– Was light faster at the beginning of the universe?
A new explanation for cosmology’s greatest conundrum. CATHAL O’CONNELL reports.
They don’t make light like they used to. Today’s light, chugging along at 299,792,458 metres per second, would have been blown away by photons of the early universe – or so says a newly beefedup theory of physics.
In 1998, Portuguese physicist João Magueijo of Imperial College London first proposed superfast light, travelling trillions of times faster than light’s current speed to explain the even temperatures measured across the early universe.
Imagine switching on a heater in a cold concert hall and having the whole space warm instantly. A similar cosmic-scale effect has been puzzling physicists since the 1960s, when the the cosmic microwave background (CMB), the oldest light in the universe, was found to be the same colour in all directions.
The colour of light is a direct measure of temperature. That means that when the CMB was emitted, 300,000 years after the Big Bang, the entire universe must have had the same temperature. How could that be if the universe’s horizons were so far apart that no light – and therefore no heat – could have travelled between them?
Unless, thought Magueijo back in 1998, light once travelled faster than it does today. Then it could have spread from edge to edge quick enough to even out the temperature. Magueijo’s idea was clever – but as it contradicted Einstein’s rule about the speed of light being constant across all space and time, it never quite caught on.
Instead, most physicists favoured the theory of cosmic inflation, which says the universe went through an incredible growth spurt in its first fraction of a second – a rapid stretch that smoothed out the universe’s temperature end to end.
Yet physicists still don’t have the foggiest about what caused inflation, nor why it stopped. And physicists haven’t found any direct evidence to back it up.
Now, writing in Physical Review D, Magueijo and colleague Niayesh Afshordi, have hit back with a refined superfast light theory—along with a testable prediction that seems right on the money.
The refinement proposes that at first, light travelled far faster than gravity before settling down to its current speed.
The new theory spits out an exact prediction for a feature of the cosmic microwave background called the spectral index. Although the CMB is ‘pretty much’ the same in all directions, there are some slight variations--the picture is a teensy bit splotchy. The spectral index captures these variations. The prediction proposes a precise number for the spectral index.
Laying out a testable prediction is a badge of honour in the somewhat nebulous world of theoretical physics, where theories can often be retrofitted to match the latest data. Their predicted splotchiness is close to the latest data from the Planck satellite. But Magueijo and Afshordi know better than to get too excited: “Improved observations will soon vindicate or disprove this model,” they note.
A conundrum: the even temperature of the early universe, evidenced by this map of the cosmic microwave background, suggests light once trarvelled much faster than it does today.