14 Sep 2015
Michael D. Lemonick
The failure so far to find gravitational waves has some cosmologists wondering if the ‘inflationary’ theory of the Big Bang is right. Michael D. Lemonick explains.
On 17 March, 2014, the Harvard-Smithsonian Centre for Astrophysics held a press conference to announce “a major discovery”. It was not an exaggeration. A team of astrophysicists had detected evidence of gravitational waves from a time when the Universe was almost indescribably young.
It was the most powerful confirmation yet of the 30-year-old theory of inflation which explains why the cosmos looks the way it does. The distribution of galaxies, the relative proportions of ordinary matter and dark matter3, the curvature of space-time, the fact that the cosmos looks essentially the same no matter where you look – all of this can be understood if you assume that the entire visible Universe expanded for the briefest interval from something about the size of a proton to something about the size of a grapefruit at faster than the speed of light when it was less than a billionth of a trillionth of a trillionth of a second old. In the words of University of California, Santa Cruz, cosmologist Joel Primack: “No theory this beautiful has ever been wrong.”
Evidently, it had been proven right. Using an exquisitely sensitive microwave telescope known as BICEP2 located at the South Pole, Harvard’s John Kovac and a team of observers had detected a twist in the orientation of microwaves generated about 300,000 years after the Big Bang. Known as B-mode polarisation, it had been predicted by inflation theory. The fantastic energy released by an inflating Universe would have rippled space-time itself.
Alternative theories about how the Universe got its structure – such as the one developed by Princeton University’s Paul Steinhardt – did not predict these ripples. “If this is correct, we’re finished,” Steinhardt commented. He had been one of the pioneers of inflation theory but had since abandoned it in favour of his own competing theory.
The swirls detected in this picture of the Milky Way were initially believed to be caused by gravitational waves, but later measurements showed cosmic dust could create the same effect.Credit: ESA / Planck Collaboration
The announcement at the Harvard press conference reverberated in headlines around the world. “Space Ripples Reveal Big Bang’s Smoking Gun,” trumpeted the New York Times. “Primordial gravitational wave discovery heralds ‘whole new era’ in physics,” declared the Guardian. Like virtually every other story that appeared on that day, there were dutiful caveats along the lines of “The results will require confirmation …” They barely dented the feverish tone.
Within days of the announcement the reporters were wishing they’d been more than merely dutiful. Kovac’s scientific report (revealed online on arXiv – a forum for work to be published soon) wasn’t released until the press conference. Once other astrophysicists got a look at it, they became suspicious. Primordial gravitational waves aren’t the only thing that could polarise microwaves. The Milky Way’s swirling dust clouds could do it too – “schmutz”, Princeton’s David Spergel called it, using a Yiddish word meaning “dirt.”
As independent physicists scrutinised the report more closely, they became increasingly sceptical as to whether the Harvard team had seen gravitational waves at all. Finally in February 2015, a combined analysis of the data from Kovac’s BICEP2 team; the Keck Array (located next to BICEP2 at the South Pole); and Planck, the European Space Agency’s orbiting space observatory, left the researchers in no doubt. “What we see”, Kovac conceded “is compatible with no inflationary gravitational waves”.
That hardly means that inflation is dead. What these three very sensitive instruments saw is also compatible with inflationary gravitational waves hiding within the dust. Inflation, moreover, isn’t a single theory: it’s a class of theories, and many predict gravitational waves 10 orders of magnitude lower than any existing instrument is capable of detecting. “Am I worried?” asks Stanford University theorist Andrei Linde, one of the founders of inflation theory. “Why should I be?”
But for a small number of theoretical astrophysicists, the failure to detect gravitational waves raises the stock for an alternative theory of the birth of the Universe. Known as the cyclic model, it was first proposed in 2003 by Princeton’s Steinhardt and Neil Turok, then at the University of Cambridge (now director of Canada’s Perimeter Institute for Theoretical Physics). These days it is championed by a handful of theorists mostly in the US and the UK. It posits that the observable Universe has gone through alternating phases of expansion and contraction – perhaps forever.
This model of cosmology explains everything we know about the Universe just as well as inflation does, they say. A major point of departure though, is that primordial gravitational waves are not part of the cyclical model.
