Physicists searching—unsuccessfully—for today's most favored candidate for
dark matter, the axion, have been looking in the wrong place, according to a
new supercomputer simulation of how axions were produced shortly after the
Big Bang 13.6 billion years ago.
Using new calculational techniques and one of the world's largest computers,
Benjamin Safdi, assistant professor of physics at the University of
California, Berkeley; Malte Buschmann, a postdoctoral research associate at
Princeton University; and colleagues at MIT and Lawrence Berkeley National
Laboratory simulated the era when axions would have been produced,
approximately a billionth of a billionth of a billionth of a second after
the universe came into existence and after the epoch of cosmic inflation.
The simulation at Berkeley Lab's National Research Scientific Computing
Center (NERSC) found the axion's mass to be more than twice as big as
theorists and experimenters have thought: between 40 and 180 microelectron
volts (micro-eV, or μeV), or about one 10-billionth the mass of the
electron. There are indications, Safdi said, that the mass is close to 65
μeV. Since physicists began looking for the axion 40 years ago, estimates of
the mass have ranged widely, from a few μeV to 500 μeV.
"We provide over a thousandfold improvement in the dynamic range of our
axion simulations relative to prior work and clear up a 40-year old question
regarding the axion mass and axion cosmology," Safdi said.
The more definitive mass means that the most common type of experiment to
detect these elusive particles—a microwave resonance chamber containing a
strong magnetic field, in which scientists hope to snag the conversion of an
axion into a faint electromagnetic wave—won't be able to detect them, no
matter how much the experiment is tweaked. The chamber would have to be
smaller than a few centimeters on a side to detect the higher-frequency wave
from a higher-mass axion, Safdi said, and that volume would be too small to
capture enough axions for the signal to rise above the noise.
"Our work provides the most precise estimate to date of the axion mass and
points to a specific range of masses that is not currently being explored in
the laboratory," he said. "I really do think it makes sense to focus
experimental efforts on 40 to 180 μeV axion masses, but there's a lot of
work gearing up to go after that mass range."
One newer type of experiment, a plasma haloscope, which looks for axion
excitations in a metamaterial—a solid-state plasma—should be sensitive to an
axion particle of this mass, and could potentially detect one.
"The basic studies of these three-dimensional arrays of fine wires have
worked out amazingly well, much better than we ever expected," said Karl van
Bibber, a UC Berkeley professor of nuclear engineering who is building a
prototype of the plasma haloscope while also participating in a microwave
cavity axion search called the HAYSTAC experiment. "Ben's latest result is
very exciting. If the post-inflation scenario is right, after four decades,
discovery of the axion could be greatly accelerated."
If axions really exist.
The work will be published Feb. 25 in the journal Nature Communications.
Axion top candidate for dark matter
Dark matter is a mysterious substance that astronomers know exists—it
affects the movements of every star and galaxy—but which interacts so weakly
with the stuff of stars and galaxies that it has eluded detection. That
doesn't mean dark matter can't be studied and even weighed. Astronomers know
quite precisely how much dark matter exists in the Milky Way Galaxy and even
in the entire universe: 85% of all matter in the cosmos.
To date, dark matter searches have focused on massive compact objects in the
halo of our galaxy (called massive compact halo objects, or MACHOs), weakly
interacting massive particles (WIMPs) and even unseen black holes. None
turned up a likely candidate.
"Dark matter is most of the matter in the universe, and we have no idea what
it is. One of the most outstanding questions in all of science is, 'What is
dark matter?'" Safdi said. "We suspect it is a new particle we don't know
about, and the axion could be that particle. It could be created in
abundance in the Big Bang and be floating out there explaining observations
that have been made in astrophysics."
Though not strictly a WIMP, the axion also interacts weakly with normal
matter. It passes easily through the earth without disruption. It was
proposed in 1978 as a new elementary particle that could explain why the
neutron's spin does not precess or wobble in an electric field. The axion,
according to theory, suppresses this precession in the neutron.
Zooming in on a small part of the supercomputer simulation of the early
universe shows the formation of topological defects called strings
(yellow), which writhe and vibrate at speeds approaching the speed of
light. As the strings twist, vibrate and shrink, they emit radiation in
the form of axions (blue). This axion radiation may then become the dark
matter in our universe. The goal of this simulation is to precisely
measure how much axion radiation is produced by the shrinking string
network, and from that calculate the expected mass of the axion
particle. Credit: Malte Buschmann, Princeton University
"Still to this day, the axion is the best idea we have about how to explain
these weird observations about the neutron," Safdi said.
