One Saturday I hired a metal detector and drove four hours to the historic gold-rush town of Bright in Victoria, Australia, where my wedding ring lies lost, somewhere on the bed of the Ovens River. I spent the evening wading through the icy waters in gumboots, uncovering such treasures as a bottle cap, a fisher’s lead weight and a bracelet caked in rust. I did not uncover the ring. But that doesn’t mean the ring is not there.
Like me, physicists around the world are in the midst of an important search that has so far proven fruitless. Their quarry is nothing less than most of the matter in the universe, so-called “dark matter”.
So far their most sensitive detectors have found – to be pithy – nada. Despite the lack of results, scientists aren’t giving up. “The frequency with which articles show up in the popular press saying ‘maybe dark matter isn’t real’ massively exceeds the frequency with which physicists or astronomers find any reason to re-examine that question,” says Katie Mack, a theoretical astrophysicist at the University of Melbourne.
In many respects, the quest for dark matter has only just begun. We can expect quite a few more null results before the real treasure turns up. So here is where we stand, and what we can expect from the next few years.
Imagine a toddler sitting on one end of a seesaw and launching her father, at the other end, high into the air. It’s a weird and unsettling image, yet we regularly observe this kind of ‘impossible’ behaviour in the universe at large. Like the little girl on the seesaw, galaxies behave as if they have four or five times the mass we can see.
Our first inkling of this discrepancy came in the 1930s, when the Swiss astronomer Fritz Zwicky noticed odd movements among the Coma cluster of galaxies. Zwicky’s anomaly was largely ignored until the 1970s, when astrophysicist Vera Rubin, based at the Carnegie Institute in Washington, noticed that the way galaxies spin did not tally with the laws of physics. The meticulous observations by Rubin (who passed away in December 2016) convinced most of the astronomical community something was amiss. There were two possible answers to the problem: either galaxies were a lot heavier than they appeared, or our theory of gravity was kaput when it came to galaxy-scale movements.
From the outset, astronomers preferred the first explanation. At first they thought the missing matter was probably nothing too weird – just regular astronomical objects (like planets, black holes and stars) too dim for us to see. But as we surveyed the sky with ever bigger telescopes, these so-called ‘massive compact halo objects’ (or MACHOs) never turned up in the numbers needed to explain all the extra mass.
Other astrophysicists, such as the Mordehai Milgrom at Israel’s Weizmann Institute, explored models where gravity behaved differently at cosmic scales. They were not successful.
Slowly astronomers realised they had something radically different on their hands – a new kind of stuff they called ‘dark matter’, which must outweigh the universe’s regular matter by about five to one. “Certainly, when all the evidence is taken together,” Mack says, “there’s no competing idea right now that comes anywhere close to explaining it as well.”
We know four main facts about dark matter. First, it has gravity. Second, it doesn’t emit, absorb or reflect light. Third, it moves slowly. Fourth, it doesn’t seem to interact with anything, even itself.
Like detectives in a TV murder mystery, physicists have compiled a list of suspects. Topping the list are three hypothetical particles already wanted on other charges: axions, sterile neutrinos and WIMPs. Besides nailing dark matter, each would help explain a grand mystery of their own.
The axion is a particle proposed by Roberto Peccei and Helen Quinn back in 1977 to explain a quirk of the strong force (namely, why it can’t distinguish left from right, the way the weak force does). Thirty years on, axions are still our best explanation for that puzzle.
Axions could have any mass, but if – and it is a big ‘if’ – they have a mass about 100 billion times lighter than an electron, theorists have calculated they would have been created in the Big Bang in such vast numbers that they could account for the universe’s dark matter. Like detectives with a dragnet, physicists are searching through different possible masses in an attempt to close in from both ends and corner the axion.
The Axion Dark Matter eXperiment (ADMX), based at the University of Washington, is dragging the lightest end of the range. Since 2010 the project has been triying to catch axions by turning them into photons using strong magnetic fields. So far ADMX has ruled out the featherweight mass range between 150 to 270 billion times lighter than the electron.
The CERN Axion Solar Telescope (CAST) is dragging the heavyweight end of the range looking for axions that are a few tens of millions to about a million times lighter than the electron. The theorised source of these hefty axions is the Sun, where they might be created by X-rays in the presence of strong electric fields. In an example of recycling at its big-science best, CAST was assembled from a piece of the Large Hadron Collider -– a giant test magnet. It aims to detect solar axions by turning them back into X-rays. It has been running since 2003. The search goes on.
Sterile neutrinos are the hypothetical heavier, lazier brothers of neutrinos – the ghostly, fast-moving particles created in nuclear reactions and in the centre of the Sun. They are called ‘sterile’ or ‘inactive’ because they only interact via gravity.
Besides being a dark-matter candidate, sterile neutrinos would plug a number of holes in the Standard Model, which, like a subatomic version of the periodic table, has had great success in predicting the properties of the fundamental building blocks of the universe. For instance, sterile neutrinos could explain why neutrinos are so light, and why every neutrino we’ve ever seen has a ‘left-handed’ spin; sterile neutrinos would be the missing ‘right-handed’ partners.