While most physicists are not even close to abandoning inflation, they don’t rule out that this beautiful theory may also be wrong. “Paul has a bunch of concerns about the inflation theory, which I think are valid,” says Charles Bennett, an experimental physicist at Johns Hopkins University.
Joanna Dunkley, a cosmologist at the University of Cambridge, agrees the failure to detect gravitational waves “should make us think more seriously about whether inflation is the only option”.
“I think most of the community is focused on inflationary models, and I think some of that is fashion,” adds David Spergel, Steinhardt’s Princeton colleague.
Neil Turok argues the case for a cyclic view of the Universe.Credit: Peter Power
Fashion explains some of their focus, perhaps, but hardly all of it. When inflation theory first emerged in the 1980s, it was nothing short of breathtaking in the way it explained a series of problems that had bedevilled cosmologists since the 1964 discovery of the cosmic microwave background (CMB) radiation.
At the time, there were two competing theories about how the Universe began. One was the Steady State, which posited that the Universe has always been expanding, and that new matter is created to fill in the gaps as existing matter spreads apart.
The other was the Big Bang, ironically coined by English astronomer Sir Fred Hoyle as a term of ridicule – he was the leading proponent of the Steady State. The original idea here was that the Universe was born out of the violent expansion of an extremely dense, hot gas cloud (a modern version holds that it began from a singularity – a pinpoint of sub-atomic proportions) which has been expanding ever since. If that were true, then the brilliant light generated by that bang should still be echoing through the Universe – except the expansion of the Universe would have stretched the light into the microwave region of the electromagnetic spectrum.
Paul Steinhardt is another insurgent challenging the theory of inflation.Credit: Beverly Schaefer
In 1964, radio astronomers Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in New Jersey, stumbled across that stretched ancient light. They were experimenting with Bell Lab’s giant radio antenna – originally built to track satellites – to see if they could repurpose it to peer at the Universe.
Robert Woodrow Wilson (left) with Arno Allan Penzias at the Bell Labs giant radio antenna
Annoyingly, their efforts were thwarted by a mysterious microwave frequency hiss in the antenna. When the meticulous pair had ruled out all other explanations (including pigeon poop) they suggested the hiss was cosmic in origin. Around the same time, just an hour’s drive to the west, Robert Dicke and several other physicists at Princeton University were setting out to look for relic microwaves from the Big Bang. Penzias and Wilson heard about Dicke’s project and called during one of his group meetings. As those who were present recall, Dicke listened patiently, hung up and said “boys, we’ve been scooped”.
Both groups published simultaneously in The Astrophysical Journal in 1965 (only Penzias and Wilson got the Nobel, however). The discovery tipped the scales firmly in favour of the Big Bang.
Cosmologists leapt at the opportunity to study the CMB in detail – it was the first glimpse of our youthful, 400,000-year-old Universe. It turned out to be a mysterious place. For one thing, they were struck by its uncannily uniform temperature – it hovers at 2.725° above absolute zero, varying by no more than one part in 100,000 in either direction, no matter where in the sky you look. The turbulent super-heated gas cloud from which the Universe erupted would have had spots that varied significantly in temperature and density and some of that messiness should have been on show in the structure of the early expanded Universe.
Another problem was that while the strapping 400,000-year-old Universe was as smooth and even as a baby’s bottom, the mature universe is wrinkled with features such as galaxies. But how did these age-related wrinkles arise?
Physicists were also worried by the apparent topology of the early Universe. Over large scales, their measurements showed that it appeared to be geometrically “flat”. And it was unclear why monopoles – particles with either a north or south magnetic charge but not both – had never been found.
Cosmologists scratched their heads for more than a decade. Then in 1980 a young physicist named Alan Guth figured out these conundrums would vanish if a proton-sized Universe experienced an ultra-fast expansion in its very earliest moments.
A proton-sized beginning that suddenly inflated would explain the evenness of the Universe. It would have ballooned out so fast there was no time for any fluctuations to wrinkle the expanding fabric of space-time.
On the other hand, the fact that the entire Universe was once sub-atomic in size made it subject to quantum effects such as “uncertainty” – a state in which physical variables can fluctuate unpredictably. These random quantum fluctuations seeded the wrinkles that gave rise to features such as galaxies.
