In a simulation of the early universe, shortly after the Huge Bang, tornado-like strings (darkish blue loop) throw off axion particles. These axions ought to nonetheless be round right now, and might be the darkish matter that astrophysicists have been trying to find. Credit score: Malte Buschmann, Princeton College
Utilizing adaptive mesh refinement, supercomputer simulation narrows axion mass vary.
Physicists looking — unsuccessfully — for right now’s most favored candidate for darkish matter, the axion, have been trying within the mistaken place, in line with a brand new supercomputer simulation of how axions have been produced shortly after the Huge Bang 13.6 billion years in the past.
Utilizing new calculational methods and one of many world’s largest computer systems, Benjamin Safdi, assistant professor of physics on the College of California, Berkeley; Malte Buschmann, a postdoctoral analysis affiliate at Princeton College; and colleagues at MIT and Lawrence Berkeley Nationwide Laboratory simulated the period when axions would have been produced, roughly a billionth of a billionth of a billionth of a second after the universe got here into existence and after the epoch of cosmic inflation.
The simulation at Berkeley Lab’s Nationwide Analysis Scientific Computing Heart (NERSC) discovered the axion’s mass to be greater than twice as massive 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 mentioned, that the mass is near 65 µeV. Since physicists started searching for the axion 40 years in the past, estimates of the mass have ranged extensively, from a number of µeV to 500 µeV.
“We offer over a thousandfold enchancment within the dynamic vary of our axion simulations relative to prior work and clear up a 40-year previous query relating to the axion mass and axion cosmology,” Safdi mentioned.
Zooming in on a small a part of the supercomputer simulation of the early universe reveals the formation of topological defects known as strings (yellow), which writhe and vibrate at speeds approaching the pace of sunshine. Because the strings twist, vibrate and shrink, they emit radiation within the type of axions (blue). This axion radiation might then turn into the darkish matter in our universe. The aim of this simulation is to exactly measure how a lot axion radiation is produced by the shrinking string community, and from that calculate the anticipated mass of the axion particle. Credit score: Malte Buschmann, Princeton College
The extra definitive mass signifies that the commonest sort of experiment to detect these elusive particles — a microwave resonance chamber containing a robust magnetic subject, wherein scientists hope to snag the conversion of an axion right into a faint electromagnetic wave — received’t have the ability to detect them, regardless of how a lot the experiment is tweaked. The chamber must be smaller than a number of centimeters on a facet to detect the higher-frequency wave from a higher-mass axion, Safdi mentioned, and that quantity can be too small to seize sufficient axions for the sign to rise above the noise.
“Our work gives essentially the most exact estimate so far of the axion mass and factors to a particular vary of plenty that isn't at present being explored within the laboratory,” he mentioned. “I actually do suppose it is sensible to focus experimental efforts on 40 to 180 µeV axion plenty, however there’s a variety of work gearing as much as go after that mass vary.”
One newer sort of experiment, a plasma haloscope, which seems for axion excitations in a metamaterial — a solid-state plasma — needs to be delicate to an axion particle of this mass, and will probably detect one.
“The essential research of those three-dimensional arrays of superb wires have labored out amazingly nicely, a lot better than we ever anticipated,” mentioned Karl van Bibber, a UC Berkeley professor of nuclear engineering who's constructing a prototype of the plasma haloscope whereas additionally taking part in a microwave cavity axion search known as the HAYSTAC experiment. “Ben’s newest outcome may be very thrilling. If the post-inflation state of affairs is true, after 4 many years, discovery of the axion might be vastly accelerated.”
If axions actually exist.
The work can be printed right now (February 25, 2022) within the journal Nature Communications.
Axion prime candidate for darkish matter
Darkish matter is a mysterious substance that astronomers know exists — it impacts the actions of each star and galaxy — however which interacts so weakly with the stuff of stars and galaxies that it has eluded detection. That doesn’t imply darkish matter can’t be studied and even weighed. Astronomers know fairly exactly how a lot darkish matter exists within the Milky Method Galaxy and even in your complete universe: 85% of all matter within the cosmos.
Up to now, darkish matter searches have targeted on large compact objects within the halo of our galaxy (known as large compact halo objects, or MACHOs), weakly interacting large particles (WIMPs) and even unseen black holes. None turned up a possible candidate.
“Darkish matter is a lot of the matter within the universe, and we don't know what it's. One of the excellent questions in all of science is, ‘What's darkish matter?'” Safdi mentioned. “We suspect it's a new particle we don’t learn about, and the axion might be that particle. It might be created in abundance within the Huge Bang and be floating on the market explaining observations which have been made in astrophysics.”
Although not strictly a WIMP, the axion additionally interacts weakly with regular matter. It passes simply by way of the earth with out disruption. It was proposed in 1978 as a brand new elementary particle that would clarify why the neutron’s spin doesn't precess or wobble in an electrical subject. The axion, in line with concept, suppresses this precession within the neutron.
“Nonetheless to at the present time, the axion is one of the best thought we've about tips on how to clarify these bizarre observations in regards to the neutron,” Safdi mentioned.
