Dark matter, a hypothetical material that does not absorb, emit or reflect
light, is thought to account for over 80 percent of the matter in the
universe. While many studies have indirectly hinted at its existence, so
far, physicists have been unable to directly detect dark matter and thus to
confidently determine what it consists of.
One factor that makes searching for dark matter particularly challenging is
that very little is known about its possible mass and composition. This
means that dark matter searches are based on great part on hypotheses and
theoretical assumptions.
Researchers at SLAC National Accelerator Laboratory and Université Paris
Saclay have recently carried out a theoretical study that could introduce a
new way of searching for dark matter. Their paper, published in Physical
Review Letters, shows that when macroscopic dark matter travels through a
star, it could produce shock waves that might reach the star's surface.
These waves could in turn lead to distinctive and transient optical, UV and
X-ray emissions that might be detectable by sophisticated telescopes.
"Most experiments have searched for dark matter made of separate particles,
each about as heavy as an atomic nucleus, or clumps about as massive as
planets or stars," Kevin Zhou, one of the researchers who carried out the
study, told Phys.org. "We were interested in the intermediate case of
asteroid-sized dark matter, which had been thought to be hard to test
experimentally, since dark asteroids would be too rare to impact Earth, but
too small to see in space."
Initially, Zhou and his colleagues started exploring the possibility that
the heat produced during the impact between a dark matter asteroid and an
ordinary star could result in the star exploding. This hypothesis was based
on past studies suggesting that energy deposition can sometimes trigger
supernova in white dwarfs. After a few weeks of calculations and
discussions, however, the team realized that the impact between a dark
matter asteroid and an ordinary star would most likely not lead to an
explosion, as ordinary stars are more stable than white dwarfs.
"We had a hunch that the energy produced by such a collision should be
visible somehow, so we brainstormed for a few months, trying and tossing out
idea after idea," Zhou explained. "Finally, we realized that the shock waves
generated by the dark asteroid's travel through the star were the most
promising signature."
Shock waves are sharp signals that are produced when an object is moving
faster than the speed of sound. For instance, a supersonic aircraft produces
a sonic boom, which can be heard from the Earth's surface even when it is
flying miles above it.
Similarly, Zhou and his colleagues predicted that the shock waves produced
by dark asteroids deep inside a star could reach a star's surface. This
would in turn result in a short-lived hot spot that could be detected using
telescopes that can examine the UV spectrum.
"We're excited that we identified a powerful new way to search for a kind of
dark matter thought to be hard to test, using telescopes that we already
have in an unexpected way," Zhou said. "The most powerful UV telescope is
the Hubble space telescope, but since stellar shock events are transients,
it helps to be able to monitor more of the sky at once."
The recent study follows a growing trend within the astrophysics community
to use astronomical objects as enormous dark matter detectors. This
promising approach to searching for dark matter unites the fields of
particle physics and astrophysics, bringing these two communities closer
together.
In the future, the recent work by this team of researchers could inspire
engineers to build new and smaller UV telescopes that can observe wider
parts of the universe. A similar telescope, dubbed ULTRASAT, is already set
to be released in 2024. Using this telescope, physicists could try searching
for dark matter by examining stellar surfaces. In their next works, the
researchers themselves plan to try to detect potential dark asteroid impact
events using UV telescope data.
"The ideal case would be to use the Hubble space telescope to monitor a
large globular cluster in the UV," Zhou said. "It would also be interesting
to consider dark asteroids impacting other astronomical objects. Since our
work, there have been papers by others considering impacts on neutron stars
and red giants, but there are probably even more promising ideas in this
direction that nobody has thought of yet."
Reference:
Anirban Das et al, Stellar Shocks from Dark Matter Asteroid Impacts, Physical
Review Letters (2022).
DOI: 10.1103/PhysRevLett.128.021101
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Physics