The nature of dark matter continues to perplex astronomers. As the search
for dark matter particles continues to turn up nothing, it's tempting to
throw out the dark matter model altogether, but indirect evidence for the
stuff continues to be strong. So what is it? One team has an idea, and
they've published the results of their first search.
The conditions of dark matter mean that it can't be regular matter. Regular
matter (atoms, molecules, and the like) easily absorbs and emits light. Even
if dark matter were clouds of molecules so cold they emitted almost no
light, they would still be visible by the light they absorb. They would
appear like dark nebulae commonly seen near the galactic plane. But there
aren't nearly enough of them to account for the effects of dark matter we
observe. We've also ruled out neutrinos. They don't interact strongly with
light, but neutrinos are a form of "hot" dark matter since neutrinos move at
nearly the speed of light. We know that most dark matter must be sluggish,
and therefore "cold." So if dark matter is out there, it must be something
else.
In this latest work, the authors argue that dark matter could be made of
particles known as scalar bosons. All known matter can be placed in two
large categories known as fermions and bosons. Which category a particle is
in depends on a quantum property known as spin. Fermions such as electrons
and quarks have fractional spin such as 1/2 or 3/2. Bosons such as photons
have an integer spin such as 1 or 0. Any particle with a spin of 0 is a
scalar boson.
While it seems like a trivial distinction, the two kinds of particles behave
very differently when brought together in large groups. Fermions can never
occupy the same quantum state, so when you try to squeeze them together,
they push back. This is why white dwarfs and neutron stars exist. Gravity
tries to push electrons or neutrons together, but the Fermi pressure is so
strong it can resist gravity (up to a point). Bosons, on the other hand, are
perfectly happy occupying the same state. So if you supercool a bunch of
bosons (such as helium-4) they can settle into a strange quantum object
known as a Bose-Einstein condensate.
The only known scalar boson is the Higgs boson. The Higgs can't be dark
matter given its known properties, but some theories propose other scalar
bosons. These would not interact strongly with light, only with gravity.
Since light can't significantly heat them up, over time, these scalar bosons
would cool and collapse into large clouds. So perhaps dark matter is made of
large, diffuse clouds of scalar bosons.
It's an interesting idea, but how could you prove it? It turns out that
since scalar bosons interact gravitationally, they also interact with
gravitational waves. Depending on their mass, scalar bosons might also decay
by emitting gravitons. As a result, scalar bosons could create long-lasting
gravitational waves that have a similar frequency. It's the gravitational
equivalent of a faint hum. So the team looked at gravitational wave data
from LIGO and Virgo. They looked for evidence of a gravitational hum in the
20 to 600 Hz range and found nothing. Based on their work, the authors
conclude that there are no young scalar boson clouds in our galaxy. There
are also no old and cold scalar boson clouds within 3,000 light-years of
Earth.
This study doesn't rule out scalar bosons completely, but it does put some
strong limits on the idea. And right now, that seems to be the story of dark
matter. In our search to discover what it is, we continue to find out what
it is not.
Reference:
R. Abbott et al, All-sky search for gravitational wave emission from scalar
boson clouds around spinning black holes in LIGO O3 data. arXiv:2111.15507v1
[astro-ph.HE],
arxiv.org/abs/2111.15507v1
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