Is Time Running Out On Dark Matter?

Time may be running out on the search for the cosmos’ exotic dark matter. Decades after the first searches for dark matter’s hypothetical exotic particle counterparts, researchers are mostly at a loss to explain why there still has been no direct detection. That is, one that could explain why such unseen, dark matter particles only appear to weakly interact with normal matter. Either some 85 percent of the universe’s matter is non-baryonic (or exotic), or there’s something awfully wrong with our current understanding of how gravity works on the largest scales. Although most researchers won’t state it publicly, more than two decades after writing my first article on the subject for the U.K.’s Financial Times newspaper, I sense an undercurrent of weariness with this vexing problem. In addition to the palpable desperation, there is also an air, among some, of religious conviction,” Stacy McGaugh, an astronomer at Case Western University in Cleveland, told me. He notes the thinking appears to be: ‘So what if we haven’t detected dark matter? We’re sure it’s there.’ Yet as McGaugh notes, all we really know is that our known laws of physics --- “normal” gravity plus “normal” particles --- don’t suffice to explain the universe. “Which of those is to blame depends on how you weigh the various lines of evidence,” said McGaugh. “That is a procedure fraught with very human bias.” Among those who are convinced that such exotic particles are part of our cosmology, there’s a defensiveness about why it must be so. “We’ve been looking for dark matter for four decades or more in the modern era,” Pasquale Blasi, a theoretical physicist at Italy’s Gran Sasso Science Institute, told me. Thus far, we’ve only had indirect detection of dark matter’s existence, Blasi told me last month at the CRISM 2018 (Cosmic Rays and the Interstellar Medium) conference in Grenoble, France. Arguably, the most compelling such evidence is dark matter’s astrophysical gravitational effects on galactic rotation and formation. Blasi says the idea is that, as a result of annihilation or decay, these dark matter particles would cause the emission of gamma rays or electrons and positrons or even other kinds of radiation. One would ideally like to look for this dark matter signal in our Milky Way’s galactic center, where theoretically there should be more of this exotic unseen matter, Blasi notes. But from an astrophysical point of view, he says the galactic center is a mess, with an observational confusion limit caused by the sheer numbers of stars there. So, to date, there’s no smoking gun for dark matter annihilation or decay from our galactic center, says Blasi. As for the solution to this conundrum? Blasi speculates that maybe we don’t understand something intrinsic about gravity. Maybe, he says, what we are seeing is that gravity at some level of acceleration or on some scales behaves differently. Some form of WIMP (Weakly-interacting Massive Particle) remains a prime hypothetical dark matter particle candidate. Is it possible that researchers are simply not looking for large enough WIMPs? This particle could be a WIMPzilla, a super heavy dark matter particle, says Blasi. But he contends it could also be a heretofore undetected very light particle. When asked about the current search for the mass ranges of dark matter particles, Blasi pauses and places his hands on the table between us. He stretches his arms to each side and then moves his hands together until they almost touch. “We’re only looking at a range of between 1 GeV (Giga electron Volts) and 100 GeV or a TeV (Terra electron Volt) or so,” said Blasi. In dark matter terms, a hypothetical dark matter particle’s mass is measured in its rest energy (or the energy equivalent of a particle at rest). We are not ruling out dark matter particles which are this heavy or this light, says Blasi, indicating unprobed mass ranges to his left and right. As for the role of cosmic rays in the hunt for dark matter? Blasi says researchers need to better understand the subtleties of the cosmic ray background very well before they can conclude that any given flux of cosmic rays is a signal of dark matter.

Over the last 30 years, all claims about dark matter discovery via cosmic rays were disprovedeither due to an underestimation of the astrophysical background that produces cosmic rays, or an overestimation of the cosmic ray measurement, David Maurin, an astrophysicist at France’s University of Grenoble Alps, told me. He says this was either due to a miscalibration of the instrument, or the fact that the instrument was pushing its limits in measuring higher energy cosmic rays.

To convince the research community of an indirect detection of dark matter annihilation or decay, Maurin says astronomers would need to see cosmic ray excesses in multiple targets on the sky as well as in several different types of cosmic ray particles.

The classical WIMP that we were thinking of, says Blasi, is very strongly limited if not ruled out.But so far nothing has ruled out a WIMPzilla, he says.

I would like it to be nonbaryonic dark matter because that would open up more research possibilities, says Blasi. “But I definitely can’t rule out the possibility that we’re looking at some modification of gravity,” he said. Perhaps the answer lies in new direct detection experiments like PandaX-II, a xenon-based experiment at China’s Jinping underground lab, the deepest in the world. A new paper just published in the journal Physical Review Letters sets what it terms “strong limits” for dark matter masses ranging from 5 GeV to 10 TeV. But specifically, this international team imposes new conditions on how dark matter may interact with normal matter’s protons and neutrons via non-gravitational means, the University of California, Riverside, reports. In the case of PandaX-II, that would mean a dark matter particle colliding with the experiments liquified xenon. That would produce signals of both electrons and photons. “[Our] paper demonstrates the possibility of testing the self-interacting nature of dark matter,” research team member Hai-Bo Yu, a University of California, Riverside physicist, told me. What's next? “The team is upgrading the detector, and we expect to collect more data in the next few years,” said Yu. In addition, we will analyze observational data from galaxy surveys to further test dark matter self-interactions, he says. But despite the progress with such new detectors, for some, the time has come for a theoretical reckoning. McGaugh says one huge problem is that while dark matter theory is ‘confirmable’ it is not ‘falsifiable’ as a scientific theory should be. “There is no clear way to know [that] what you’re looking for --- but failing to find --- doesn’t exist at all,” said McGaugh. “If you’re convinced it must [exist], you’ll go on looking forever.” There’s no exit strategy, says McGaugh. Indeed, he says his colleagues have heedlessly blown through many experiential markers which they have chosen to ignore. “The search for dark matter has become a quagmire of confirmation bias,” McGaugh concludes.