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Speedier sound may have smoothed out early universe
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Did sound once travel as fast as light? THE speed of sound might have been quicker just after the big bang. That’s the suggestion of one physicist, who says it could help explain both how galaxies formed and why distant corners of the universe have much in common. One of the things that cosmologists need to explain about the formation of the universe is called the “horizon problem”. It runs like this: no matter where you look in the universe, the background temperature is pretty much the same, but not enough time has elapsed since the big bang for radiation to zip across the universe exchanging temperature information. The usual explanation is that there was a period of rapid expansion in the early universe, called inflation, which blew adjacent regions to the opposite sides of today’s visible universe. But in the late 1990s, João Magueijo, a cosmologist at Imperial College London, and a few other researchers came up with a controversial alternative proposal. If light travelled much faster soon after the big bang, they argued, it would cover more ground than we assume and so could have exchanged energy
People power dents cosmic ‘axis of evil’ IT’S citizen science in action. More than 85,000 members of the public have knocked a dent in the cosmic “axis of evil” that threatens to bring down standard cosmology. In 2005, Kate Land and João Magueijo at Imperial College London noticed an unexpected alignment of hot and cold spots in the cosmic background radiation – the relic www.newscientist.com
between distant galaxies in the form of light radiation. The trouble was that speedy light would have been too good at racing round and smoothing out differences in the universe, says Magueijo. In particular, it wouldn’t leave behind the temperature and density fluctuations that cosmologists believe seeded the formation of galaxies.
“So far only inflation has been able to explain the cosmic microwave background. Now it has a competitor”
–Sound years apart–
Now Magueijo has studied what would happen if the speed of sound – rather than light – was much larger in the early universe, and whether a sound wave travelling through the primordial plasma of gas could have exchanged energy between disparate regions. It turns out speedy sound would be able to solve the horizon problem, just as a varying speed of light could. As well as this, Magueijo found that if the speed of sound suddenly dropped at a specific time in the early universe, it would leave behind small
fluctuations. His calculations show that these fluctuations would be just the type that are imprinted on the cosmic microwave background (CMB) – the relic radiation left behind by the big bang (www. arxiv.org/abs/0803.0859). “So far, only inflation has been able to explain these patterns and so it has been accepted by default,” says Magueijo. “Now it has a competitor.” Magueijo says that further observations will help to test his idea. For example, his speedy sound model does not predict the existence of gravitational waves,
unlike many inflation models. “It’s a great strength of the model that it is falsifiable,” says Andrew Liddle, a cosmologist at the University of Sussex in Falmer, UK. However, Liddle doubts that Magueijo has done enough to appease critics who were uncomfortable about breaching light’s speed limit. He points out that Magueijo’s scenario may require the speed of sound to have been greater than the speed of light in the early universe. “This would end up violating the speed limit once again,” says Liddle. Zeeya Merali G
radiation of the big bang. The alignment seemed to defy standard cosmology, which predicts that the universe is isotropic and so should look the same in every direction. The case for this so-called axis of evil grew stronger in 2007, when Michael Longo at the University of Michigan in Ann Arbor studied 1660 spiral galaxies and found the majority appear to line up with the axis, and a higher-than-expected number rotate in a certain direction. To check Longo’s claim, Land, now at the University of Oxford, and her colleagues recruited the public to classify the orientation of almost
900,000 other galaxies as part of an online project called Galaxy Zoo. At first, things looked good for the axis. The volunteers found a lot more anticlockwise than clockwise galaxies in the Northern hemisphere of the sky – matching Longo’s findings. But to double-check the findings, Land’s team flipped the images of the some of the galaxies and ran them past a subset of the volunteers again. It turned out people have a preference when picking orientation: despite the mirroring, 52 per cent of the galaxies were still described as anticlockwise. “Rather than the universe being odd, it
might be that people are odd,” says Land. The team has submitted the findings to Monthly Notices of the Royal Astronomical Society (www. arxiv.org/0803.3247). Longo, however, is unconvinced. The mirroring analysis was only carried out for 5 per cent of the galaxies studied and he believes this sample is too small to justify rejecting the original excess that users spotted, which corroborated the existence of the axis. “[Land and colleagues] have done an impressive job of organising the Galaxy Zoo project, but I believe their analysis is flawed,” he says. Zeeya Merali G 5 April 2008 | NewScientist | 11
