Black hole galaxies emit energy and
light, including gamma rays (γ) (left panel). If axions exist, some of the
γ-rays would, hypothetically, turn into axions (a) as they travel through the
magnetic fields surrounding clusters of galaxies. But since these magnetic
fields are very intricate, it is impossible to predict such a conversion to
axions in a single observation, resulting in messy data (center panel). But by
combining knowledge from many of these galactic pairs, the step-like signature
appears (right panel). Credit: Lidiia Zadorozhna
Physicists from the University of
Copenhagen have begun using the gigantic magnetic fields of galaxy clusters to
observe distant black holes in their search for an elusive particle that has
stumped scientists for decades.
It is a story of extremes that are
hard to fathom.
The heaviest structures in the
universe, clusters of galaxies, are a quadrillion times more massive than the
sun. And axions, mysterious theoretical particles, are much, much lighter than
even the lightest atom.
The axion is a hypothetical
elementary particle that could hold the key to understanding dark matter, an
unknown material that is believed to take up about 80% of the mass in our
universe.
No one has yet proven the existence
of axions, which has eluded researchers for decades. But with a clever trick
involving faraway galaxies, physicists from the University of Copenhagen
possibly came closer than ever before.
Instead of using a particle accelerator on Earth, like the one at CERN, the researchers turned to the cosmos
and used it as a kind of gigantic particle accelerator. Specifically, they
searched for electromagnetic radiation emitting from the cores of distant and
very bright galaxies, each with a supermassive black hole at its center.
They then observed this radiation
as it passed through the vast magnetic fields found in galaxy clusters, where some of it could hypothetically transform into axions. This
transformation would leave behind tiny, random fluctuations in the data. But
each signal is so faint that, on its own, it gets lost in the background noise
of the universe.
So, the researchers introduced a
novel concept. Instead, they observed a total of 32 supermassive black holes
positioned behind galaxy clusters and then combined the data from their
observations.
When the researchers examined the
data, they were surprised to discover a pattern that resembled the signature of
the elusive axion particle.
"Normally, the signal from
such particles is unpredictable and appears as random noise. But we realized
that by combining data from many different sources, we had transformed all that
noise into a clear, recognizable pattern," explains Oleg Ruchayskiy,
Associate Professor at the Niels Bohr Institute of the University of Copenhagen
and senior author of a paper in Nature Astronomy titled "Constraints on axion-like particles from active
galactic nuclei seen through galaxy clusters" that tries to study the axion.
He adds, "It shows up like a
unique step-like pattern that shows what this conversion could look like. We
only see it as a hint of a signal in our data, but it is still very tantalizing
and exciting. You could call it a cosmic whisper, now loud enough to
hear."
Closer to discovering dark matter
While the pattern revealed by the
scientists is not definite proof of the existence of axions, the research by
Ruchayskiy and colleagues brings us closer to understanding what dark matter
is.
"This method has greatly
increased what we know about axions. It essentially enabled us to map a large
area that we know does not contain the axion, which narrows down the space where it can be found," says Postdoc
Lidiia Zadorozhna, a Marie Curie fellow at the Niels Bohr Institute, who is one
of the leading authors of the new paper.
While this experiment focused on a
specific type of electromagnetic radiation known as gamma rays, the method can also be used on other types of
radiation, like X-rays.
"We are so excited, because it
is not a one-time advancement. This method allows us to go beyond previous
experimental limits and has opened a new path to studying these elusive
particles. The technique can be repeated by us, by other groups, across a broad
range of masses and energies. That way we can add more pieces to the puzzle of
explaining dark matter," says Zadorozhna.
Source: Intergalactic experiment: Researchers hunt for mysterious dark matter particle with clever new trick
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