four fundamental forces of nature, there are two, gravity and electromagnetism, that act on a macroscopic scale and explain the structure of stellar black holes that are "bald." The other two forces, called weak and strong, act only on a microscopic scale, inside atoms. Can these two fundamental forces influence the structure of black holes?

The physical theories that describe these forces are quite complicated to study, and that is why physicists first focused on simplified theoretical models. It is thanks to these simplified models that the so-called hairy black holes were discovered, that is, surrounded by a shell of matter intrinsically linked to them and thus characterized by additional parameters (other than mass, charge, rotation speed) that allow them to be distinguished from one another.

Since their first discovery in 1989, many examples of hairy black holes have been found by theoretical physicists, but always within the framework of simplified or, conversely, extremely speculative theories. Such black holes exist on paper as solutions to mathematical equations, but nothing allows us to affirm that they really exist in our Universe.

Black holes with "electroweak hair"

In our study, we considered the unification of three exact, non-simplified, and experimentally confirmed theories, which bring together three of the four fundamental forces: gravity, electromagnetism, and the weak nuclear force (the latter two together form the electroweak force).

The solutions we obtained by solving the equations of these combined theories describe magnetically charged black holes surrounded by "hair" in the form of rings.

These rings are composed of elementary particles (more precisely, W, Z, and Higgs bosons), in the form of a Bose-Einstein condensate—a particular state of matter appearing in certain situations. In the laboratory, it has been observed for cold atoms trapped using lasers (which earned a Nobel Prize in 2001 for its discoverers).

In our case, it is the intense magnetic field of the charged black hole that produces the electroweak condensate, and since the latter is also magnetically charged, it is repelled from the black hole by the magnetic force and thus does not fall inside. However, it is not ejected further either, as it is attracted to the black hole by the gravitational force. It therefore remains trapped outside the black hole.

Our ringed black holes, of a new type, can be macroscopic in size, about one centimeter, while the elementary particles that make up their rings normally appear on the scale of the infinitely small.

Since these black holes are described by experimentally confirmed theories, this strongly suggests that they exist not only as mathematical solutions but also as real objects in the Universe.

Could these black holes be detected?

It is clear that these hairy black holes could not form today. However, the favorable conditions for their formation could have been encountered in the first moments of the Universe, in the extremely dense and fluctuating primordial plasma. They would therefore be primordial black holes.

It is important to note that these black holes are stable, as the presence of the rings reduces the mass of the black hole, so that getting rid of them would be energetically unfavorable. They could therefore survive until today and be part of the dark matter, this substance whose exact nature remains unknown to this day and which is detected only by its gravitational influence.

These hairy black holes could be detected by their interaction with rotating neutron stars (pulsars), because if they are absorbed by one of them (which can happen as they are much smaller and lighter), then the star continues to exist with the black hole inside but this must abruptly change its rotation period, which could be detectable.