Delving deep for the dark arts of physics

PUBLISHED : Tuesday, 28 December, 2010, 12:00am
UPDATED : Tuesday, 28 December, 2010, 12:00am

Chinese scientists are digging deep to win a Nobel prize. Enveloped in rock more than 2.5 kilometres below the earth's surface, the Jinping Deep Underground Laboratory in southwestern Sichuan is the world's deepest scientific facility, according to Science magazine.

Its purpose is to find dark matter - and at that depth, the lab enjoys almost perfect radio silence.

'If you are looking for a place to detect dark matter with minimum disturbance, this is the place to go,' said Professor Huang Qingguo of the Chinese Academy of Sciences' Institute of Theoretical Physics. 'And if you are lucky to get some high-quality WIMPs, congratulations. The Nobel prize is yours.'

WIMP is short for weakly interacting massive particles, the form of dark matter most commonly believed to exist, though it is still hypothetical. But despite the costly construction at Jinping, a recent study in Germany has raised grave concern that the experiment might be looking for nothing.

The study's lead scientist, Professor Pavel Kroupa, says in an e-mail interview that the Jinping lab has no chance of finding dark matter particles because the latest astronomical observations suggest WIMPs do not exist. If Kroupa is right, it will be bad news for the dark matter search globally. There are nine detection sites at work - in western Europe, North America and East Asia. The blow to China would be particularly hard, as its lab only opened this month.

The Jinping lab is an unplanned child of a massive project called the Ertan Hydroelectric Plant. To facilitate the transport of construction materials, the Ertan plant dug two parallel tunnels, each nearly two kilometres long, through Mount Jinping.

Before that, China had been unable to find a site for a dark matter lab. Coal and gold mines were examined and an underground aviation museum near Beijing was considered. But they were too shallow or inaccessible.

Then it was realised that by hollowing out numerous chambers in the middle of the Ertan tunnels, where the rocky envelope is thickest, enough space could be provided to house two detectors run by two competing universities.

Tsinghua University's detector, CDEX, is a cryogenic Ge detector that uses 20 grams of germanium to intercept any passing WIMPs. It is not the most sensitive detector of its kind, but researchers still hope to get some results because of the Jinping lab's natural advantage. The Panda-X programme of Shanghai Jiaotong University uses a different kind of detector with liquefied xenon as the interceptor. CDEX is already running; the Panda-X will be ready next year.

Professor Ni Kaixuan , a member of the Panda-X programme, said they were not ready to release detailed information about it, but some people in the scientific community believe that, when completed, Panda-X will be an impressive and advanced WIMP hunter.

'A dark matter detector works like a soccer goal net,' Ni said. 'The interaction point between dark matter and the net is so small that every day billions of WIMPs just pass through the detector without leaving a trace.'

But Kroupa, head of the Stellar Populations and Dynamics research group at the University of Bonn, said no matter how sensitive the detectors were, the experiments were doomed.

When Kroupa's team applied the mainstream cosmological model in a supercomputer to simulate the formation of the universe, they discovered that the computer-created universe, which allowed for the existence of dark matter, looked very different from the real one.

For instance, faint satellite galaxies of the Milky Way weigh more or less the same as bright ones in the computer simulation. Existing theory assumes that dark matter is distributed evenly throughout the universe regardless of light intensity, but in reality, the brighter galaxies do turn out to be heavier.

Their research, published in the journal Astronomy & Astrophysics last month, echoed a joint study by Princeton University and the Israel Institute of Technology published in Nature in June. It concluded that when nearby galaxies were observed in greatest detail, the existing models - with dark matter as a key building block - failed.

In Germany, the study has stirred fierce debate, with some leading scientists repositioning themselves and calling for a rethink on dark matter.

Professor Volker Springel of the Max Planck Institute for Astrophysics said in an article in Spektrum der Wissenschaft, an influential scientific journal in Germany, it was possible the current standard model of cosmology was false and that scientists have been looking for a phantom.

Kroupa said the dark matter problem originated in a mistake made by Einstein in 1915. That year, Einstein brilliantly introduced the idea that matter curves space and time. To describe the idea, he used a set of 10 equations, which Kroupa says could be wrong in detail.

'Einstein's field equation is merely the simplest form that can be written down. Other types of field equations are possible,' he said.

But astronomers and physicists assumed Einstein's field equations to be correct as they interpreted observations of distant galaxies, and when the field equations failed to fit the data, theoreticians 'dream up' explanations. Dark matter was just one of them, Kroupa said.

For instance, to explain why the universe started from a singularity but became spatially flat in the next instant, physicists came up with a mathematical solution called inflation, which was not understood from a particle physics point of view, he said. And to explain why the expansion of the universe accelerates, rather than slows down, physicists conjured up something called dark energy, which, according to Nasa, is still a complete mystery.

The biggest problem is that if Einstein's field equations are correct, dark matter and dark energy should make up more than 95 per cent of our universe. Even after decades of feverish search and study, we don't have the faintest idea what they are.

In 1998, Italian researchers claimed they had detected dark matter particles in a lab deep in Gran Sasso d'Italia, a mountain in Italy's Abruzzo region, and 10 years later they reaffirmed the claim.

Unfortunately, their observations could not be repeated by other projects using more sensitive detectors in less noisy environments, so most scientists took the results as noise rather than WIMPs.

Kroupa said the fact no particle had yet been found was merely a logical conclusion from the astronomical evidence.

'The problem with dark matter particle searches is that whenever a new, expensive experiment fails to find a particle, theoreticians can always say that the particles exist, but one needs even more sensitive experiments. That is, the dark matter particle hypothesis is not falsifiable experimentally,' he said. 'This is, therefore, not a good way to proceed.'

Ni, from the Panda-X project, said the existence of dark matter was widely accepted by mainstream scientists and a large number of astronomical observations had marginalised the sceptics in recent years.

'Though indirectly, evidence such as cosmic microwave background radiation and gravitational lensing has confirmed the existence of dark matter,' he said. 'It is only a matter of time before particle physicists find it.'