Finally, inflation explained why the visible cosmos appears so flat. Perhaps it started off with significant curvature like the surface of a balloon. Imagine that you’re a fly balancing on the ball. Suddenly, it expands to the size of the Sun. You’re still standing on a curved surface, but to you it now looks utterly flat as far as the eye can see. Without the rapid expansion, the balloon wouldn’t have expanded sufficiently to create the flatness we observe.
Guth’s original version of inflation left some gaps but they were filled by Linde, turning the theory into a robust set of predictions that cosmologists have been testing ever since.
There was a problem, however.
“We discovered early on that we completely misunderstood something at the beginning,” says Steinhardt, who was one of the pioneers of inflation theory. “We thought that inflation was essentially a story about stretching the Universe. And then we thought if you add a little bit of quantum mechanics to explain why the Universe isn’t perfectly uniform” – why it has galaxies and clusters of galaxies – “we seem to have a consistent story”.
However, there’s no such thing as a little bit of quantum mechanics, says Steinhardt. “Quantum physics is constantly producing fluctuations in all forms of energy, including the energy that’s driving inflation, so that it ends in some places a little bit later than others,” he says.
He and others soon realised that quantum uncertainty complicated matters.
In our patch of the Universe, for instance, inflation stopped billions of years ago, but in some other patches it’s still going on. Given inflation’s breakneck expansion rate, these regions would now be unimaginably large – as though bits of the original balloon had bulged outward to form gigantic protuberances, much larger than the original. “This will occur over and over and over again,” Steinhardt explains. Linde, who is mostly responsible for this idea, calls it “chaotic inflation” or “eternal inflation”. It means that our own visible Universe is just one patch in a far larger multiverse – a patch within a patch within a patch, ad infinitum – and each patch could have its own unique laws of physics. “The multiverse will explore every conceivable physical property and possibility and produce every conceivable outcome,” says Steinhardt.
And that’s the problem. “What can you predict from such a theory?” Steinhardt asks. “Nothing. Literally nothing, since anything that’s physically possible will occur.” But it’s worse than that: since an infinite number of patches exist with an infinite variety of physical laws and constants, the fundamental question that physicists have been trying to answer since the time of Aristotle – why is the Universe the way it is? – becomes meaningless. It’s the way it is because the Universe is every possible way all at once. Ours happens to look the way it does because that’s the part we happen to be living in. This is what’s known as the anthropic principle, and since it says in essence that there’s no explanation for anything, it pulls the rug out from under science. That doesn’t make it wrong, but physicists tend to abhor it.
There’s a second problem as well. “It’s remarkable that we have a theory that can describe what’s going on and match the observations so beautifully,” says Spergel. “But it doesn’t explain how it got into that phase.” In other words inflation might have happened but nobody knows why it started. Inflationary theorists say that’s a problem to be solved later, says Steinhardt. “But it’s a big problem to be solved later,” he says, “because we’ve been trying to solve that problem and we think the conditions under which inflation could begin are very, very rare.” Unless you believe in a Creator, that’s not a good place to be.
There was a third problem: dark energy. In 1999 cosmologists confirmed that this mysterious force is ballooning out the Universe at an ever-accelerating rate. Inflation theory, conceived in the 1980s, was blissfully unaware of dark energy.
“It was a total surprise,” says the Perimeter Institute’s Turok. “Inflation was already something of an artificial add-on to the Big Bang, and now you’ve got this new add-on, which has nothing to do with inflation.” Turok says you also have to account for the fact that inflation dominated the earliest moments of our part of the Universe, then went away – and that dark energy (tiny compared with the energy of inflation) would emerge billions of years later to dominate the Universe.
Inflationists consider dark energy to be something entirely different from inflation – a second expansionary force that only became significant many billions of years after inflation ran out of steam. The fact that you need to explain not one, but two different forces makes Steinhardt and Turok uncomfortable with the inflation model. “It’s horribly fine-tuned,” says Turok.
For this pair of physicists, dark energy had finally robbed inflationary theory of its beautiful shine. There had to be a simpler, better theory. After several years of intensive work, they came up with the cyclic model.
In the cyclic model, dark energy doesn’t suddenly turn off after the creation of the Universe and then return. Instead, it is dark energy – which we can observe as opposed to inflation which is theoretical – that drives the initial expansion of the Universe and continues the process, strengthening as the Universe ages.
