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 know 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.