Sky & Telescope - January 2021 - 21

atom could produce gravitational waves
angular momentum as it passes. If axions
in two ways. First, in a direct parallel to
exist with the right range of masses then,
electrons in an atom, axions could fall
like a speaker amplifying a sound wave,
from higher energy states to lower ones.
superradiance will spontaneously and
In an everyday atom, an electron making
exponentially grow the number of axions
this transition emits a photon. But in the
around the black hole. Their numbers will
Axion-like particles are,
gravitational atom, the axion would emit a
balloon from zero to a few percent of the
unsurprisingly, just like
gravitational wave. Second, unlike elecblack hole's mass in just a few years. And
axions: They are predicted
trons, axions are their own antiparticle, so
these axion clouds will form states around
to be very light and
two axions from the same energy level can
the black hole that look similar to electron
weakly interacting, and
annihilate to produce gravitational waves.
orbitals in a hydrogen atom, earning such
they perform the same
"This doesn't happen in a regular hydrogen
black holes the term gravitational atoms.
photon identity-swapping
atom," says Baryakhtar. "But it's the most
"Gravitational atoms are huge - hunas axions. The difference
common source of gravitational waves in
dreds of kilometers across - and can
is that, instead of being
gravitational atoms."
contain astronomical amounts of axions,
motivated by QCD, they
With a potentially colossal number of
up to 1080!" remarks Baryakhtar. But for
fall out of the equations of
axions at any one energy level in a gravitathe black hole, forming a gravitational
different versions of string
tional atom's axion cloud, the gravitationalatom takes a heavy toll. As the axions
theory and other particle
wave signal could be large and easy to spot.
sap the black hole's energy, there quickly
physics frameworks that
"So far there have not been any positive
comes a point when it is no longer spinning
go beyond the Standard
signals that could potentially be coming
with enough energy to induce superradiModel. This means they
from gravitational atoms, so we are mostly
ance. This rapid spin-down effect is key
do not solve the strong
thinking about how to properly use these
for Baryakhtar: She can plot the mass and
nuclear force conundrum
searches to place constraints on axions,"
spin of known black holes and compare
that axions do. Efforts to
she says. "But I think it is a very promisthis to what she would expect if superradifind axion-like particles
ing direction and expect the prospects to
ance spins black holes down. The black hole
and axions often overlap.
improve as LIGO and Virgo achieve even
masses and spins that superradiance allows
Should a promising parmore incredible sensitivity to gravitational
are directly related to the axion mass. If a
ticle be discovered, calcuwaves."
black hole exists that is spinning too fast
lations will soon reveal if it
Future detectors like ESA's three Laser
for a given axion mass, she can rule out
solves the strong CP probInterferometer Space Antenna (LISA)
that axion mass. She and her colleagues
lem and is therefore a QCD
spacecraft promise even more sensitivity to
have already used this technique to exclude
axion, or not and instead
axion-derived gravitational waves, if axions
a range of axion masses.
an equally intriguing axionexist. And the outlook for other axion huntThough Baryakhtar is currently reﬁning
like particle. Its mass and
ing methods is just as bright. With a large
this technique and adding new black hole
interaction strength will
swathe of axions' potential parameters
measurements as they come in with the
quickly answer whether it
still unexplored, researchers are throwing
aim of further tightening the constraints
can account for some or
the book at the problem using acronymon axion mass, she has her eye on a more
all of the universe's dark
heavy Earth-based detectors such as IAXO,
direct way of identifying axions from black
matter.
ABRACADABRA (A Broadband/Resonant
holes: gravitational waves.
Approach to Cosmic Axion Detection with
Researchers at the Laser Interferometer
an Amplifying B-ﬁeld Ring Apparatus),
Gravitational-Wave Observatory (LIGO)
MADMAX (Magnetized Disc and Mirror Axion Experitwin sites in Washington and Louisiana, alongside scientists
ment), and CASPER (Cosmic Axion Spin Precession Experibased at LIGO's European cousin Virgo in Italy, have so far
ment). At the same time, scientists are leveraging increased
conﬁdently witnessed 10 events in which two black holes
understanding of stellar evolution to further constrain axion
collide and merge, as well as one binary neutron star merger
and dozens more candidate events. Since the ﬁrst detection in parameters. They will also soon wield the power of the Square
Kilometre Array (SKA) radio telescope to search for signals
2015 that proved gravitational waves' existence, these ripples
from the elusive particles.
in spacetime have been used to gain a deeper understanding
"Right now, there is a bit of a Wild West mentality," says
of the sizes and nature of black holes and neutron stars, to
Safdi. "It really is an exciting time . . . any of these searches
gauge how common black holes are in the galaxy and unicould lead to a discovery."
verse, and to test whether the speed of light and gravity are
the same (spoiler alert, they are!).
Baryakhtar thinks scientists could soon wield gravitational BENJAMIN SKUSE is a science writer based in Somerset,
waves to identify gravitational atoms as well. A gravitational
United Kingdom.

An Axion or
Axion-like
Particle?

sk yandtelescope.org * JANUARY 2 021

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Sky & Telescope - January 2021

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