Axions are the most favored candidate for dark matter today, and several experiments are being conducted to find them in microwave cavities, where the axion should seldom change into an electromagnetic wave. However, a new simulation of axion formation in the early cosmos provides a more accurate mass estimate as well as a higher frequency for the EM wave, which is outside the scope of our tests. Adaptive mesh refinement in supercomputer simulations generates the new mass.
According to a recent supercomputer simulation of how axions were formed just after the Big Bang 13.6 billion years ago, physicists seeking for today’s most popular candidate for dark matter, the axion, have been looking in the wrong location.
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 increased mass means that the most common type of experiment for detecting these elusive particles – a microwave resonance chamber with a strong magnetic field, in which scientists hope to capture the conversion of an axion into a faint electromagnetic wave – will be unable to detect them, no matter how much the experiment is tweaked. To detect the higher-frequency wave from a higher-mass axion, the chamber would have to be smaller than a few millimeters on a side, according to Safdi, and that volume would be insufficient 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 ?eV axion masses, but there’s a lot of work gearing up to go after that mass range.”
A plasma haloscope, which looks for axion excitations in a metamaterial — a solid-state plasma — could be sensitive to and potentially identify an axion particle of this mass.
“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 nuclear engineering professor who is building a prototype of the plasma haloscope while also participating in the HAYSTAC experiment for microwave cavity axion search. “Ben’s most recent achievement is incredibly exciting. If the post-inflation scenario is correct, the discovery of the axion could be considerably hastened after four decades.”
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.
Benjamin Safdi
Axion top candidate for dark matter
Dark matter is a mystery element that astronomers know exists because it influences the movements of every star and galaxy, yet it interacts so weakly with the stuff of stars and galaxies that it has escaped discovery. This is not to say that dark matter cannot be researched or even weighed. Astronomers know exactly how much dark matter exists in the Milky Way Galaxy, and even the entire universe: it accounts for 85 percent of all matter in the universe.
To date, dark matter searches have concentrated on massive compact objects (called MACHOs) in our galaxy’s halo, weakly interacting massive particles (WIMPs), and even undetected black holes. None of them seemed like a good fit.
“The majority of matter in the cosmos is dark matter, and we have no idea what it is. ‘What is dark matter?’ is one of science’s most intriguing questions.” Safdi stated. “We believe it is a new particle that we are unaware of, and the axion could be that particle. It could have been generated in abundance during the Big Bang and be floating around explaining astrophysical data.”
Though not precisely a WIMP, the axion interacts with conventional matter in a weak way. It effortlessly goes through the earth without causing any disruption. In 1978, it was postulated as a new elementary particle that could explain why the spin of a neutron does not precess or wobble in an electric field. The axion, according to theory, suppresses this precession in the neutron.
“Still to this day, the axion is the best idea we have about how to explain these weird observations about the neutron,” Safdi said.
The axion became popular as a candidate for dark matter in the 1980s, and the first attempts to detect axions were made. It is possible to calculate the axion’s precise mass 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, but the equations are so difficult that we only have estimates, which have varied enormously. Because the mass is so imprecisely known, searches using microwave cavities (basically complex radio receivers) must tune through millions of frequency channels in order to discover 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 colleagues produced the most recent, if inaccurate, axion mass estimate that experimentalists are now aiming for. However, as they worked on better simulations, they approached a Berkeley Lab team that had developed a specialized code for a superior simulation technique known as adaptive mesh refinement. During simulations, a small portion of the expanding cosmos is represented by a three-dimensional grid on which equations are solved. The grid is made more detailed around areas of interest and less detailed around parts of space where nothing much happens in adaptive mesh refinement. This concentrates computational power on the simulation’s most relevant sections.
Safdi’s simulation was able to see thousands of times more information around the locations where axions are generated, allowing for a more precise measurement of the total number of axions produced and, given the entire quantity of dark matter in the universe, the axion mass. The simulation used 69,632 actual computer processing unit (CPU) cores of the Cori supercomputer and approximately 100 terabytes of random access memory (RAM), making it one of the largest dark matter simulations ever performed.
The simulation revealed that following the inflationary era, little tornadoes, or vortices, develop in the early cosmos like ropey strings and toss off axions like riders bucked from a bronco. 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 in a more creative approach, as well as by applying all of the computational power we could possibly locate to this problem,” Safdi explained. “Because our world is so vast, we could never simulate it in its entirety. However, we do not need to arouse the entire universe. We simply need to mimic a large enough patch of the cosmos for a long enough time to capture all of the dynamics that we know are contained within that box.”
The team is collaborating with a new supercomputing cluster being created at Berkeley Lab, which will enable simulations with even more precision. The next-generation supercomputer named Perlmutter after Saul Perlmutter, a UC Berkeley and Berkeley Lab physicist who won the Nobel Prize in Physics in 2011 for discovering the rapid expansion of the cosmos caused by so-called dark energy, will double the computing capability of NERSC.
“We want to run larger simulations at higher resolution, which will allow us to narrow these error bars, hopefully down to 10%, so we can tell you a very accurate value, 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.
The new mass estimate, according to van Bibber, who was not a member of Safdi’s simulation team, challenges the limitations of microwave cavities, which perform less well at high frequencies. So, even if the bottom limit of the mass range is still detectable by the HAYSTAC experiment, he is excited about the plasma haloscope.