Ultimately it also reverses direction, a possibility that other theorists had considered even before the cyclic Universe scenario was proposed. The reversal takes a long time – perhaps as much as 10500 years. But eventually the Universe collapses to a tiny size (the model doesn’t specify precisely how small, but it’s far larger than inflation calls for). Then the dark energy reverses direction again, the Universe begins to expand, and a new cycle bounces into being. “In this model,” Turok says, “there is no inflation, and dark energy isn’t a bizarre add-on: it’s essential.”
By positing a Universe that expands for many billions of years, then contracts then expands again, perhaps infinitely many times, Steinhardt’s and Turok’s theory addresses many of the same mysteries inflation appeared to solve.
For example, because the cosmos has gone through many, many cycles, it has had ample time for different regions to have come into temperature equilibrium, so there’s no problem with the fact that opposite sides of the visible Universe look essentially the same. And the topological “flatness” of the visible Universe might emerge not from ultra-fast expansion but from the effect of dark energy during the contraction. Precisely how the reversal happens is something Turok and Steinhardt haven’t worked out yet. “There’s a lot of effort in the field right now,” says Steinhardt, “different approaches for thinking about these bounces, but they all have the feature that they are continuous processes, meaning there can’t be anything too crazy that happens as you’re going through them”– for example, nothing as crazy as the singularity where density becomes infinite and physics breaks down – a state that appears inevitable if the Universe expands only once.
While both physicists are convinced that the cyclic theory is more straightforward and plausible than the inflationary model, they realise their arguments won’t be enough to wean their colleagues away from inflation. Both theories match existing observations very well, and neither Steinhardt nor Turok is prepared to say the cyclic model is clearly better at this point. But there’s one observation that could decide between them. Gravitational waves are predicted by inflation; cyclic models say they shouldn’t exist.
If the BICEP2 telescope had actually found the signal its scientists claimed last spring, that would have been the end of the road for Steinhardt’s ideas. The fact that it didn’t, he says, should inspire other physicists and astrophysicists to take another look at cyclic models.
For Steinhardt, cosmology is experiencing a challenge akin to that faced by planetary astronomers of the mid-1500s. Ptolemy’s Earth-centred Solar System was the reigning view but contested by Copernicus’ Sun-centered theory. “Copernicus could explain some things conceptually that Ptolemy couldn’t”, says Steinhardt, “and vice versa”. It was only when Kepler realised the planets follow elliptical rather than circular paths that Copernicus’ model pulled ahead. In Steinhardt’s view this is a Kepler moment.
Most physicists aren’t quite ready for that. “It’s still possible with the BICEP2 and Planck data that there could be a whopping great gravitational wave signature,” says Cambridge’s Joanna Dunkley. “It’s not that BICEP2 has got no signal at all, it’s just the signal is much more likely to be dust than the Big Bang.” As observers continue to refine their observations of the dust, however, it will become easier for them to subtract the dust signal electronically and see if there are any truly primordial polarised microwaves hiding behind it – much as they do now when observing vanishingly dim galaxies through the Earth’s atmosphere.
And even if no inflation signal emerges out of the dust, the waves could well be out there but beyond the limits of current detectors to find them. “There’s a very large spectrum of possibilities for the intensity of those gravity waves,” says Guth.
That could change over the next few years, however, as Planck satellite data continues to be analysed and as other ground-based CMB detectors continue their watch for signals from the ancient Universe. They include the balloon-borne SPIDER detector, which just completed a loop around Antarctica; the Atacama Cosmology Telescope, the POLARBEAR experiment and the Cosmology Large Angular Scale Surveyor in Chile; the South Pole Telescope; the Harvard group’s Keck Array, and more. All of them are looking for polarised light – some scanning larger patches of sky in less detail, others looking at small patches more intensively. “A lot of people are thinking up new ways to measure this very, very tiny signal,” says Lyman Page, Steinhardt’s Princeton colleague “and we’ve been thinking about it for years”.
Each instrument will make valuable observations in its own right, says Bill Jones, a Princeton physicist who works with the SPIDER experiment. “It’s sort of like a force multiplier in the sense that we can take advantage of the different strengths that they have in order to really nail the signal,” he says.
Like most of his colleagues, Jones acknowledges that the cyclic models are interesting –even intriguing. But he adds: “I think that when the average cosmologist wakes up in the morning, he or she probably still thinks something like inflation happened.”
Steinhardt, Turok and the other crusaders for the cyclic model are fine with that. For now.
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