“There’s something happening in the data we don’t understand,” said Rebecca Leane, a theoretical physicist at the Massachusetts Institute of Technology said about the Milky Way galaxy’s strange glow of its central region. Astrophysicists have been baffled by what could be powering this excess of energetic light. “Something about our understanding of the gamma rays is missing at this stage,” she added. “It’s possible to hide a dark matter signal, if it were really there.”
In March of 2018, The Galaxy reported that according to predictions by the Max Planck Institute for Gravitational Physics and the Max Planck Institute for Radio Astronomy, the observed excess of high-energy gamma-radiation glow from the central region of the Milky Way could be due to a hidden population of thousands of millisecond pulsars. While only a handful might be detectable with current large radio telescopes, gamma-ray searches might have a better chance of finding more of these sources.
But a re-examination of the pulsar theory as the source of the glow posted on the scientific preprint site arxiv.org, revealed the possibility that space-based instruments have found the first direct evidence of the invisible “dark matter” thought to pervade the universe.
Pulsars or Dark Matter?
The problem first appeared in 2009, writes Charlie Wood in “Dark Matter Gets a Reprieve” for Quanta, when Dan Hooper, an astrophysicist at the University of Chicago noticed that NASA’s Fermi Gamma-ray Space Telescope appeared to be picking up too many of the energetic photons known as gamma rays.
“Hooper,” continues Wood in Quanta, “suggested the anomaly could originate from a theoretical jumbling throng of dark matter particles in the galaxy’s center. While dark matter doesn’t shine or fraternize with known particles, in the right sort of collision these particles could annihilate in a shower of familiar matter and antimatter that would then go out with a puff of gamma rays. A measurement of these offshoots would represent the first evidence of dark matter that wasn’t exclusively gravitational in nature.”
But according to earlier studies the glow could be powered by cosmic undiscovered beacons known as millisecond pulsars -magnetically charged neutron stars that make a thousand turns each second- bathing the center of the galaxy in extra gamma rays.
The two studies released in 2015 observed that the Fermi data looked grainy. It had bright pixels, writes Wood, “suggestive of multiple millisecond pulsars, and dim pixels suggestive of no pulsars. If dark matter was the culprit, it should have colored all pixels more evenly. The dark matter interpretation, it seemed, was dying.”
Now Leane and Tracy Slatyer, a theoretical physicist at MIT and a co-author of one of those 2015 pulsar studies, have found that any dark matter in the galactic center would have been misidentified as pulsars by earlier studies.
tainted the analysis, leading to the erroneous conclusion that our dark matter–infused Milky Way had little dark matter.
Their digital model was proof of concept for how surprising additions could shift results, but to see if something similar might be affecting our real galaxy, the MIT team added hypothetical dark-matter data to actual Fermi data finding that their data analysis did not correctly identify the added dark matter giving instead too much weight to grainy, pulsar-like points. The technique failed to detect any of the fake dark matter until the researchers had injected enough to account for the observed gamma-ray glow five times over.
Researchers emphasize that this result contains no new evidence for dark matter. Rather, it weakens the competing explanation for the galaxy’s gamma ray glow. “It puts the dark matter explanation into slightly better shape,” said Martin Winkler, a physicist at Stockholm University.
Recent data analyzed from the International Space Station’s Alpha Magnetic Spectrometer (AMS) experiment show that it has detected higher than expected levels of antiprotons — another possible remnant of dark matter collisions.
The kind of dark matter particles that would be needed to produce the AMS data, write Wood, “are approximately the same kind of dark matter particles that would be needed to produce the observed gamma-ray glow in the center of the galaxy. The overlap has encouraged some astrophysicists to believe that they may be looking at a two-for-one explanation.”
“If you told me the background model we were using is actual reality, I’d be running around screaming, ‘Dark matter!’ right now,” says Tim Linden, an astrophysicist at Ohio State University who co-authored the second of the recent papers about the inconclusive nature of the AMS data.
A clearer point for dark matter, however, would be if AMS or another high-altitude experiment found heavier, lumbering antimatter particles, which cosmic rays rarely produce. “That would be a smoking gun,” said Winkler.
“If the galactic center excess is back in the game,” Leane said of the antiproton measurements, “potentially we are seeing the first signal of dark matter.”
Other efforts to detect the Milky Way’s dark matter have been undertaken following the Gaia Spacecraft’s April 2018 data release. Astronomers have unveiled a Milky Way teeming with astounding surprises, including hints of dark-matter clumps that might eventually provide a better grasp of the elusive material’s properties. These early, easy-to-spot findings, reports Adam Mann in Nature, have been “transformational.” Astronomer Vasily Belokurov at the University of Cambridge, UK, said they are merely a glimpse of what is to come: “How we see the Milky Way has clearly changed.”
Gaia Spacecraft and Search for Milky Way’s Dark Matter
Theorists suspect that our Galaxy sits inside an enormous, roughly spherical halo of dark matter that, much like ordinary matter, has clumped together into smaller structures thanks to gravity. Cosmological simulations suggest that thousands of large dark-matter clumps orbit the Galaxy, occasionally getting eaten by a mass of dark matter at the center, in a process akin to the Milky Way’s consumption of its small satellite dwarf galaxies.
Gaia’s precision will ultimately be 100 times greater than any previous effort. And thanks to its sensitivity, it can probe deeper into the Galaxy: some 99% of the more than 1 billion stars it observes have never had their distances accurately determined. Knowing where each star is, reports Nature, “and where it’s going allows researchers to tease out hidden Milky Way history”.
Gaia is forcing researchers to take a second look at some of the canonical assumptions that are used to simplify models, says astrophysicist Adrian Price-Whelan at Princeton University. “We knew those assumptions were wrong,” he adds. “Gaia has now shown us how wrong they were.
The vast majority of the dark-matter substructures are thought to contain few or no stars make them hard to detect, but Gaia might have found a hint of one in GD-1, a long stream of stars discovered in 2006 that stretches across half of the northern sky. Gaia enabled Price-Whelan and astronomer Ana Bonaca at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, to pick out true members of the group. The two study how the tidal field of the Milky Way galaxy disrupts globular clusters, and what the resulting debris can tell us about the underlying distribution of dark matter.
Last November 2018, Price-Whelan and Bonaca and two other colleagues identified a distinct gap, that could be the scar of an encounter with a massive object some 500 million years ago. “As the dark object sped past the stream, it might have separated the train of stars by gravitationally tugging on some, reports Mann, allowing them to pull ahead of their companions.
“The most likely culprit seems to be a dense dark-matter clump, probably somewhere between 1 million and 100 million times the mass of the Sun,” says Bonaca, an estimate could have implications for physical models of dark matter. A dark-matter particle’s mass helps to dictate how fast it can move and, in turn, the size of clusters it is liable to form. The GD-1 perturber’s size is in an interesting range, says Bonaca, that could eliminate hypothesized dark-matter candidates that are relatively low in mass.
Bonaca and her team are now interested in using Gaia data to determine the orbit of the ancient dark-matter object. “If they can ascertain where it could be found today,” reports Nature, “they might be able to detect its gravitational effects on other material. Or perhaps they could train γ-ray telescopes on the spot to look for evidence of dark-matter particles annihilating one another or decaying, processes that could emit energetic photons. Either technique could offer a more-direct probe of the invisible substance’s physical properties.”
Image at the top of the page: Excess glow at the galactic center. The intrepid researchers in the post above have argued that the signal might hint at hypothetical dark matter particles. However, it could also have other cosmic origins. (NASA; A. Mellinger/Central Michigan University; T. Linden/University of Chicago)