Nearby dark-matter-free zone poses cosmic conundrum

2019-03-04 12:15:10

By Lisa Grossman THE invisible dark matter credited with providing over 80 per cent of the universe’s mass is by its nature inconspicuous – but what if it doesn’t exist at all? A new survey of the stars within 13,000 light years of Earth suggests our patch of the Milky Way could be free of the elusive stuff, yet that would contradict otherwise successful explanations for how galaxies stick together and how our universe formed. “The claim is that in this volume of space, there is no dark matter,” says Christian Moni Bidin at the University of Concepción in Chile, who led the survey. Two other recent surveys also suggest a local absence of dark matter. The surprising findings come as physicists wrestle with conflicting results from experiments designed to detect dark matter directly on Earth (see “The ongoing WIMP war”). The results also fit with the view of a fiery minority who have long disputed the existence of dark matter. The need for dark matter emerged from the way that our galaxy rotates. If the only matter in the Milky Way is the visible stuff like stars and planets, then stars at the edge are moving too quickly to be held together just by our galaxy’s gravity. Some extra mass must be creating the gravity needed to hold them. A similar situation has since been observed in other galaxies, and it is calculated that the missing mass would amount to about 83 per cent of the total mass of the universe. This dark matter has fed into our understanding of how the universe formed, leading to predictions for how dark matter must be distributed. For example, supercomputer simulations of the entire history of the universe that include these distributions do an excellent job of reproducing the structures we see in the universe today. There are also predictions for how the stuff is distributed within the Milky Way, which should have a measurable effect on the motions of its stars. These velocities can be measured with Earthly telescopes, but until now most surveys considered only stars that zip around the Milky Way radially, like ants riding on a vinyl record. These motions are easy to chart. “Most surveys considered only stars zipping around the Milky Way radially, like ants riding a vinyl record” By contrast, stars moving towards or away from Earth are much harder to measure. Moni Bidin’s team did it by combining historical survey data and new observations from telescopes at the La Silla Observatory and the Las Campanas Observatory, both in Chile. The result was a record of the motions of more than 400 stars within 13,000 light years of the sun. These measurements could be used to estimate the mass of matter, both dark and visible, in a volume four times larger than had been considered before at this level of precision. The standard dark matter theory predicts at least as much dark matter as visible matter in this region, but the researchers were able to account for all the stars’ motions that they measured with just the normal visible matter. “The result matches the visible mass strongly,” says Moni Bidin. The work, which will be published in The Astrophysical Journal, could explain why experiments on Earth hoping to catch particles of dark matter have turned up confusing results. It’s too soon to give up on dark matter, though. We still need that extra mass to explain why galaxies hold together – and to set in motion the processes that led to many structures in the universe, from dwarf galaxies to superclusters. “We have many independent lines of reasoning that lead us to the conclusion that we have substantial amounts of dark matter in the local part of our galaxy,” says dark matter theorist Dan Hooper at Fermilab in Batavia, Illinois. “This is not going to be easily abandoned as an idea. I’m not saying they’re wrong, just that you’re going to have to work really hard to convince me.” Moni Bidin’s colleague Rory Smith, also at the University of Concepción, agrees that dark matter is still needed: “It explains an enormous number of things famously well, and puts them together into one framework.” “Dark matter explains a number of things famously well and puts them into one framework” There are other signs that dark matter is misbehaving, however. According to standard cosmology, dark matter drew together under its own gravity to form small clusters shortly after the big bang. Those clusters snowballed in size, and galaxies as we see them today grew up inside colossal, near-spherical blobs of dark matter. If this is true, streams of stars, clusters and small galaxies that orbit the Milky Way should be distributed randomly in a sphere around the main disc. But Pavel Kroupa at the University of Bonn in Germany reports in a paper to appear in Publications of the Astronomical Society of Australia that most of them are clustered in an enormous disc that rotates in a plane perpendicular to that of the Milky Way. That disc could be the remnants of another galaxy that collided with the Milky Way some 11 billion years ago, but it could not be the result of dark matter, Kroupa says. Meanwhile, a study to be published in Astrophysical Bulletin that used just the radial velocities of stars found, similar to Moni Bidin’s team, that much less dark matter than expected was required to explain the motions of stars in the local universe. An alternative explanation for the high speeds of stars on the outer edge of the Milky Way could account for all three anomalies. Called modified Newtonian dynamics, it is supported by a significant minority of physicists, including Kroupa. MOND has it that gravity works differently at vast cosmological scales than on the scales we are used to. Most physicists, however, think dark matter is too successful to abandon, and MOND too immature to replace it. Moni Bidin’s team prefers to wait for clarification from star surveys like the upcoming Gaia mission, which will collect data on the positions and velocities of about a billion stars in and around our galaxy. “MOND has done an incredible job explaining rotation curves really accurately,” says Smith. “But at how well it’s been tested, at the moment it’s not in a place to replace dark matter.” Even for dark matter’s biggest fans, there’s trouble brewing. Labs across the world are waiting with detectors wide open for hypothetical dark matter particles called WIMPs (weakly interacting massive particles), but what they are seeing is not easy to explain. The “WIMP wars” have raged since 1998, when the DAMA experiment in the Gran Sasso lab in Italy claimed its detector was sparkling with particles that could be WIMPs. The team reported the same in 2008, but no other experiment had seen anything. The plot thickened in 2011, when two other experiments – CRESST II, also in Gran Sasso, and CoGeNT, housed in the Soudan mine in Minnesota – reported flickers of dark matter in their detectors, too. That would have made a pretty strong case, if not for two similar experiments that have seen no dark matter at all. Xenon100, a tub of liquid xenon in Gran Sasso, and CDMS II, next door to CoGeNT, have so far come up empty. CDMS presented its most recent disappointing results in February at the Dark Matter 2012 conference in Los Angeles. Subsequently, Juan Collar, a member of the rival CoGeNT team at the University of Chicago noticed that their results only considered WIMPs above a certain energy. When he re-analysed the CoGeNT data for WIMPs of lower energies too, he found a signal that might be consistent with particles of dark matter ( Sadly, the signal still differs from the detections by the other groups, so Collar is not announcing evidence for dark matter yet. “I’m personally not ready to believe that we’re seeing WIMPs,