(Image: Julien Vallée www.valleeduhamel.com)
SUITABLE clients at Milliways, Douglas Adams’s Restaurant at the End of the Universe, would have experienced six impossible things before breakfast. In the past decade or so, cosmologists have looked like they were halfway there.
Strike one: dark matter. Galaxies are whirling round faster than normal gravity alone can explain, so 80 per cent plus of the universe’s matter is in a form neither we nor, so far, our detectors can see.
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Strike two: dark energy. Contrary to all expectations, the universe’s expansion is apparently accelerating, so the inwards gravitational tug of both normal and dark matter is being trumped by the effect of another substance so exotic no one knows for certain what it might be.
Strike three: inflation. After all that, the universe still looks rather less crinkly than we would expect, so it must have been smoothed out by a spontaneous faster-than-light expansion in the earliest phase of its existence, which then just stopped.
Extraordinary claims, requiring extraordinary evidence. So you could almost feel the collective sigh of relief last month when the team running the , situated at the South Pole, announced that they had seen a sign of one of this terrible trio. Distinctive patterns of light polarisation in the cosmic microwave background (CMB) radiation were in fact two for the price of one. They represented an apparently unmistakable signature of inflation, but also provided indirect evidence for the existence of gravitational waves from the same era – another theoretically predicted phenomenon that has so far shied away from the limelight.
If the result is confirmed, it is heady stuff. Cosmologists will be able to begin whittling down a forest of ideas about how inflation might have happened, and close in on an understanding of the first microscopic moments of the universe’s history.
But there is sobriety amid the popping of champagne corks. Besides the question of that all-important confirmation, there are wider-ranging considerations. “Although this is a historic advance, it is also a limit,” says , a cosmologist at the Royal Observatory in Edinburgh, UK. “We have now seen as far back in time as it is possible to see, and it is not to the very beginning.” We may now know more about the moments following the big bang than ever – at the price of never knowing any more about the event itself.
Cosmic inflation was dreamed up in the 1980s to overcome the problem that the overall temperature and density of the universe is much more uniform than models predicted. At the big bang, space-time was squashed in on itself like a screwed-up piece of paper. Even expanding it to the size of the currently observable universe would not have erased all of its creases.
Inflation violently unfurls that scrunched-up ball. The basic idea is that in the universe’s first instants, the vacuum of space held vast reserves of energy. Quantum fluctuations jolted the vacuum to start shedding this energy in what became a cascade that spread right across the infant universe. This drove an exponential expansion that doubled the size of the universe about eighty times – from just 10-28 metres across to no more than a centimetre across – in just 10-36 seconds. The result was a featureless, flat sheet of space-time in which stars and galaxies could begin to form.
This process also generated the CMB radiation that the BICEP2 team scrutinised. This oldest light in the universe suffuses all of space. Originally trapped by the sheer density of matter in the early universe, it was finally released to flee in all directions some 380,000 years on, when the universe had cooled enough for the first atoms to form.
The expansion of the universe in the 13.8 billion years since has stretched the CMB and cooled it from its initial, stupendously high, energy to feeble microwaves capable of heating molecules to only about 2.7 kelvin – a value often referred to as the temperature of space. On large scales, this temperature and other characteristics are more or less evenly distributed. Take a closer look, however, and things appear rather different. Fixing its eye on one patch of the sky, the BICEP2 telescope saw the scars of inflation imprinted on the CMB in the form of patterns known as polarisation B-modes.
These scars have their origin in Einstein’s general theory of relativity, which describes how the presence of any matter and energy curves space and time. One consequence is that the same energetic quantum popping and fizzing that caused inflation would have set space-time jumping around in response, sending out ripples known as primordial gravitational waves.
If, and only if, inflation happened, the naturally minuscule gravitational ripplings at the moment it kicked off would have been vastly amplified. Traversing these enlarged ripples twisted the photons of the CMB, creating the B-mode polarisation. A moment from the ultra-early universe became writ large across the whole sky, like an image blown up so large all you can see is blocks of pixels.
“A moment of the Early universe was writ large, like a picture seen only in pixels”
This finding is what the BICEP2 team announced at the Harvard Smithsonian Center for Astrophysics on 17 March. “Detecting primordial gravitational waves is the closest thing to a proof of inflation that we are ever going to get,” says , a cosmologist at University College London.
A great leap backwards
It is, potentially, huge. Peacock, who was not involved with the work, points out that the CMB was released at a time when the universe was smaller by a factor of 1100 than it is now – but the signal of inflation imprinted on the CMB could date back to when it was as much as a staggering 10-55 times smaller. “This is just a wee bit of a step forward,” he says. “Assuming this is confirmed, I think it has to be the greatest development in science in my lifetime.”
Assuming it is confirmed. Such a potential breakthrough is in urgent need of corroboration, not least because as things stand not everything adds up. BICEP2’s result implies that the size of the primordial gravitational waves, characterised by a number called the r-value, was much bigger than anyone expected. “The r-figure was a bit of a shock to people because we thought such a value had been ruled out,” says Duncan Hanson of McGill University in Montreal, Quebec, Canada. He works with the , which sits next to BICEP2 and complements its observations of the CMB. “Some sort of confirmation is definitely needed.”
The conflict is with data on temperature variations in the CMB released in March last year by the European Space Agency’s . Whereas BICEP2 looked at just a small part of the CMB, Planck spent four-and-a-half years up till last year recording the CMB over the entire sky with unprecedented accuracy.
