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Has the dark energy particle been spotted?
This week–
Faster, faster said the chameleon ZEEYA MERALI
MAYBE there’s a good reason why it has been so difficult to pin down the nature of dark energy – the mysterious force that is making the universe expand faster. Dark energy could be mediated by a chameleon-like particle that changes character to suit its surroundings. Ever since dark energy was suggested in the late 1990s, cosmologists have struggled to explain just how it can counter gravity on the cosmological scale, but apparently leave it untouched here on Earth. In 2003, Justin Khoury of the Perimeter Institute in Waterloo, Canada, and Amanda Weltman of the University of Cape Town in South Africa suggested the existence of a curious particle that could provide the answer. Dubbed “chameleons”, these are theoretical particles that mimic their immediate surroundings: they are heavy in regions that are packed with matter, and light where matter is in short supply. Khoury and Weltman’s colleagues calculated that on cosmological scales, the chameleon would be nearly massless, and would
mediate a force that reproduced the effects of dark energy. But here on Earth, chameleons would be too heavy to produce any measurable force – making them undetectable in experiments designed to test deviations from Newtonian gravity on Earth. It has been tough to find evidence for particles that are, by definition, adept at hiding. However, it seems the elusive chameleon may have been spotted by the PVLAS experiment at the National Laboratories of Legnaro in Italy. The experiment sends a linearly polarised laser beam through a vacuum chamber bathed in an intense magnetic field and measures changes in the polarisation of the photons as they leave the vacuum chamber. PVLAS hit the headlines in July 2006, when researchers announced that they had measured a rotation in the polarisation of a laser beam that was 10,000 times larger than could be explained by standard physics, along with an unexpected change from linear to elliptical polarisation. Almost immediately, the results were ascribed to the disappearance or conversion of
a tiny fraction of the photons to axions, the much sought-after hypothetical particles that are candidates for dark matter (New Scientist, 18 July 2006, p 35). At the same time, AnneChristine Davis at the University of Cambridge and her colleagues began to wonder if a dark energy particle, the chameleon, was
PARTICLES THAT PASS THROUGH PLATES Carlo Rizzo at the French national research agency (CNRS) in Toulouse and his team passed a laser beam through a vacuum chamber in the presence of a magnetic field. Unlike the PVLAS experiment in Italy, the team blocked the far end of the vacuum chamber with an aluminium plate. The plate would block the photons from the laser beam, but if photons were being converted to axions – candidate particles for dark matter – then they would be able to pass
10 | NewScientist | 21 July 2007
through the plate and change back into photons. No such photons were detected by the team, ruling out axions (www.arxiv.org/abs/0707.1296). This doesn’t mean chameleons are the only answer. So-called minicharged particles (MCPs) – predicted to have between 10-15 and 10-7 times the charge of an electron – can also explain the latest PVLAS results, without falling foul of Rizzo’s experiment. If they were being produced in the vacuum chamber, then after passing through
the aluminium plate, pairs of MCPs would spiral apart and not recombine to create new photons, says Andreas Ringwald, with the DESY collaboration in Hamburg, Germany. In theory, MCPs can travel through solid objects – potentially allowing for cable-free telecommunication directly through Earth. “MCPs are more exciting [than chameleons] because their discovery could actually change people’s day-to-day lives,” says Ringwald.
responsible for the shift in polarisation. Within the nearperfect vacuum of the PVLAS chamber the chameleon, in keeping with its shifting nature, would have a mass very close to zero. The team’s calculations showed that it would be very easy for photons in the laser beam to transform into chameleons in such a vacuum. But more detailed calculations revealed problems. According to the team’s model, chameleon particles would remain trapped in the vacuum chamber, bouncing back and forth, and cancelling out any rotation in polarisation in the laser beam rather than creating the observed huge rotation. It seemed the chameleon could not, after all, explain the PVLAS results. Then there came a surprising twist. Last month, researchers ruled out the axion explanation (see “Particles that pass through plates”). A few days later, the www.newscientist.com
www.newscientist.com
How a megaflood carved the English Channel HALF a million years ago, Britain was connected to mainland Europe by a broad chalk ridge across what is now the Dover strait. Somehow that ridge was then destroyed, creating the English Channel. Now we know what destroyed it: a megaflood. Sanjeev Gupta and Jenny Collier at Imperial College London came to this conclusion after studying images from an ultra-high-resolution sonar survey of the channel’s bedrock by the UK Hydrographic Office. This showed deep valleys that could only have been gouged out by vast volumes of water. “All the bedrock landforms we see are characteristic of a megaflood,” says Gupta. He and Collier calculated that 1 million cubic metres of water per second must have flowed for several months sometime between 200,000 and 450,000 years ago. The flood carved the sea-floor valleys, some of which are up to 10 kilometres wide and 50 metres deep, with a flow rate 100 times the average flow rate of the Mississippi river today. The megaflood’s source was a vast glacial lake that took up most of what is now the southern North Sea.
