Whether witnessed in the birth and growth of individual galaxies or in the formation of galaxy clusters and other mind-numbingly enormous cosmic structures—so much of what we see in the cosmos is governed by what we don’t see: dark matter. “Not only is dark matter in every single galaxy in the universe, but we need it to explain how the universe came to be. It’s really the seed of all that we see; it’s a main player behind the formation of all the structures in the universe,” says Miguel Escudero Abenza of CERN, the European laboratory for particle physics near Geneva.
Other than the obvious fact that it’s, well, dark—not emitting light—and that its gravitational bulk serves as cosmic scaffolding for galaxies, most aspects of dark matter are unknown. But axions (or more generally axionlike particles) are among the most popular and well studied of the many different candidates theorists have devised for what dark matter really is.
First introduced in the 1970s as a possible solution to an entirely different problem in physics having nothing to do with dark matter, axions are hypothetical ultralightweight particles that possess vanishingly minuscule mass—the value of which is only theorized but can fall anywhere between about 10–5 to 10–22 electron volts, or tens of thousands of times to billions of trillions of times lighter than the elusive neutrino. Axions and their “axionlike” theoretical ilk would naturally be “dark”—that is, electrically neutral, so they only interact with other matter through gravity, and cold, meaning that they are not moving around very much or very quickly—two properties that overlap with scientists’ best models of dark matter. This makes axions a great dark matter candidate but also renders them difficult to detect in a direct way, which is why despite being proposed nearly a half century ago, they remain entirely theoretical and as yet unconfirmed to actually exist.
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But if axionlike particles exist and are indeed dark matter’s main constituent, then a pair of papers published in Physical Review D in February suggest a new indirect way to detect them.
Merging Stars
Beyond being cold and electrically neutral, dark matter generally follows a third requirement: it needs to be stable—or at least, if it decays, that process needs to be longer than the current 13.8-billion-year estimated age of the cosmos. Otherwise dark matter just wouldn’t exist anymore, and astronomers wouldn’t see its effects playing out across the observable universe. In this regard, axions are a little different than other dark matter candidates. Axions should decay—albeit very slowly. When they do, they should emit photons, leading to potential observational effects. “But in order to explore the cosmological signatures of this decay, we need to know how many of them there are,” says Escudero Abenza, who is an author on both papers and led one of the two studies.
If dark matter is made up of axions, then something special should reside at the center of each “halo” of dark matter, which astronomers have surmised envelop most every galaxy. That special something at the heart of each halo would be an axion star, which is “like a star because it’s a compact, dense and spherical object, but it does not emit any light because it’s dark matter,” says Xiaolong Du, a postdoctoral researcher at the University of California, Los Angeles. Du led the second of the two studies, which focused on how frequently such “stars” could form and collide.
Theorists have already calculated the maximum possible mass for an axion star—a threshold beyond which these objects would decay into photons. In their paper, Du and his co-authors point out that if two merging dark matter halos (attached to merging galaxies or galaxy clusters, for example) each contain an axion star below this critical mass, that union should eventually merge the axion stars together, too, generating a new oversized axion star that promptly—and explosively—decays. This, Du says, is one of the few scenarios for today’s nearly 14-billion-year-old universe in which an axion star would be expected to decay, because any star that’s otherwise above this mass limit cannot exist.
A typical axion star should weigh in at somewhere around one ten-thousandth the mass of our sun, but the exact mass threshold for such an object’s decay is still unknown. Given a specific theoretical critical mass and the empirically measured density and distribution of galaxies in space, Du’s team calculated strong constraints for how frequently axion stars should form, undergo major mergers and explode.
Using that work as its basis, Escudero Albenza’s paper explores the cosmological consequences of axion stars as dark matter.
Cosmic Suppression
Shortly after the big bang, of course, no stars of any kind existed at all—back then, the early universe was filled with a very hot and dense plasma composed of electrons and protons. This all-pervading plasma was effectively opaque, scattering photons rather than letting them pass unimpeded. Yet the plasma cooled as the universe expanded, eventually becoming cold enough for the electrons and protons to bind together to form atoms. The emergence of atoms was like the lifting of a fog, freeing the light emitted from the last vestiges of fiery plasma to stream across the universe; we see that residual glow today as the all-sky cosmic microwave background (CMB). The subsequent formation of stars—and all the radiance they unleashed—ultimately reionized most of the normal matter in the universe to form the rarefied plasma that exists between galaxies, the so-called intergalactic medium.
In their paper, Escudero Albenza and his co-authors argue that if dark matter halos and axion stars also formed around this time, the axion stars’ explosive decay would have injected additional energy into the cosmos, contributing to the reionization and heating of the intergalactic medium. This contribution would reduce the visibility of the CMB, as the additional reionization would add another layer of opacity to the intergalactic medium. The team placed constraints on how strongly axions with masses between 10–14 and 10–8 electron volts can interact with photons. Although this is only a subset of the wide span of potential axion masses, axions that fall within this smaller theoretical range would have detectable effects on the CMB’s photons through heating and ionization. “If we have additional energy injected into the universe by axion stars, this process [of suppressing the CMB] may happen earlier than expected,” says Du, who is also an author on Escudero Albenza’s paper.
Looking at how the CMB may have been altered—how much smaller its signal is than expected because of its additional suppression from axion stars—can therefore be a way to measure axionlike dark matter, albeit very indirectly.
“These papers show that even a very simple theory can lead to very complex behavior, and that complex behavior can actually be very important, even if it’s a rare phenomenon,” says Tim Tait, a theorist working on dark matter models at the University of California, Irvine, who was not involved in either paper. Both, he says, are unique in that the authors aren’t building a novel model of dark matter or introducing anything wholly new to the axion story. Instead they’re simply doing a very careful analysis of the current axion dark matter theory and the phenomena it predicts, leading to fresh insights about how these almost impossibly featherweight and as-yet-hypothetical particles may collectively act to profoundly shape the universe at large.