Did sound once travel as fast as light? THE speed of sound might have been quicker just after the big bang. That’s the suggestion of one physicist, who says it could help explain both how galaxies formed and why distant corners of the universe have much in common. One of the things that cosmologists need to explain about the formation of the universe is called the “horizon problem”. It runs like this: no matter where you look in the universe, the background temperature is pretty much the same, but not enough time has elapsed since the big bang for radiation to zip across the universe exchanging temperature information. The usual explanation is that there was a period of rapid expansion in the early universe, called inflation, which blew adjacent regions to the opposite sides of today’s visible universe. But in the late 1990s, João Magueijo, a cosmologist at Imperial College London, and a few other researchers came up with a controversial alternative proposal. If light travelled much faster soon after the big bang, they argued, it would cover more ground than we assume and so could have exchanged energy
People power dents cosmic ‘axis of evil’ IT’S citizen science in action. More than 85,000 members of the public have knocked a dent in the cosmic “axis of evil” that threatens to bring down standard cosmology. In 2005, Kate Land and João Magueijo at Imperial College London noticed an unexpected alignment of hot and cold spots in the cosmic background radiation – the relic www.newscientist.com
between distant galaxies in the form of light radiation. The trouble was that speedy light would have been too good at racing round and smoothing out differences in the universe, says Magueijo. In particular, it wouldn’t leave behind the temperature and density fluctuations that cosmologists believe seeded the formation of galaxies.
“So far only inflation has been able to explain the cosmic microwave background. Now it has a competitor”
–Sound years apart–
Now Magueijo has studied what would happen if the speed of sound – rather than light – was much larger in the early universe, and whether a sound wave travelling through the primordial plasma of gas could have exchanged energy between disparate regions. It turns out speedy sound would be able to solve the horizon problem, just as a varying speed of light could. As well as this, Magueijo found that if the speed of sound suddenly dropped at a specific time in the early universe, it would leave behind small
fluctuations. His calculations show that these fluctuations would be just the type that are imprinted on the cosmic microwave background (CMB) – the relic radiation left behind by the big bang (www. arxiv.org/abs/0803.0859). “So far, only inflation has been able to explain these patterns and so it has been accepted by default,” says Magueijo. “Now it has a competitor.” Magueijo says that further observations will help to test his idea. For example, his speedy sound model does not predict the existence of gravitational waves,
unlike many inflation models. “It’s a great strength of the model that it is falsifiable,” says Andrew Liddle, a cosmologist at the University of Sussex in Falmer, UK. However, Liddle doubts that Magueijo has done enough to appease critics who were uncomfortable about breaching light’s speed limit. He points out that Magueijo’s scenario may require the speed of sound to have been greater than the speed of light in the early universe. “This would end up violating the speed limit once again,” says Liddle. Zeeya Merali G
radiation of the big bang. The alignment seemed to defy standard cosmology, which predicts that the universe is isotropic and so should look the same in every direction. The case for this so-called axis of evil grew stronger in 2007, when Michael Longo at the University of Michigan in Ann Arbor studied 1660 spiral galaxies and found the majority appear to line up with the axis, and a higher-than-expected number rotate in a certain direction. To check Longo’s claim, Land, now at the University of Oxford, and her colleagues recruited the public to classify the orientation of almost
900,000 other galaxies as part of an online project called Galaxy Zoo. At first, things looked good for the axis. The volunteers found a lot more anticlockwise than clockwise galaxies in the Northern hemisphere of the sky – matching Longo’s findings. But to double-check the findings, Land’s team flipped the images of the some of the galaxies and ran them past a subset of the volunteers again. It turned out people have a preference when picking orientation: despite the mirroring, 52 per cent of the galaxies were still described as anticlockwise. “Rather than the universe being odd, it
might be that people are odd,” says Land. The team has submitted the findings to Monthly Notices of the Royal Astronomical Society (www. arxiv.org/0803.3247). Longo, however, is unconvinced. The mirroring analysis was only carried out for 5 per cent of the galaxies studied and he believes this sample is too small to justify rejecting the original excess that users spotted, which corroborated the existence of the axis. “[Land and colleagues] have done an impressive job of organising the Galaxy Zoo project, but I believe their analysis is flawed,” he says. Zeeya Merali G 5 April 2008 | NewScientist | 11