– Was light faster at the be­gin­ning of the uni­verse?

A new ex­pla­na­tion for cos­mol­ogy’s great­est conundrum. CATHAL O’CON­NELL re­ports.

Cosmos - - Digest -

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.

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