The speed of sound might have been quicker just after the big bang. That鈥檚 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 鈥渉orizon problem鈥�. 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鈥檚 visible universe. But in the late 1990s, Jo茫o 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 between distant galaxies in the form of light radiation.
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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鈥檛 leave behind the temperature and density fluctuations that cosmologists believe seeded the formation of galaxies.
Now Magueijo has studied what would happen if the speed of sound 鈥� rather than light 鈥� 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) 鈥� the relic radiation left behind by the big bang ().
鈥淪o far, only inflation has been able to explain these patterns and so it has been accepted by default,鈥� says Magueijo. 鈥淣ow it has a competitor.鈥�
鈥淪o far only inflation has been able to explain the cosmic microwave background. Now it has a competitor鈥�
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.
鈥淚t鈥檚 a great strength of the model that it is falsifiable,鈥� 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鈥檚 speed limit. He points out that Magueijo鈥檚 scenario may require the speed of sound to have been greater than the speed of light in the early universe. 鈥淭his would end up violating the speed limit once again,鈥� says Liddle.
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