The discovery of dark matter would mean a major breakthrough in efforts to advance our understanding of the universe. This is why any hint of such a discovery is greeted with mounting excitement. It's also why such hints should be met with scepticism, writes Richard Panek.
An announcement of the possible discovery of dark matter—for instance, the one this past June 15 from the XENON1T collaboration that generated headlines around the world—merits either of two responses.
One is “Big deal.”
The other is “Big deal.”
The actual discovery of dark matter would in fact be a really big deal, one of the biggest in the history of physics. It would solve a mystery that has been plaguing cosmologists for nearly half a century, but more importantly, it would help validate a reinterpretation of the universe that renders everything we’d previously thought to be the cosmos in its entirety (galaxies, stars, planets, people, pixels) only four or five percent of what’s actually out there.
SUGGESTED READING Cutting edge science at HowTheLightGetsIn Global By Throughout the 1970s, astronomers using new instruments found that spiral galaxies like our own Milky Way were rotating in a way that didn't make sense, at least according to the laws of gravitation. The outermost wisps of gas and dust and stars were rotating at the same rate as the innermost clots of gas and dust and stars. On the scale of our solar system, the analog would be if the outermost and innermost planets were keeping pace with each other in a race around the Sun, even though Neptune is 73 times more distant from the Sun than Mercury.
If a galaxy were actually just as fast at the rim as at the center, it would be flying apart. It couldn’t survive even a couple of rotations. Yet our own Milky Way galaxy is now on its eighteenth trip around the galactic center, at the rate of one every 250 million years.
Despite the absence of direct evidence of dark matter, the indirect evidence has only grown.
Physicists using computer simulations, however, found that they could stabilize a galaxy gravitationally if they posited a bubble of matter around it roughly ten times more massive than the galaxy.
Or, more accurately, the visible galaxy. The other matter—we’ll call it dark, they decided—seemed to be invisible in any part of the electromagnetic spectrum, from microwave to gamma ray. Physics theorists calculated the properties of such matter, then left the discovery to the physics empiricists: Design detectors to look for subatomic particles possessing specific properties—mass, cosmic density, and so on—and there the particle will be waiting, along with your Nobel Prize.
In 1980 Vera Rubin, one of the astronomers who had discovered some of the most compelling evidence of missing mass in spiral galaxies, predicted the discovery of dark matter within ten years. Ten years later, British astronomer (and future Astronomer Royal) Martin Rees predicted the discovery of dark matter within ten years. Eleven years later, in his book Our Cosmic Habitat, Rees wrote, “I think there is a good chance of achieving this goal within ten years.” Five years later, in 2006, Rees doubled down on that prediction at an American Institute of Physics symposium: Five more years, he vowed. Vera Rubin, who happened to be in the audience, stood up and announced, with an ironic edge familiar to her peers, “I know of earlier predictions.”
Despite the absence of direct evidence of dark matter, the indirect evidence has only grown. Yes, dark matter would explain the rotation rates of galaxies that astronomers discovered in the 1970s, but it would also explain subsequent observations of the universe on the largest scales. Take away the overwhelming gravitational influence of dark matter on “regular” matter, and the universe wouldn’t have evolved into the observable “cosmic web”: filaments of galaxy superclusters ranging in length from hundreds of millions to several billion light-years, all separated by vast expanses of empty space.
The 1998 discovery of evidence for dark energy—the energy part of the moniker being a linguistic extension of dark matter—made the argument for the existence of dark matter even more compelling. Two competing teams of scientists independently found that the expansion of the universe isn’t doing what you might expect an expanding universe full of matter interacting with all other matter under the influence of gravity should be doing—slowing down. Instead, it’s accelerating. As subsequent censuses of the composition of the universe have shown again and again and again, a little more than two-thirds dark matter plus a little more than a quarter dark energy plus four or five percent “normal” matter adds up to the universe we observe.
Hence the excitement attending any announcement of a possible detection. Hence, also, the second response to an announcement: a cautiously pessimistic “Big deal.”
When I learned about the XENON1T announcement in June via an email press release, I didn’t even click the hyperlink: If it’s a big deal—the really big one—I’ll know about it soon enough. But if this announcement is premature, well, it wouldn’t be the first time.
In 2009, the CDMS II (Cryogenic Dark Matter Search) collaboration scheduled a series of announcements at various institutions over the course of a single day. Surely they had something to reveal, the thinking went, and soon rumors about the result were dominating the particle physics slice of the blogosphere: “Dark matter discovered?”; “Has Dark Matter Finally Been Detected on Earth?”; “Rumor has it that the first dark matter particle has been found!”; “¿Se ha descubierto la materia oscura en el CDMS?”;“Pátrání po supersymetrické skryté hmotě”; みんな大好き“ (か、どうかは分かりませんが） dark matter を検出したという報告が出ています。”
XENON100 collaboration reported six detections of something, a number statistically significant enough to count as a detection...if the data could survive further scrutiny. It did not.
In the end, the “news” was that the CDMS II detectors, deep underground in a former iron mine in Minnesota, had registered two detections of something. Which was better than the nothing that dark-matter experiments usually revealed. But two was still a statistically insignificant number.
A premature announcement wouldn’t even be a first for the XENON team. In April 4, 2011, the XENON100 collaboration, a precursor to XENONT1, reported six detections of something. Six: a number statistically significant enough to count as a detection...if the data could survive further scrutiny.
It did not. Over the next few days the XENON100 team discovered that they had to attribute three events to electronic noise. Which still left them with three events. But they had known in advance that no matter how many or how few “detections” they found, they would have to discount two—or, more accurately, 1.8 ± 0.6, the number they had calculated that in a sample of this particular size would be due to radioactive interference. Which left them with one, a statistically insignificant number. Nine days after the initial announcement, when the collaboration posted their paper online, it included this conclusion in the abstract: “no evidence for dark matter.”
Even XENON1T’s announcement this past June 15 came with caveats. The experiment—a mile under the rocky surface of Gran Sasso, a mountain in Italy’s Appenine range—was looking for axions (a hypothetical particle that would solve some problems with the Standard Model of particle physics). But team members acknowledged that even if what they’d found were axions, they would likely be the “wrong” axions: ones that emerged several minutes ago from the Sun rather than 13.8 billion years ago from the primordial soup at the dawn of the universe. Then again, what they’d detected might match predictions of a new variation on the neutrino. Or possibly it was the result of subatomic pollution—radiation from tritium.
But if what the XENON1T collaboration found did turn out to be the kind of axion that would qualify as dark matter, at least they had staked their claim. In this sense, such announcements are a modern variation on the ciphers that their seventeenth-century ancestors used in order to establish priority.
Scientists understand these nuances. Most of the science press does, too; once you read past the breathless headlines and encounter the qualifiers, you can’t help but think, “No big deal.” But further analysis of the XENON1T’s data will follow, and while the result is likely to be disappointing, hope springs eternal: The day will come when we will get to open an email press release and say, “Yes! Big deal.”
Richard Panek has been a Guggenheim Fellow in science writing. His latest book, “The Trouble with Gravity: Solving the Mystery Beneath Our Feet,” is now available in paperback.