Corralled by glaciers to the north and to the south by the Dover strait ridge – also known as the WealdArtois ridge – the lake was fed by the Rhine and the Thames as well as glaciers. “There was so much water coming in from the Rhine, the Thames and the glacial meltwater that it had no place to go,” says Gupta. “So at some point it breached a hole in the ridge.” The water gushing through the hole tore down the ridge and scoured the sea floor to form the English Channel (Nature, vol 448, p 342). Philip Gibbard, a geologist at the University of Cambridge, thinks the Imperial team are spot on in their analysis. “That lake was like a bath with the taps running,” he says. “The package of bedrock features they describe is very convincing.” Gupta and Collier are not the first to propose that a megaflood dug the English Channel. In 1985, marine geologist Alec Smith of Bedford College in London used a lowerresolution sonar survey to draw similar conclusions – but because that survey was only accurate to 10 metres, geologists were sceptical of his claims (Marine Geology, vol 64, p 65). Smith was right all along, says Collier. And Gibbard admits he was one of those who doubted Smith. “I have already ordered my sackcloth and ashes,” he says. Paul Marks ●
CUTTING THE LINK Rising water levels between 200,000 and 450,000 years ago caused by melting ice and increased river flows eventually breached the ridge that once linked Britain to continental Europe
ICE SHEET
MELTWATER
GLACIAL LAKE R. Thames
R. Rhine
MEGAFLOOD WEALD-ARTOIS RIDGE R. Seine
21 July 2007 | NewScientist | 11
IEKH9;0?CF;H?7B9EBB;=;BED:ED
NASA/ESA/STSCI/AVRA
by astronomers on the cosmological scale. “We got the accelerating universe thrown in for free,” says Davis, who presented her work at a conference on particles, strings and cosmology at Imperial College London on 5 July. Astrophysicist Andy Albrecht at the University of California, Davis, is impressed that the group has proposed a solution for dark energy that can be tested. “There is so much unconstrained speculation about dark energy, I cannot help but feel that this is a breath of fresh air,” he says. And while Glennys Farrar, an astrophysicist at New York University, is intrigued by the link between the PVLAS findings and dark energy, she is cautious about the reliability of the latest PVLAS results, given that the researchers retracted their original claims. PVLAS team member Giovanni Cantatore is also keen to play down claims that the dark energy particle has been discovered. “We won’t say that we have definitely found it,” he says. “But the impact –Dark energy sorted?– of such a discovery would be enormous, so I encourage independent groups to look into PVLAS collaboration retracted this further.” their original findings. They had Carlo Rizzo at the French upgraded their equipment and national research agency could not reproduce their earlier (CNRS) in Toulouse agrees that the results. The new experiment no PVLAS retraction is “troubling”. longer showed that the photon He and his colleagues are setting polarisation had rotated, but it up their own experiment to was still elliptical at low settings double-check the latest PVLAS of the magnetic field (www.arxiv. results and will also test for the org/abs/0706.3419). chameleon. “We could flood the vacuum chamber with a dense gas “On cosmological scales the during the experiment,” says chameleon would mediate Rizzo. “Perhaps then we will see a force that reproduced the the chameleon changing effects of dark energy” character before our eyes.” Space tests for the chameleon are also being lined up. Precise This was good news for Davis tests of the force of gravity carried and her team. They redid their out on the SEE, MICROSCOPE, calculations and found that the Galileo Galilei and STEP satellites chameleon – whose presence should reveal if a significant predicted a change in ellipticity chameleon force does indeed but no rotation – now fitted the come into play in regions bill. Also, the strength of the of sparse matter. “It will be chameleon field needed to very tough for the chameleon explain the PVLAS result was just to keep on hiding out there,” the right size to account for the says Weltman. ● density of dark energy observed
Faster, faster said the chameleon ZEEYA MERALI
MAYBE there’s a good reason why it has been so difficult to pin down the nature of dark energy – the mysterious force that is making the universe expand faster. Dark energy could be mediated by a chameleon-like particle that changes character to suit its surroundings. Ever since dark energy was suggested in the late 1990s, cosmologists have struggled to explain just how it can counter gravity on the cosmological scale, but apparently leave it untouched here on Earth. In 2003, Justin Khoury of the Perimeter Institute in Waterloo, Canada, and Amanda Weltman of the University of Cape Town in South Africa suggested the existence of a curious particle that could provide the answer. Dubbed “chameleons”, these are theoretical particles that mimic their immediate surroundings: they are heavy in regions that are packed with matter, and light where matter is in short supply. Khoury and Weltman’s colleagues calculated that on cosmological scales, the chameleon would be nearly massless, and would