Inflation evens out the temperature across the CMB, but the same quantum fluctuations that created the primordial gravitational waves cause small variations in it. Other theories can give rise to similar patterns, so their existence is not itself proof of inflation. But assuming the variations are caused by inflation allows an upper limit for calculating the r-value of the gravitational waves. With the Planck data, it came out at 0.1; BICEP2’s value is twice that.
The simplest, but most deflating, resolution is that there is a problem in the BICEP2 analysis. There is certainly any number of bear traps the telescope’s analysis team might have fallen into. Above all, other things besides primordial gravitational waves from inflation can produce B-mode polarisation signals. The gravity of galaxy clusters could have twisted the light at a later date, as could dust grains trapped in our galaxy’s magnetic field. Already it has been suggested that dust grains from supernovae could have created the pattern (see “Star dust casts doubt on recent big bang wave result“).
Targeting a small patch of sky far away from the central plane of our galaxy and from any large galaxy clusters, as BICEP2 has done, cuts down this contamination. But Peiris, who works on the Planck mission, still has concerns. “I am amazed by their accomplishment. This is like a one-in-fifty-years discovery, right?” she says. “But as a scientist, I have to be sceptical.”
She worries that a single sequence of joined-up software routines, together known as a pipeline, was used to analyse the BICEP2 data. On Planck, at least two and sometimes as many as five independent pipelines reduced the risk of an undetected bug skewing the result. “To verify the BICEP2 results requires a different pipeline, different data and different part of the sky,” says Peiris.
BICEP2 could do that itself, by developing a second pipeline to put its data through. Other ground-based telescopes could also offer independent insights, among them Hanson’s South Pole Telescope and , which is located in Chile. Their respective teams are already scouring their data or planning observations that could corroborate or contradict BICEP2. Unsurprisingly, they are not yet prepared to comment.
The heavy hitter remains Planck. Its full-sky map cuts the likelihood that any pattern it sees is a localised blip. What’s more, it took readings at nine microwave wavelengths. This makes it easier to subtract foreground polarisation contamination reliably, as the scattering of microwaves by intervening dust varies with wavelength.
When the Planck team released their temperature map last year, they promised to publish polarisation data this year – in November, if the latest rumours are right. But not all is sweetness and light with the Planck data, either. If the temperature fluctuations it saw are born of inflation, they should be the same over all distance scales. In fact, they are greater on smaller scales. Researchers have tended to sweep this anomaly under the carpet, referring to it as a “tension”. That may no longer be so tenable. “If you take BICEP2’s r-value at face value, that would actually make the tension worse,” says Hanson. “We may have to extend the cosmology.”
By that he means dreaming up even more outlandish versions of inflation that can generate differently sized gravitational waves on different scales. Theorists are already busy at their laptops slinging together papers to do just that. Others take a less radical line, believing that BICEP2 probably has detected primordial gravitational waves and will come up with a smaller r-value as more data becomes available.
Homing in on the right r-value might also help solve the big question of what sort of engine drove inflation. Or it might not, says theorist of the University of Portsmouth, UK. “Inflation is a way of avoiding a lot of the difficult problems we encounter when discussing the big bang. It means that we can describe a lot of the properties of the universe without having to push back to the big bang itself.”
Worse than that: inflation effectively erases the details of what went before, smearing it out in the exponential expansion or driving it so far away that we cannot see it. Inflation can take almost any imaginable beginning and transform it into the universe we see. That means we can’t even be sure there was a big bang, an expansion from a singularity of infinite density and heat, immediately before. “A big bang singularity might indeed have been only 10-36 seconds before these gravitational waves were generated, or it might have been a trillion years,” says Peacock.
“Inflation can take almost any imaginable beginning and make it into the universe we see, So we can’t now even be certain there was a big bang”
Trapped in the past
It might not just be the big bang itself that is forever hidden from view. Most cosmologists and physicists believe that describing the universe’s origin requires a theory linking gravity, as described by general relativity, to quantum theory, which describes the other three forces of nature. The effects of quantum gravity would manifest themselves only at energies or densities far higher than those we could create in a particle accelerator. But there was still hope that direct observational evidence from the early universe would help us to pin down such a theory.
This could be the catch-22 of the BICEP2 result. According to Peacock, confirmation of the BICEP2 discovery would tell us that a quantum theory of gravity does exist. “The gravitational waves would only be generated with this amplitude if quantum mechanics applies to gravity as well as other fields such as electromagnetism,” he says. But the era of quantum gravity would now be firmly trapped behind the featureless mask of inflation, ruling out direct cosmological tests.
If so, the early sighs of relief may turn into screams of frustration. “Now we have proof that some such quantum theory of gravity must exist: it’s just that we can’t see how to probe it,” says Peacock.
Peiris is less definitive, and thinks there might still be a chance of finding out something of what came before. “I wouldn’t actually rule out that we’ll never get any further information ever,” she says. “It is still early days for the theory of inflation. Perhaps some further prediction will come out of it.”
Let’s hope so, because otherwise 17 March 2014 might come to be seen as both a beginning and an end for cosmology. If there really is a limit on how much we can know about how and why it all began, cosmologists will have to comfort themselves that there are still enough mysteries this side of the inflationary barrier to occupy them. Dark matter and dark energy, anyone?
Leader: Cosmic inflation seen? Don’t get hopes up too quickly
This article appeared in print under the headline “The end of the beginning”
