Artistic representation of a dark dwarf.
Credit: Sissa Medialab
Celestial
objects known as dark dwarfs may be hiding at the center of our galaxy and
could offer key clues to uncover the nature of one of the most mysterious and
fundamental phenomena in contemporary cosmology: dark matter.
A paper published in the Journal of Cosmology and Astroparticle Physics by
a team of researchers based in the UK and Hawaii describes these objects for
the first time and proposes how to verify their existence using current
observational tools such as the James Webb Space Telescope. The paper is titled
"Dark Dwarfs: Dark Matter-Powered Sub-Stellar Objects Awaiting Discovery
at the Galactic Center."
The Anglo-U.S. team behind the study
named them dark dwarfs. Not because they are dark bodies—on the contrary—but
because of their special link with dark matter, one of the most central topics
in current cosmology and astrophysics research.
"We think that 25% of the universe
is composed of a type of matter that doesn't emit light, making it invisible to
our eyes and telescopes. We only detect it through its gravitational effects.
That's why we call it dark matter," explains Jeremy Sakstein, Professor of
Physics at the University of Hawai'i and one of the study's authors.
What we know today about dark matter is
that it exists and how it behaves—but not yet what it actually is. Over the
past 50 years, several hypotheses have been proposed, but none have yet
gathered enough experimental evidence to prevail. Studies like the one by
Sakstein and colleagues are important because they offer concrete tools to
break this deadlock.
Among the most well-known dark matter
candidates are the Weakly Interacting Massive Particles (WIMPs)—very massive
particles that interact very weakly with ordinary matter: they pass through things unnoticed, don't emit light
and don't respond to electromagnetic forces (so they don't reflect light and
remain invisible), and reveal themselves only through their gravitational
effects. This type of dark matter would be necessary for dark dwarfs to exist.
"Dark matter interacts
gravitationally, so it could be captured by stars and accumulate inside them.
If that happens, it might also interact with itself and annihilate, releasing
energy that heats the star," Sakstein explains.
Ordinary stars—like our sun—shine
because nuclear fusion processes
occur in their cores, generating large amounts of heat and energy. Fusion
happens when a star's mass is large enough that gravitational forces compress
matter toward the center with such intensity that they trigger reactions
between atomic nuclei. This process releases a huge amount of energy, which we
see as light. Dark dwarfs also emit light—but not because of nuclear fusion.
"Dark dwarfs are very low mass
objects, about 8% of the sun's mass," Sakstein explains. Such a small mass
is not sufficient to trigger fusion reactions.
For this reason, such objects—although
very common in the universe—usually only emit a faint light (due to the energy
produced by their relatively small gravitational contraction) and are known to
scientists as brown dwarfs.
However, if brown dwarfs are located in regions where dark matter is
particularly abundant—such as the center of our galaxy—they can transform into
something else.
"These objects collect the dark
matter that helps them become a dark dwarf. The more dark matter you have
around, the more you can capture," Sakstein explains. "And, the more
dark matter ends up inside the star, the more energy will be produced through
its annihilation."
But
all of this relies on a specific type of dark matter. "For dark dwarfs to
exist, dark matter has to be made of WIMPs, or any heavy particle that
interacts with itself so strongly to produce visible matter," Sakstein
says.
Other candidates proposed to explain
dark matter—such as axions, fuzzy ultralight particles, or sterile
neutrinos—are all too light to produce the expected effect in these objects.
Only massive particles,
capable of interacting with each other and annihilating into visible energy,
could power a dark dwarf.
This entire hypothesis, however, would
have little value if there weren't a concrete way to identify a dark dwarf. For
this reason, Sakstein and colleagues propose a distinctive marker. "There
were a few markers, but we suggested Lithium-7 because it would really be a
unique effect," the scientist explains.
Lithium-7 burns very easily and is
quickly consumed in ordinary stars. "So if you were able to find an object
which looked like a dark dwarf, you could look for the presence of this lithium
because it wouldn't be there if it was a brown dwarf or a similar object."
Tools like the James Webb Space
Telescope might already be able to detect extremely cold celestial objects like dark dwarfs. But, according to Sakstein,
there's another possibility. "The other thing you could do is to look at a
whole population of objects and ask, in a statistical manner, if it is better
described by having a sub-population of dark dwarfs or not."
If in the coming years we manage to
identify one or more dark dwarfs, how strong would that clue be in support of
the hypothesis that dark matter is made of WIMPs?
"Reasonably strong. With light dark
matter candidates, something like an axion, I don't think you'd be able to get
something like a dark dwarf. They don't accumulate inside stars. If we manage
to find a dark dwarf, it would provide compelling evidence that dark matter is
heavy and interacts strongly with itself, but only weakly with the Standard
Model. This includes classes of WIMPs, but it would include some other more
exotic models as well," concludes Sakstein.
Observing a dark dwarf wouldn't conclusively tell us that dark matter is a WIMP, but it would mean that it is either a WIMP or something that, for all intents and purposes, behaves like a WIMP.
Source: Dark dwarfs lurking at the center of our galaxy might hint at the nature of dark matter
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