mediate a force that reproduced the effects of dark energy. But here on Earth, chameleons would be too heavy to produce any measurable force – making them undetectable in experiments designed to test deviations from Newtonian gravity on Earth. It has been tough to find evidence for particles that are, by definition, adept at hiding. However, it seems the elusive chameleon may have been spotted by the PVLAS experiment at the National Laboratories of Legnaro in Italy. The experiment sends a linearly polarised laser beam through a vacuum chamber bathed in an intense magnetic field and measures changes in the polarisation of the photons as they leave the vacuum chamber. PVLAS hit the headlines in July 2006, when researchers announced that they had measured a rotation in the polarisation of a laser beam that was 10,000 times larger than could be explained by standard physics, along with an unexpected change from linear to elliptical polarisation. Almost immediately, the results were ascribed to the disappearance or conversion of
a tiny fraction of the photons to axions, the much sought-after hypothetical particles that are candidates for dark matter (New Scientist, 18 July 2006, p 35). At the same time, AnneChristine Davis at the University of Cambridge and her colleagues began to wonder if a dark energy particle, the chameleon, was
PARTICLES THAT PASS THROUGH PLATES Carlo Rizzo at the French national research agency (CNRS) in Toulouse and his team passed a laser beam through a vacuum chamber in the presence of a magnetic field. Unlike the PVLAS experiment in Italy, the team blocked the far end of the vacuum chamber with an aluminium plate. The plate would block the photons from the laser beam, but if photons were being converted to axions – candidate particles for dark matter – then they would be able to pass
10 | NewScientist | 21 July 2007
through the plate and change back into photons. No such photons were detected by the team, ruling out axions (www.arxiv.org/abs/0707.1296). This doesn’t mean chameleons are the only answer. So-called minicharged particles (MCPs) – predicted to have between 10-15 and 10-7 times the charge of an electron – can also explain the latest PVLAS results, without falling foul of Rizzo’s experiment. If they were being produced in the vacuum chamber, then after passing through
the aluminium plate, pairs of MCPs would spiral apart and not recombine to create new photons, says Andreas Ringwald, with the DESY collaboration in Hamburg, Germany. In theory, MCPs can travel through solid objects – potentially allowing for cable-free telecommunication directly through Earth. “MCPs are more exciting [than chameleons] because their discovery could actually change people’s day-to-day lives,” says Ringwald.
responsible for the shift in polarisation. Within the nearperfect vacuum of the PVLAS chamber the chameleon, in keeping with its shifting nature, would have a mass very close to zero. The team’s calculations showed that it would be very easy for photons in the laser beam to transform into chameleons in such a vacuum. But more detailed calculations revealed problems. According to the team’s model, chameleon particles would remain trapped in the vacuum chamber, bouncing back and forth, and cancelling out any rotation in polarisation in the laser beam rather than creating the observed huge rotation. It seemed the chameleon could not, after all, explain the PVLAS results. Then there came a surprising twist. Last month, researchers ruled out the axion explanation (see “Particles that pass through plates”). A few days later, the www.newscientist.com
www.newscientist.com
How a megaflood carved the English Channel HALF a million years ago, Britain was connected to mainland Europe by a broad chalk ridge across what is now the Dover strait. Somehow that ridge was then destroyed, creating the English Channel. Now we know what destroyed it: a megaflood. Sanjeev Gupta and Jenny Collier at Imperial College London came to this conclusion after studying images from an ultra-high-resolution sonar survey of the channel’s bedrock by the UK Hydrographic Office. This showed deep valleys that could only have been gouged out by vast volumes of water. “All the bedrock landforms we see are characteristic of a megaflood,” says Gupta. He and Collier calculated that 1 million cubic metres of water per second must have flowed for several months sometime between 200,000 and 450,000 years ago. The flood carved the sea-floor valleys, some of which are up to 10 kilometres wide and 50 metres deep, with a flow rate 100 times the average flow rate of the Mississippi river today. The megaflood’s source was a vast glacial lake that took up most of what is now the southern North Sea.