Physicists are trying to detect sterile neutrinos in different ways, including searching deep space for the X-rays emitted when they decay. NASA’s Chandra X-ray telescope has picked up an excess of X-rays from the Perseus cluster of galaxies, which is so far unexplained. Meanwhile, regular neutrino detectors based at nuclear reactors, such as Daya Bay in China, have noticed anomalies that might be explained by sterile neutrinos.
“Like Elvis, people see hints of the sterile neutrino everywhere,” quipped Francis Halzen in August 2016, when he and his colleagues at the IceCube Neutrino Observatory announced the disappointing results of their own search. Their detector, buried up to 2.5 km deep in ice near the South Pole, found no evidence of the elusive sterile neutrino – a result that seems to rule out the Daya Bay reactor sightings. For a conclusive answer, we’ll need to wait for the next neutrino searches, such as the Precision Reactor Antineutrino Oscillation and Spectrum Measurement (PROSPECT) under construction at the US National Institute of Standards and Technology (NIST) in Maryland.
The third and most popular suspect is WIMPs – weakly interacting massive particles. The name covers a broad range of hypothetical particles that would interact via the weak force. They pop naturally out of the ideas of supersymmetry, an extension proposed to tidy up the loose ends of the Standard Model.
Physicists calculate that the simplest possible WIMP, with a mass of about 100 billion electron volts, would have been created in the Big Bang at just the right numbers to explain dark matter: the so-called ‘WIMP miracle’.
WIMP detectors are typically deep underground, watching for a telltale flash given out when a particle of dark matter bumps into an atomic nucleus.
The most sensitive WIMP experiment yet is LUX, a bathtub-sized vat holding 370 kg of liquid xenon at the Sanford Underground Research Facility in South Dakota. In 2016, the LUX team announced it had discovered no dark matter signals during its first 20-month-long search. Undeterred, the LUX team plan to upgrade to a 7,000-kg vat, LUX-ZEPLIN, by 2020.
The most intriguing dark matter result so far comes from the DAMA/LIBRA experiment in Italy. Using a detector made of highly purified sodium-iodide crystals, 1.5 km beneath Italy’s Gran Sasso mountain, scientists believe they have seen evidence of dark matter every year for the past 14 years (see Cosmos 65, p60). Their evidence comes in an annual rise and fall in background detections. Such a pattern might reflect the Earth’s relative speed through the dark-matter cloud that surrounds the Milky Way; while our planet moves around the Sun at 30 km/s, the Solar System as a whole is travelling at 230 km/s around the centre of the Milky Way.
For half of the year the Earth’s orbital speed would add to the speed of the Solar System, increasing the rate of dark-matter interactions. For the other half, the speeds would subtract and the rate of interactions decrease. The problem is that lots of other things change with the seasons too, such as the thickness of the atmosphere. To rule out terrestrial effects, astronomers are setting up two identical detectors, called SABRE, in opposite hemispheres – so that one is collecting data in winter and the other in summer.
One detector will be based at Gran Sasso, the other in Australia, in an abandoned gold mine near Stawell, Victoria. Each detector will be made of 50 kg of sodium iodide, and have noise levels 10 times lower than DAMA/LIBRA. Construction on each is under way, and could be finished this year.
Rather than detecting dark matter, others are trying to make it. The closest we can get to the conditions of the Big Bang – where dark matter was presumably created – is in the collision chambers of the Large Hadron Collider, CERN’s 27-km long particle smasher. These chambers are ringed by sensors that can pick up the energies of millions of particles generated in each smash-up, and tally this against the known collision energy. If some energy is missing, it might indicate the creation of a particle that could not be detected by any sensors: dark matter.
So far, notwithstanding a brief, hallucinatory blip in late 2015, the LHC has not discovered anything that might constitute a dark matter particle such as a WIMP. But the LHC has only collected about 1% of the data it is due to produce before it is retired in 2025. So it is too early to throw in the towel on producing dark matter yet. Plans are afoot for the LHC’s successor, which will be able to probe far higher energies.
Snowmelt from the Alpine ranges had swelled the Ovens River. I had to hug the shore with my metal detector, where the water was shallow and easy to sweep. I searched those parts that I could search as thoroughly as possible. If I did not find my prize, I wanted to at least be able to point to the map and say with confidence where the ring was not.
The map that physicists search has coordinates of energy levels and interaction strengths. Each new search sweeps out a new territory, so even a null result is valuable information. So far, in our search for the three primary candidates – axions, sterile neutrinos and WIMPs – we have only probed the most shallow, accessible waters. “There’s nothing really that says they have to be easy to detect,” Mack says. “It may just be that their interactions with our detectors are smaller than expected.”
It took almost 50 years for the Higgs boson to be discovered. Gravitational waves took almost a century. Let’s not give up on dark matter just yet.
I certainly won’t be giving up my own search. Next summer, when the Ovens dries, I will return to Bright and sweep the next unprobed area of the riverbed. I’d say wish me luck, but the point is to be so rigorous that luck has nothing to do with it.
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