杏吧原创

(Image: Joseba Elorza)

IT IS the night of 24 August, 2014. The moon has set and the sky is dark. Johannes Eser and Matt Rodencal are in a helicopter 3 kilometres above Ontario, Canada, and Rodencal is giving instructions to the pilot over the headset. He wants the helicopter to fly in circles below a balloon carrying their experiment. The trouble is, the balloon is floating at the edge of Earth鈥檚 atmosphere nearly 35 kilometres above them.

While Rodencal instructs the pilot, Eser is firing an ultraviolet laser, invisible to the naked eye, horizontally away from the helicopter. The physicists are hoping that a detector on the balloon will see the laser light scattering off the nitrogen molecules and aerosols in the air. It is all being done blind. There are no lights on the helicopter or the balloon: Eser and Rodencal just can鈥檛 risk overwhelming the detector with photons.

This aerobatic adventure might sound like something from a James Bond film, but it was designed to test an instrument that may one day be deployed on the International Space Station. No one had done anything like this before. 鈥淚 was very nervous about whether it鈥檇 work or not,鈥� says Eser, a PhD student at the Colorado School of Mines in Golden.

To their credit, it did. 鈥淚t was a really big success,鈥� Eser says. And that success is no small beer: it means we are now getting closer to searching space for monster particles from the dawn of time.

In 1998, a trio of researchers from the Universities of Chicago and Michigan, and CERN, published a paper called, simply, 鈥溾�� The paper suggested space might be filled with huge particles, each one at least a trillion times heavier than a proton. WIMPZILLAs are a proposed explanation for dark matter, the mysterious, unseen stuff that physicists believe makes up a significant proportion of the universe鈥檚 mass.

The strongest evidence for dark matter鈥檚 existence comes from observations of how galaxies rotate and how clusters of galaxies behave. If the only matter in the universe were normal matter, then galaxies and clusters would have long since come apart 鈥� there wouldn鈥檛 be enough mass, and therefore gravity, to hold them together. And there must be a lot of the dark stuff: our best estimates suggest that it accounts for 85 to 90 per cent of the matter in the universe. But because it mainly interacts via gravity 鈥� the weakest fundamental force 鈥� that makes it extremely hard to detect.

Dark data

Working from theory and observations tells us what this stuff must be like. The favoured candidate is a type of particle designated a WIMP: a weakly interacting massive particle. There are theoretical reasons to believe in WIMPs. For instance, these hypothetical particles would have been produced early in the universe鈥檚 history, and in about the right amount to match astronomical data on dark matter. That鈥檚 why they have been the main focus of dark matter searches so far. But, although there have been tantalising hints of sightings, in the end they have always proved elusive.

鈥淎lthough there have been tantalising hints, WIMPs have proved elusive鈥�

Take the results from the DAMA/LIBRA detector experiment, for example. It has been running for nearly two decades at the Gran Sasso National Laboratory in Italy, using a matrix of sodium iodide crystals that give out a burst of faint light if hit by a WIMP. It has found a strange signal that peaks every June. This is exactly what would happen if Earth were regularly ploughing through a sea of WIMPs on its way round the sun. The signal is far enough above the detector鈥檚 noise level to get some physicists excited. What鈥檚 more, another experiment called CoGENT has seen a similar annual modulation, although theirs is not statistically significant.

The trouble is, if these signals are due to dark matter, myriad other dark matter detectors should have seen it too. 鈥淚 honestly have no idea what DAMA is seeing,鈥� says of Fermilab near Batavia, Illinois. 鈥淚t鈥檚 clear that their signal modulates, [but] I鈥檓 pretty convinced that it鈥檚 not dark matter.鈥�

What鈥檚 more, if WIMPs were the answer, CERN鈥檚 Large Hadron Collider should have produced at least a hint of them in its particle collisions. And yet 鈥� nothing, nada, zip.

Could we be looking for the wrong particles? 鈥淣ature has its own mind, and maybe [dark matter] isn鈥檛 WIMPs,鈥� says of the University of Chicago. 鈥淭here is no theorem that says it has to be WIMPs.鈥�

Born of inflation

This has reinvigorated the quest for alternative dark matter explanations. 鈥淚t is time to explore other scenarios,鈥� says Carsten Rott, a dark matter hunter who works at Sungkyunkwan University in South Korea.

WIMPZILLAs and other types of 鈥渟uperheavy dark matter鈥� are strong contenders. However, they present a weighty problem: creating them takes a lot of energy 鈥� far more than creating WIMPs. In fact, the sort of energies required would only have been conceivable in the supremely hot and dense early universe, during the 鈥�inflationary epoch鈥� moments after the big bang.

鈥淚f superheavy dark matter does decay, we should be able to detect the results鈥�

Until recently, we didn鈥檛 know for sure whether the inflationary epoch was energetic enough. However, by combining data from two telescopes examining the cosmic microwave background 鈥� radiation left over from the big bang 鈥� researchers can put an upper limit on this energy scale. Results from the and the tell us the energy available during this period could have been as high as 10²⁴ electronvolts 鈥� sufficient to produce superheavy dark matter particles.

During inflation, the universe expanded exponentially fast thanks to the action of a hypothetical particle known as an inflaton. One way in which superheavy dark matter particles could have been produced is by the decay of the inflaton. In theory, an inflaton could decay into other particles and 鈥� maybe 鈥� superheavy dark matter.

Another mechanism could have been similar to what happens in the modern universe. Today, the vacuum of space is filled with fluctuating electromagnetic fields. Thanks to the equivalence of mass and energy laid out in Einstein鈥檚 famous E=mc² equation, those fields can spontaneously give rise to particles such as electrons and their antimatter equivalents, positrons. In the moments of inflation, intense fluctuating gravitational fields could have also created particles. And the enormous energies involved would have made them superheavy.