Corralled by glaciers to the north and to the south by the Dover strait ridge – also known as the WealdArtois ridge – the lake was fed by the Rhine and the Thames as well as glaciers. “There was so much water coming in from the Rhine, the Thames and the glacial meltwater that it had no place to go,” says Gupta. “So at some point it breached a hole in the ridge.” The water gushing through the hole tore down the ridge and scoured the sea floor to form the English Channel (Nature, vol 448, p 342). Philip Gibbard, a geologist at the University of Cambridge, thinks the Imperial team are spot on in their analysis. “That lake was like a bath with the taps running,” he says. “The package of bedrock features they describe is very convincing.” Gupta and Collier are not the first to propose that a megaflood dug the English Channel. In 1985, marine geologist Alec Smith of Bedford College in London used a lowerresolution sonar survey to draw similar conclusions – but because that survey was only accurate to 10 metres, geologists were sceptical of his claims (Marine Geology, vol 64, p 65). Smith was right all along, says Collier. And Gibbard admits he was one of those who doubted Smith. “I have already ordered my sackcloth and ashes,” he says. Paul Marks ●
CUTTING THE LINK Rising water levels between 200,000 and 450,000 years ago caused by melting ice and increased river flows eventually breached the ridge that once linked Britain to continental Europe
ICE SHEET
MELTWATER
GLACIAL LAKE R. Thames
R. Rhine
MEGAFLOOD WEALD-ARTOIS RIDGE R. Seine
21 July 2007 | NewScientist | 11
IEKH9;0?CF;H?7B9EBB;=;BED:ED
NASA/ESA/STSCI/AVRA
by astronomers on the cosmological scale. “We got the accelerating universe thrown in for free,” says Davis, who presented her work at a conference on particles, strings and cosmology at Imperial College London on 5 July. Astrophysicist Andy Albrecht at the University of California, Davis, is impressed that the group has proposed a solution for dark energy that can be tested. “There is so much unconstrained speculation about dark energy, I cannot help but feel that this is a breath of fresh air,” he says. And while Glennys Farrar, an astrophysicist at New York University, is intrigued by the link between the PVLAS findings and dark energy, she is cautious about the reliability of the latest PVLAS results, given that the researchers retracted their original claims. PVLAS team member Giovanni Cantatore is also keen to play down claims that the dark energy particle has been discovered. “We won’t say that we have definitely found it,” he says. “But the impact –Dark energy sorted?– of such a discovery would be enormous, so I encourage independent groups to look into PVLAS collaboration retracted this further.” their original findings. They had Carlo Rizzo at the French upgraded their equipment and national research agency could not reproduce their earlier (CNRS) in Toulouse agrees that the results. The new experiment no PVLAS retraction is “troubling”. longer showed that the photon He and his colleagues are setting polarisation had rotated, but it up their own experiment to was still elliptical at low settings double-check the latest PVLAS of the magnetic field (www.arxiv. results and will also test for the org/abs/0706.3419). chameleon. “We could flood the vacuum chamber with a dense gas “On cosmological scales the during the experiment,” says chameleon would mediate Rizzo. “Perhaps then we will see a force that reproduced the the chameleon changing effects of dark energy” character before our eyes.” Space tests for the chameleon are also being lined up. Precise This was good news for Davis tests of the force of gravity carried and her team. They redid their out on the SEE, MICROSCOPE, calculations and found that the Galileo Galilei and STEP satellites chameleon – whose presence should reveal if a significant predicted a change in ellipticity chameleon force does indeed but no rotation – now fitted the come into play in regions bill. Also, the strength of the of sparse matter. “It will be chameleon field needed to very tough for the chameleon explain the PVLAS result was just to keep on hiding out there,” the right size to account for the says Weltman. ● density of dark energy observed