Within the Eighties, the axion started to be seen additionally as a candidate for darkish matter, and the primary makes an attempt to detect axions have been launched. Utilizing the equations of the well-vetted concept of basic particle interactions, the so-called Normal Mannequin, along with the speculation of the Huge Bang, the Normal Cosmological Mannequin, it's potential to calculate the axion’s exact mass, however the equations are so tough that so far we've solely estimates, which have different immensely. Because the mass is thought so imprecisely, searches using microwave cavities — primarily elaborate radio receivers — should tune by way of hundreds of thousands of frequency channels to attempt to discover the one comparable to the axion mass.
“With these axion experiments, they don’t know what station they’re alleged to be tuning to, in order that they must scan over many alternative potentialities,” Safdi mentioned.
Safdi and his group produced the latest, although incorrect, axion mass estimate that experimentalists are at present concentrating on. However as they labored on improved simulations, they approached a group from Berkeley Lab that had developed a specialised code for a greater simulation method known as adaptive mesh refinement. Throughout simulations, a small a part of the increasing universe is represented by a three-dimensional grid over which the equations are solved. In adaptive mesh refinement, the grid is made extra detailed round areas of curiosity and fewer detailed round areas of house the place nothing a lot occurs. This concentrates computing energy on a very powerful elements of the simulation.
The method allowed Safdi’s simulation to see 1000's of occasions extra element across the areas the place axions are generated, permitting a extra exact willpower of the whole variety of axions produced and, given the whole mass of darkish matter within the universe, the axion mass. The simulation employed 69,632 bodily laptop processing unit (CPU) cores of the Cori supercomputer with almost 100 terabytes of random entry reminiscence (RAM), making the simulation one of many largest darkish matter simulations of any sort so far.
The simulation confirmed that after the inflationary epoch, little tornadoes, or vortices, type like ropey strings within the early universe and throw off axions like riders bucked from a bronco.
“You possibly can consider these strings as composed of axions hugging the vortices whereas these strings whip round forming loops, connecting, present process a variety of violent dynamical processes in the course of the enlargement of our universe, and the axions hugging the edges of those strings are attempting to carry on for the experience,” Safdi mentioned. “However when one thing too violent occurs, they only get thrown off and whip away from these strings. And people axions which get thrown off of the strings find yourself changing into the darkish matter a lot in a while.”
By retaining observe of the axions which might be whipped off, researchers are in a position to predict the quantity of darkish matter that was created.
Adaptive mesh refinement allowed the researchers to simulate the universe for much longer than earlier simulations and over a a lot greater patch of the universe than earlier simulations.
“We clear up for the axion mass each in a extra intelligent method and likewise by throwing simply as a lot computing energy as we might presumably discover onto this downside,” Safdi mentioned. “We might by no means simulate our whole universe as a result of it’s too massive. However we don’t have to stimulate our whole universe. We simply have to simulate a sufficiently big patch of the universe for a protracted sufficient time frame, such that we seize the entire dynamics that we all know are contained inside that field.”
The group is working with a brand new supercomputing cluster now being constructed at Berkeley Lab that may allow simulations that may present an much more exact mass. Known as Perlmutter, after Saul Perlmutter, a UC Berkeley and Berkeley Lab physicist who received the 2011 Nobel Prize in Physics for locating the accelerating enlargement of the universe pushed by so-called darkish power, the next-generation supercomputer will quadruple the computing energy of NERSC.
“We need to make even greater simulations at even larger decision, which can enable us to shrink these error bars, hopefully all the way down to the ten% stage, so we will let you know a really exact quantity, like 65 plus or minus 2 micro-eV. That then actually modifications the sport experimentally, as a result of then it might turn into a neater experiment to confirm or exclude the axion in such a slim mass vary,” Safdi mentioned.
For van Bibber, who was not a member of Safdi’s simulation group, the brand new mass estimate exams the bounds of microwave cavities, which work much less nicely at excessive frequencies. So, whereas the decrease restrict of the mass vary continues to be throughout the potential of the HAYSTAC experiment to detect, he's enthused in regards to the plasma haloscope.
“Over time, new theoretical understanding has loosened the constraints on the axion mass; it may be wherever inside 15 orders of magnitude, if you happen to think about the chance that axions shaped earlier than inflation. It’s turn into an insane job for experimentalists,” mentioned van Bibber, who holds UC Berkeley’s Shankar Sastry Chair of Management and Innovation. “However a current paper by Frank Wilczek’s Stockholm concept group might have resolved the conundrum in making a resonator which might be concurrently each very massive in quantity and really excessive in frequency. An precise resonator for an actual experiment continues to be some methods away, however this might be the best way to go to get to Safdi’s predicted mass.”
As soon as simulations give an much more exact mass, the axion might, in reality, be simple to search out.
“It was actually essential that we teamed up with this laptop science group at Berkeley Lab,” Safdi mentioned. “We actually expanded past the physics subject and truly made this a computing science downside.”
Reference: “Darkish matter from axion strings with adaptive mesh refinement” 25 February 2022, Nature Communications.
DOI: 10.1038/s41467-022-28669-y
Safdi’s colleagues embrace Malte Buschmann of Princeton; MIT postdoctoral fellow Joshua Foster; Anson Hook of the College of Maryland; and Adam Peterson, Don Willcox and Weiqun Zhang of Berkeley Lab’s Heart for Computational Sciences and Engineering. The analysis was largely funded by the U.S. Division of Power by way of the Exascale Computing Undertaking (17-SC-20-SC) and thru the Early Profession program (DESC0019225).
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