The excitement has hardly died down before theorists have begun to mutter that there could be another explanation
Last week, the world of science was ablaze with the news that astrophysicists had found the first evidence of ripples in spacetime from the instants after the Big Bang. The discovery of these gravitational waves would be big news by itself but there was another aspect to this work that was even more significant.
The work also found crucial evidence for a process known as inflation: that in the instants after the Big Bang the universe expanded explosively—by some 20 orders of magnitude in a fraction of a second—making it the size we see today.
But even before the sound of champagne corks popping has died down, theorists are beginning to question the new result. Everyone agrees that the data shows important evidence of gravitational waves. The question is whether these waves could have been created after inflation, rather than before it. If so, then they do not provide any evidence that the early universe expanded so quickly and the celebrations have been premature.
First some background. One of the great challenges for cosmologists is to explain why the universe is the size we see today. They known that the universe is about 14 billion years old so it must have grown to its current size in that time.
But when astronomers measure the universe’s rate of expansion, they quickly come up against a problem. The universe is expanding far too slowly to have grown this big in 14 billion years.
In other words, if you run time backwards so that the universe contracts, the universe does not shrink back to a point. Not by a long way. The discrepancy is some 20 orders of magnitude.
Inflation is the 30-year old theory that explains this mystery. The idea is that in the instants after the Big Bang, high energy quantum processes caused the universe to grow by 20 orders of magnitude in the blink of an eye.
That’s why the universe is so big. Without inflation, the universe could not have grown to its current size, at its current rate of expansion.
Cosmologists have always found this a compelling idea. However, they’ve never had any evidence to back it up. It’s not possible to see that far back in time because light could not travel through the universe at that early time.
There is another way to look back in time though—by looking for ripples in spacetime. So-called gravitational waves are created by extremely violent events such as black holes colliding. This creates ripples in spacetime like a stone landing in water.
The turbulence in the early universe before inflation would also have created gravitational waves, like the surface of water during a storm. The thinking is that, as the universe expanded during inflation, these waves would have dramatically expanded too. Then, when the first light started to spread through the young universe, these waves would have polarised it in ways that ought to be visible now.
Today, we call this light the cosmic microwave background but the way it is polarised is extremely weak. That’s why it has taken so long to spot. But this is exactly the announcement that researchers working on an experiment called BICEP2 made last week, citing it as evidence of gravitational waves from before inflation.
But here’s the crucial question. How do we know that the polarisation is the result of a process that happened before inflation and not one that occurred much later, after inflation?
The BICEP2 team have spent some time studying this question. For the last three years, they’ve tried to rule out everything they can think of from the effects of dust in our galaxy to the gravitational distortions of distant galaxies down to distortions in the telescope used to capture the data. And they’ve managed to rule out all these .
But today, James Dent at the University of Louisiana at Lafayette and a couple of pals say the BICEP2 team has overlooked something. They say one possibility is that the polarisation of the cosmic microwave background was indeed caused by gravitational waves just as the BICEP2 team claim, but that these waves formed after inflation, not before it.
Here’s how. For the last few years, cosmologists have discussed what happened in the moments after inflation, as the universe began to cool. It was during this time that the universe we know ‘condensed’ out of the high energy maelstrom generated in the Big Bang.
In particular, as the universe cooled, the fundamental forces we see now, such as the weak and strong nuclear forces and the electromagnetic force, formed in processes called phase changes, just as ice forms as water cools below a critical temperature or as a magnetic field within a material aligns as it cools below a critical temperature.
What’s interesting about phase changes is that they don’t form across the entire material at the same instant. In a cooling magnet, for example, the magnetic field forms in different regions which then spontaneously align when the temperature cools below the critical point. Only then does a uniform field fill the entire material.
Many cosmologists think that the same kind of phases changes occurred in the universe after inflation. Each phase change began in different regions at slightly different times.
But as the entire universe cooled, the fields in these regions would have spontaneously aligned, filling the universe with the same properties at that instant.
This self-ordering process would have been hugely violent, generating its own gravitational waves that rippled through spacetime, albeit after inflation. Could this process be responsible for the polarisation that the BICEP2 team has measured?
According to Dent and co, it could. “Unfortunately, the [BICEP2 measurement] falls just short of ruling out this other source as the dominant contribution of the observed effect,” they say.
So the big announcement last week was premature. And before it can be confirmed, the BICEP2 team has some work ahead of it to rule out the possibility that self ordering in the early universe could be responsible.
Dent and co point out that it’s actually a close run thing—that a small improvement in the data could firmly rule out self-ordering as the origin of the signal.
And they are sympathetic. “It is perhaps frustrating that the current observation cannot unambiguously rule out this toy model straw man as a source of gravitational waves that could polarize the Cosmic Microwave Background signal as observed by BICEP2,” they say.
But that is the process of science. It’s impossible to prove that an explanation for a set of data is true. However, it is possible to rule out other explanations because they don’t match the data. So the BICEP2 team has to rule out all other possible explanations until there is only one left.
Only then can there be unambiguous agreement.
It’s important to recognise that the new new dispute is in no way evidence that inflation did not occur. Indeed, most cosmologists including Dent and co, think it is the most likely explanation for the universe we see today.
However, the BICEP2 data does not yet provide unambiguous evidence for it. As Dent and co put it: “It is important to demonstrate that other possible sources cannot account for the current BICEP2 data before definitely claiming Inflation has been proved.”
And,that may require significantly more data, painstakingly gathered and analysed by the BICEP2 team and others.
In other words, put the champagne back in the fridge.
Ref: arxiv.org/abs/1403.5166 : Killing the Straw Man: Does BICEP Prove Inflation?