Dark dwarfs lurking at the center of our galaxy might hint at the nature of dark matter

Celestial objects known as dark dwarfs can hide in the center of our galaxy and could offer key clues to discover the nature of one of the most mysterious and fundamental phenomena of contemporary cosmology: dark matter.

An article published in the Journal of Cosmology and Astroparticle Physics By a team of researchers based in the United Kingdom and Hawaii describes these objects for the first time and offers how to check their existence using current observation tools such as the James Webb space telescope. The paper is entitled “Dark Nwarfs: Dark Matter-Awed Objects awaiting discovery at the Galactic Center.”

The Anglo-Us team behind the study named them black dwarfs. Not because they are dark bodies – on the contrary – but because of their special bond with dark matter, one of the most central subjects of current research on cosmology and astrophysics.

“We think that 25% of the universe is made up of a type of matter that does not emit light, which makes it invisible to us and our telescopes. We detect it only by its gravitational effects. This is why we call it dark matter,” explains Jeremy Sakstein, professor of physics at the University of Hawai’i and one of the authors of the study.

What we know today about dark matter is that it exists and how it behaves, but not yet what it is. Over the past 50 years, several hypotheses have been proposed, but none has yet gathered enough experimental evidence to prevail. Studies like that of Sakstein and his colleagues are important because they offer concrete tools to break this dead end.

Among the most known black matter candidates appear the massive particles weakly in interaction (WIMP) – very massive particles which interact very weakly with ordinary matter: they cross things unnoticed, do not emit light and do not respond to electromagnetic forces (therefore they do not reflect light and do not remain invisible), and are only reacting by their gravitational effects. This type of dark matter would be necessary for dark dwarfs to exist.

“The dark matter interacts gravitally, it could therefore be captured by the stars and accumulate inside. If this happens, it could also interact with itself and destroy, releasing the energy that heats the star,” explains Sakstein.

Ordinary stars – like our sun – have explained themselves because the nuclear fusion processes occur in their nuclei, generating large amounts of heat and energy. The merger occurs when the mass of a star is large enough for gravitational forces to compress the material towards the center with such intensity that they trigger reactions between atomic nuclei. This process releases an enormous amount of energy, which we consider a light. The dark dwarves also emit light, but not because of nuclear fusion.

“The dark dwarfs are very low mass objects, about 8% of the mass of the sun,” explains Sakstein. Such a small mass is not sufficient to trigger fusion reactions.

For this reason, these objects – although very common in the universe – generally emit only weak light (due to the energy produced by their relatively low gravitational contraction) and are known to scientists under the name of brown dwarfs.

However, if brown dwarfs are located in regions where dark matter is particularly abundant – like the center of our galaxy – they can turn into something else.

“These objects collect the dark matter which helps them to become a dark dwarf. The more dark matter you, the more you can capture,” explains Sakstein. “And the more dark matter is found inside the star, the more energy there will be through its annihilation.”

But all of this is based on a specific type of dark matter. “For black dwarves to exist, dark matter must be made of Wimp, or any heavy particle that interacts with itself to produce visible matter,” explains Sakstein.

Other candidates proposed to explain dark matter – such as axes, blurred ultralic particles or sterile neutrinos – are too light to produce the expected effect in these objects. Only the massive particles, capable of interacting with each other and of visible energy annihilation, could supply a dark dwarf.

However, this whole hypothesis would have little value if there was no concrete way to identify a dark dwarf. For this reason, Sakstein and his colleagues offer a distinctive marker. “There were a few markers, but we suggested lithium-7 because it would really be a unique effect,” said the scientist.

Lithium-7 burns very easily and is quickly consumed in ordinary stars. “So, if you could find an object that looked like a dark dwarf, you can seek the presence of this lithium because it would not be there if it was a brown dwarf or a similar object.”

Tools like the James Webb space telescope could already detect extremely cold celestial objects like dark dwarfs. But, according to Sakstein, there is another possibility. “The other thing you might do is to look at a whole population of objects and ask, in a statistical way, if it is better described by having a subpopulation of dark dwarfs or not.”

If in the coming years, we manage to identify one or more dark dwarfs, how strong this index would be in favor of the hypothesis that dark matter is made of Wimp?

“Reasonably strong. With candidates of clear dark matter, something like an axion, I do not think you would be able to get something like a dark dwarf. They do not accumulate inside the stars. If we manage to find a dark dwarf, it would provide convincing evidence that the dark matter is heavy and strongly interacts with itself, but only weakly with the standard model. Wimps, but that it would include other other exotic models, “Sakse conclusions, but that it would not understand other other exotic models than well,” Sakse conclusions, but that would only understand others than exotic models.

The observation of a dark dwarf would not tell us in a conclusive way that dark matter is a wimp, but that would mean that it is a wimp or something which, for all purposes, behaves like a wimp.