Ghostly particles that may explain the universe

This autumn’s claims that neutrinos travelled faster than light – and in the process broke one of the firmest rules of modern physics – have drawn attention to the universe’s most abundant and at the same time most elusive subatomic particles.

The scientific jury will take several more months to come to a judgment about whether neutrinos really exceeded the speed of light on their 730km trip from Cern, the European nuclear physics centre near Geneva, to the Gran Sasso underground detector in central Italy. But even if it ends in the anticlimactic conclusion that measurement errors exaggerated the particles’ speed, neutrinos will still provide a rich field of research for physicists seeking to understand the fundamental nature of the universe.

“Neutrinos are extremely important for answering many of our biggest questions,” says Pier Oddone, director of Fermilab, the leading US particle physics centre. “For example, we don’t know why the universe is full of matter rather than antimatter. Finding out more about neutrinos will help us solve some of these mysteries.”

So Oddone and his colleagues have made neutrinos a focus of research at Fermilab, following the recent closure of the lab’s big atom smasher, the Tevatron.

Although neutrinos were originally proposed on theoretical grounds in the 1930s and first detected at a nuclear reactor in the 1950s, there are still many more unknowns than hard facts about these ghostly particles.

We know that neutrinos make up one per cent of the universe, even though each one has an infinitesimally small mass. Indeed, the whole Earth is almost transparent to neutrinos – which poses a challenge for scientists trying to detect them.

The solution is to build vast detectors to spot those incredibly rare collisions between neutrinos and ordinary matter. For instance, Fermilab is building a 15,000-ton instrument for its Nova experiment, to sit at the end of a neutrino beam generated 810km away. It contains 385,000 plastic cells filled with a special oil; when a neutrino hits an atomic nucleus in the liquid, it emits a flash of light which is then recorded.

Neutrinos come in three types. They are known as electron, muon and tau neutrinos, three of the 16 fundamental particles in the “standard model” of physics.

But neutrinos oscillate constantly between the three forms – or, to be more accurate, each particle is a constantly changing combination of all three types, with a particular one predominant at any time.

Oscillation was discovered initially in the 1990s as the solution to a mystery: why were most of the electron neutrinos, known by theorists to be produced by nuclear fusion in the sun, missing when detectors looked for them on earth? The answer was that they had turned into the other two types of neutrino.

Much neutrino research today aims to study the oscillation process by producing neutrinos of a known type in a particle accelerator and recording what arrives at a detector hundreds of miles away. That was the primary purpose of Europe’s Opera experiment before the faster-than-light excitement this year.

One scientific goal of oscillation experiments is to work out the masses of the different neutrinos. “At present we know that all three must have an extremely small mass,” says Jim Strait, who is planning the Long Baseline Neutrino Experiment, a $1bn project to send intense neutrino beams from Fermilab to a detector 1,300km away.

Another goal is to find out whether each neutrino is paired with a distinct antiparticle and, if so, whether these antineutrinos behave differently to the corresponding neutrino. The answers will help cosmologists work out why the symmetry between matter and antimatter was broken in or soon after the Big Bang, creating a universe dominated by matter.

“Those are some of the known unknowns about neutrinos,” says Oddone. “But there will also be unknown unknowns. Neutrinos have been and I’m sure will continue to be a source of surprises.”

Shedding light on dark energy

The world’s largest and most sensitive digital camera was delivered to a mountaintop telescope in Chile last week. The Dark Energy Camera, or Decam, will survey the sky in unprecedented depth, to help cosmologists understand the force that is driving the universe apart at an accelerating rate.

Astronomers discovered in 1998 that the cosmic expansion was gathering pace, rather than slowing under the influence of gravity as almost everyone had believed. The discovery, recognised with a Nobel prize this year, was made by observing light from supernovae (exploding stars in distant galaxies) and has since been confirmed by other observations.

But the nature of the acceleration – dubbed dark energy – remains a total mystery, which the new camera is designed to investigate.

The Decam is a $35m instrument built by an international consortium co-ordinated from Fermilab. Decam’s five huge lenses, up to 1m in diameter, were designed and assembled at University College London.

The 570-megapixel camera is designed to record the images of about 300 million galaxies covering about one-eighth of the southern sky. It will be attached to the 4m Blanco telescope, which has been chosen because of its exceptionally wide field of view.

“Its CCDs [photosensitive chips] are similar to those in any digital camera but they are far more sensitive to faint light, particularly in the red and infra-red wavelengths,” says Tom Diehl, a Decam scientist at Fermilab. “Light from distant galaxies is shifted toward the red because of the expansion of the universe.”

Over the next five years Decam will provide a three-dimensional survey of the sky in unprecedented detail, mapping galaxies as far away as nine billion light years – back to a time when the universe was just a third of its present age. At the same time, it is expected to detect several thousand more supernovae.

All these pictures will be analysed to show changes in the distribution of galaxies over billions of years, in the hope of providing a deeper analysis of dark energy than has been possible so far. The data may enable scientists to start discriminating between different theories about dark energy.

One priority is to confirm that dark energy is a force separate from gravity and acting in the opposite direction, repelling rather than attracting. An alternative view is that there is something wrong with our theory of gravity itself. Might Einstein and his successors be wrong in assuming gravity has remained constant over time?

If dark energy is indeed a distinct force, then it seems likely to be a property of empty space. Quantum physics shows that even a complete vacuum is not really empty, in the sense that it seethes with temporary or virtual particles, constantly appearing and disappearing.

Of course Decam will not be the last word on dark energy surveys. The US is planning the Large Synoptic Sky Telescope, an entirely new instrument to be installed in Chile to scan the whole sky. “In three nights LSST will see more of the universe than all previous telescopes put together,” says Ian Shipsey, physics professor at Purdue University.

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