The electron and positrons created by today鈥檚 electromagnetic fields tend to annihilate one another, disappearing in a puff of energy. But with superheavy dark matter, there is no annihilation. As long as those intense gravitational fields were still around, space would have continued to fill with the stuff. As space-time expanded, these particles would become part of the matter scattered through the universe 鈥� but still invisible to us or our detectors. So how do we find them?

The simple answer is, with great difficulty. For starters, given our estimate of the amount of dark matter in the cosmos, there just aren鈥檛 going to be many of these particles to catch 鈥� far, far fewer than if dark matter were made of WIMPs. It鈥檚 a simple calculation. 鈥淲hen they are that heavy, there just aren鈥檛 very many,鈥� says Hooper. 鈥淚f I want a kilogram of one kilogram rocks, I want just one. But if I want one kilogram of paper clips I need a lot.鈥�

What鈥檚 more, there is no way to get a WIMPZILLA or its superheavy cousins to interact directly with a detector on Earth: they would pass right through normal matter. So we have to rely on indirect detections, and search for products of their decay: things like photons, protons and neutrinos.

That鈥檚 assuming that these particles will eventually decay. 鈥淚f they live forever, then we are in a very sad scenario 鈥� then they are just sitting there and it鈥檚 going to be hard for us to ever detect them,鈥� Olinto says.

If they do decay, we should be capable of detecting the results (see diagram). The high mass of superheavy dark matter means its decay products will exist at superhigh energies. That鈥檚 good news: we know how to track these speeding bullets. In fact, the is already on this trail, and has spotted something tantalising.

Over the past few years, IceCube has seen 137 neutrinos with energies of a few tens of teraelectronvolts and three with energies in the region of a few petaelectrovolts. The three with the highest energies are interesting, Rott says, because they could be decay products of superheavy dark matter.

Strange source

No one yet knows where these neutrinos came from. We know that neutrinos are created in the aftermath of supernova explosions, in gamma ray bursts and in bodies known as active galactic nuclei. However, standard physics suggests that these sources produce fewer and fewer neutrinos with higher and higher energies. Physicists certainly don鈥檛 know of any astrophysical source that would produce a clump of TeV neutrinos, produce nothing at slightly higher energies, then produce PeV neutrinos. Those highest energy neutrinos seem to have come from something else. It could be that this is a statistical fluctuation. But maybe not. 鈥淚t could be pointing at decaying dark matter,鈥� says Rott.

If these highest energy neutrinos are indeed the decay products of superheavy dark matter, that is fortunate. Rott鈥檚 team has calculated that they would have an incredibly long average lifetime: about 10¹¹ seconds times the age of the universe. Some will live much longer, but some should be out there, ready to decay right now. 鈥淭his is certainly an interesting scenario that we should keep an eye on,鈥� he says.

Future observations with IceCube could help firm up this scenario. If these high-energy neutrinos, and the gap between them and their lower energy counterparts persists, it strengthens the case for an exotic source such as superheavy dark matter.

It will also complement another way to detect superheavy dark matter. When these particles decay, they could also emit ultra-high-energy cosmic rays and, with suitable detectors, we could see these by-products too.

In theory, there are a couple of detectors that could be up to the task: the , which is spread over 3000 square kilometres of grasslands in Argentina, and the Telescope Array, which observes the sky over about 800 square kilometres of high desert in Utah. So far, they have not seen any ultra-high-energy cosmic rays. However, that could be because they are not big enough.

Ultra-high-energy cosmic rays create a shower of particles when they smash into Earth鈥檚 atmosphere. That sounds like it should be easy to detect, but these are very rare events: they occur at less than one particle per square kilometre per century, on average. So perhaps our ground-based detectors just haven鈥檛 been observing enough of Earth鈥檚 atmosphere. According to Olinto, the better option is to put a detector in space, looking down at Earth. Such a detector would be able to observe about 50 to 250 times the area being scanned by the Pierre Auger Observatory, the largest ground-based cosmic ray detector.

The IceCube telescope may have seen hints of heavy dark matter (Image: Keith Vanderlinde/NSF)

鈥淭he better option is to put a detector in space that can look down at Earth鈥�

And that鈥檚 where Eser and Rodencal鈥檚 daredevil flying stunts come in. The balloon at the edge of space was a test bed for the Japanese Experimental Module 鈥� Extreme Universe Space Observatory () that is being designed to fly on the ISS. Olinto, who is the US principal investigator for the project, is testing prototypes with her team.

Last August鈥檚 flight was to determine whether the detector on the balloon would see the ultraviolet laser light when it scattered off the air molecules. The eventual detector on JEM-EUSO would look for similar light created when ultra-high-energy cosmic rays hit Earth鈥檚 upper atmosphere. Now that the initial experiment has succeeded, the next step is to fly another prototype on NASA鈥檚 pressurised long-duration balloons, which can stay aloft for weeks on end. 鈥淲e are building a payload that can go 100 days,鈥� says Olinto. Their flight is scheduled for March 2017.

As yet, no one is calling off the search for WIMPs. But if either JEM-EUSO, IceCube or future neutrino telescopes see particles with energies that cannot be explained by standard astrophysics, then the case for superheavy dark matter 鈥� whether WIMPZILLAs or something equally heavy 鈥� becomes more compelling. Particles left over from the infant universe may prove to be cosmological fossils, giving clues to what went on in the mysterious era of inflation. 鈥淢aybe nature helps us by having relics, and maybe the relic is dark matter,鈥� says Olinto. 鈥淭hat鈥檚 what makes me wake up and move forward.鈥�

Leader: 鈥�Chasing shadows: How long can we keep looking for dark matter?鈥�

This article appeared in print under the headline 鈥淚t鈥檚 behind you鈥