In the 1980s, the axion began to be seen also as a candidate for dark
matter, and the first attempts to detect axions were launched. Using the
equations of the well-vetted theory of fundamental particle interactions,
the so-called Standard Model, in addition to the theory of the Big Bang, the
Standard Cosmological Model, it is possible to calculate the axion's precise
mass, but the equations are so difficult that to date we have only
estimates, which have varied immensely. Since the mass is known so
imprecisely, searches employing microwave cavities—essentially elaborate
radio receivers—must tune through millions of frequency channels to try to
find the one corresponding to the axion mass.
"With these axion experiments, they don't know what station they're supposed
to be tuning to, so they have to scan over many different possibilities,"
Safdi said.
Safdi and his team produced the most recent, though incorrect, axion mass
estimate that experimentalists are currently targeting. But as they worked
on improved simulations, they approached a team from Berkeley Lab that had
developed a specialized code for a better simulation technique called
adaptive mesh refinement. During simulations, a small part of the expanding
universe is represented by a three-dimensional grid over which the equations
are solved. In adaptive mesh refinement, the grid is made more detailed
around areas of interest and less detailed around areas of space where
nothing much happens. This concentrates computing power on the most
important parts of the simulation.
The technique allowed Safdi's simulation to see thousands of times more
detail around the areas where axions are generated, allowing a more precise
determination of the total number of axions produced and, given the total
mass of dark matter in the universe, the axion mass. The simulation employed
69,632 physical computer processing unit (CPU) cores of the Cori
supercomputer with nearly 100 terabytes of random access memory (RAM),
making the simulation one of the largest dark matter simulations of any kind
to date.
The simulation showed that after the inflationary epoch, little tornadoes,
or vortices, form like ropey strings in the early universe and throw off
axions like riders bucked from a bronco.
"You can think of these strings as composed of axions hugging the vortices
while these strings whip around forming loops, connecting, undergoing a lot
of violent dynamical processes during the expansion of our universe, and the
axions hugging the sides of these strings are trying to hold on for the
ride," Safdi said. "But when something too violent happens, they just get
thrown off and whip away from these strings. And those axions which get
thrown off of the strings end up becoming the dark matter much later on."
By keeping track of the axions that are whipped off, researchers are able to
predict the amount of dark matter that was created.
Adaptive mesh refinement allowed the researchers to simulate the universe
much longer than previous simulations and over a much bigger patch of the
universe than previous simulations.
"We solve for the axion mass both in a more clever way and also by throwing
just as much computing power as we could possibly find onto this problem,"
Safdi said. "We could never simulate our entire universe because it's too
big. But we don't need to stimulate our entire universe. We just need to
simulate a big enough patch of the universe for a long enough period of
time, such that we capture all of the dynamics that we know are contained
within that box."
The team is working with a new supercomputing cluster now being built at
Berkeley Lab that will enable simulations that will provide an even more
precise mass. Called Perlmutter, after Saul Perlmutter, a UC Berkeley and
Berkeley Lab physicist who won the 2011 Nobel Prize in Physics for
discovering the accelerating expansion of the universe driven by so-called
dark energy, the next-generation supercomputer will quadruple the computing
power of NERSC.
"We want to make even bigger simulations at even higher resolution, which
will allow us to shrink these error bars, hopefully down to the 10% level,
so we can tell you a very precise number, like 65 plus or minus 2 micro-eV.
That then really changes the game experimentally, because then it would
become an easier experiment to verify or exclude the axion in such a narrow
mass range," Safdi said.
For van Bibber, who was not a member of Safdi's simulation team, the new
mass estimate tests the limits of microwave cavities, which work less well
at high frequencies. So, while the lower limit of the mass range is still
within the ability of the HAYSTAC experiment to detect, he is enthused about
the plasma haloscope.
"Over the years, new theoretical understanding has loosened the constraints
on the axion mass; it can be anywhere within 15 orders of magnitude, if you
consider the possibility that axions formed before inflation. It's become an
insane task for experimentalists," said van Bibber, who holds UC Berkeley's
Shankar Sastry Chair of Leadership and Innovation. "But a recent paper by
Frank Wilczek's Stockholm theory group may have resolved the conundrum in
making a resonator which could be simultaneously both very large in volume
and very high in frequency. An actual resonator for a real experiment is
still some ways away, but this could be the way to go to get to Safdi's
predicted mass."
Once simulations give an even more precise mass, the axion may, in fact, be
easy to find.
"It was really crucial that we teamed up with this computer science team at
Berkeley Lab," Safdi said. "We really expanded beyond the physics field and
actually made this a computing science problem."
Reference:
Dark matter from axion strings with adaptive mesh refinement, Nature
Communications (2022).
DOI: 10.1038/s41467-022-28669-y
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Physics