Physicists looking out — unsuccessfully — for immediately’s most favored candidate for darkish matter, the axion, have been wanting within the mistaken place, in keeping with a brand new supercomputer simulation of how axions had been produced shortly after the Massive 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 large as theorists and experimenters have thought: between 40 and 180 microelectron volts (micro-eV, or ?eV), or about one 10-billionth the mass of the electron. There are indications, Safdi stated, that the mass is near 65 ?eV. Since physicists started on the lookout for the axion 40 years in the past, estimates of the mass have ranged extensively, from a couple of ?eV to 500 ?eV.
“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 stated.
The extra definitive mass signifies that the most typical kind of experiment to detect these elusive particles — a microwave resonance chamber containing a robust magnetic area, during which scientists hope to snag the conversion of an axion right into a faint electromagnetic wave — will not be capable to detect them, irrespective of how a lot the experiment is tweaked. The chamber must be smaller than a couple of centimeters on a facet to detect the higher-frequency wave from a higher-mass axion, Safdi stated, and that quantity could be too small to seize sufficient axions for the sign to rise above the noise.
“Our work offers essentially the most exact estimate thus far of the axion mass and factors to a selected vary of plenty that’s not presently being explored within the laboratory,” he stated. “I actually do suppose it is smart to focus experimental efforts on 40 to 180 ?eV axion plenty, however there’s quite a lot of work gearing as much as go after that mass vary.”
One newer kind of experiment, a plasma haloscope, which seems to be for axion excitations in a metamaterial — a solid-state plasma — ought to be delicate to an axion particle of this mass, and will probably detect one.
“The fundamental research of those three-dimensional arrays of high-quality wires have labored out amazingly nicely, a lot better than we ever anticipated,” stated 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 end result could be very thrilling. If the post-inflation situation is true, after 4 a long time, discovery of the axion may very well be drastically accelerated.”
If axions actually exist.
The work will probably be revealed Feb. 25 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 does not imply darkish matter cannot be studied and even weighed. Astronomers know fairly exactly how a lot darkish matter exists within the Milky Manner Galaxy and even in your complete universe: 85% of all matter within the cosmos.
To this point, darkish matter searches have targeted on huge compact objects within the halo of our galaxy (known as huge compact halo objects, or MACHOs), weakly interacting huge particles (WIMPs) and even unseen black holes. None turned up a probable candidate.
“Darkish matter is a lot of the matter within the universe, and we do not know what it’s. Some of the excellent questions in all of science is, ‘What’s darkish matter?'” Safdi stated. “We suspect it’s a new particle we do not learn about, and the axion may very well be that particle. It may very well be created in abundance within the Massive Bang and be floating on the market explaining observations which were made in astrophysics.”
Although not strictly a WIMP, the axion additionally interacts weakly with regular matter. It passes simply by means 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 area. The axion, in keeping with principle, suppresses this precession within the neutron.
“Nonetheless to this present day, the axion is one of the best thought we’ve about how you can clarify these bizarre observations in regards to the neutron,” Safdi stated.
Within the Nineteen Eighties, the axion started to be seen additionally as a candidate for darkish matter, and the primary makes an attempt to detect axions had been launched. Utilizing the equations of the well-vetted principle of basic particle interactions, the so-called Customary Mannequin, along with the idea of the Massive Bang, the Customary Cosmological Mannequin, it’s attainable to calculate the axion’s exact mass, however the equations are so tough that thus far we’ve solely estimates, which have assorted immensely. Because the mass is understood so imprecisely, searches using microwave cavities — basically elaborate radio receivers — should tune by means of hundreds of thousands of frequency channels to attempt to discover the one comparable to the axion mass.
“With these axion experiments, they do not know what station they’re alleged to be tuning to, so that they should scan over many alternative potentialities,” Safdi stated.
Safdi and his workforce produced the newest, although incorrect, axion mass estimate that experimentalists are presently focusing on. However as they labored on improved simulations, they approached a workforce from Berkeley Lab that had developed a specialised code for a greater simulation approach 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 components of the simulation.
The approach allowed Safdi’s simulation to see hundreds of instances extra element across the areas the place axions are generated, permitting a extra exact dedication of the overall variety of axions produced and, given the overall 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 form thus 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’ll be able to consider these strings as composed of axions hugging the vortices whereas these strings whip round forming loops, connecting, present process quite a lot of violent dynamical processes throughout the enlargement of our universe, and the axions hugging the edges of those strings are attempting to carry on for the journey,” Safdi stated. “However when one thing too violent occurs, they simply get thrown off and whip away from these strings. And people axions which get thrown off of the strings find yourself turning into the darkish matter a lot in a while.”
By protecting 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 larger patch of the universe than earlier simulations.
“We resolve for the axion mass each in a extra intelligent manner and in addition by throwing simply as a lot computing energy as we might probably discover onto this downside,” Safdi stated. “We might by no means simulate our total universe as a result of it is too large. However we needn’t stimulate our total universe. We simply have to simulate a sufficiently big patch of the universe for a protracted sufficient time period, such that we seize all the dynamics that we all know are contained inside that field.”
The workforce 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 gained 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 wish to make even larger 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 are able to let you know a really exact quantity, like 65 plus or minus 2 micro-eV. That then actually adjustments the sport experimentally, as a result of then it could turn out to be a better experiment to confirm or exclude the axion in such a slender mass vary,” Safdi stated.
For van Bibber, who was not a member of Safdi’s simulation workforce, the brand new mass estimate checks the boundaries of microwave cavities, which work much less nicely at excessive frequencies. So, whereas the decrease restrict of the mass vary continues to be inside the skill of the HAYSTAC experiment to detect, he’s enthused in regards to the plasma haloscope.
“Through the years, new theoretical understanding has loosened the constraints on the axion mass; it may be anyplace inside 15 orders of magnitude, when you take into account the chance that axions fashioned earlier than inflation. It is turn out to be an insane activity for experimentalists,” stated van Bibber, who holds UC Berkeley’s Shankar Sastry Chair of Management and Innovation. “However a current paper by Frank Wilczek’s Stockholm principle group might have resolved the conundrum in making a resonator which may very well be concurrently each very giant in quantity and really excessive in frequency. An precise resonator for an actual experiment continues to be some methods away, however this may very well be the way in which to go to get to Safdi’s predicted mass.”
As soon as simulations give an much more exact mass, the axion might, the truth is, be straightforward to search out.
“It was actually essential that we teamed up with this laptop science workforce at Berkeley Lab,” Safdi stated. “We actually expanded past the physics area and truly made this a computing science downside.”
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 Vitality by means of the Exascale Computing Venture (17-SC-20-SC) and thru the Early Profession program (DESC0019225).
Video on measuring an axion: https://youtu.be/hikmvEbO-vA