First glimpse of big bang ripples from universe's birth
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14:54 17 March 2014 by Lisa Grossman
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Waves in the very fabric of the cosmos are allowing us to peer (scruter) further back in time than anyone thought
possible, showing us what was happening in the first slivers (éclat) of a second after the big bang. If confirmed, the
discovery of these primordial waves will have rippling (ondulation) effects throughout science. It backs up key
predictions for how the universe began and operates, and offers a glimmer (lueur) of hope for tying together two
foundational theories of modern physics. It might even net the discoverers a Nobel prize.
The waves in question are called gravitational waves and are produced when a massive object accelerates through the
fabric of space-time, causing ripples. They appear in Einstein's highly successful theory of general relativity, although
they have never been directly detected.
Today, scientists working with the BICEP2 collaboration at the South Pole announced the first clear sign of gravitational
waves, found in maps of the earliest light emitted after the big bang. The distinctive swirls (tourbillons) made by the
waves are more pronounced than the team expected, because models had suggested that gravitational waves from
this early era would be incredibly weak and perhaps even undetectable.
The team has spent three years ruling out alternate explanations, such as dust in our own galaxy, distortions caused by
the gravity of more distant galaxies and errors introduced by the telescope itself. In a pair of papers published online
today, they report a confidence level greater than 5 sigma. In other words, the odds (chances) of seeing this signal by
chance are less than 1 in 3.5 million.
Nobel-prize material
"It is absolutely mind-boggling (pas croyable) that we've actually found it," says team member Clement Pryke at the
University of Minnesota in Minneapolis. "In my heart, I did not expect it. I thought we would do this because there's
good physics to be done, and we'd prove that the signal was so small that it wasn't worth trying any harder. Instead, it
is loud and clear."
The papers have not yet been formally reviewed for publication in a journal, although they will appear on the widely
used physics preprint server arxiv.org and then will be submitted for publication. The results also still need to be
confirmed by other experiments. But physicists who have seen the papers say that so far, the results look convincing.
"No experiment should be taken too seriously until there's more than one that can vouch (se porter garant) for it," says
Alan Guth at the Massachusetts Institute of Technology. "But it does seem to me that this is a very reliable group and
what they've seen is very definitive."
Marc Kamionkowski at Johns Hopkins University in Baltimore, Maryland, is even more effusive. "This is the greatest
discovery of the century," he says. "If it sticks, which I think it will, it's Nobel-prize material."
Swirling signal
Based on his theory of general relativity, Einstein predicted that gravity from massive objects interacting, such as black
holes merging, would create ripples in space-time that would propagate outwards. Several experiments have been
searching for the telltale distortions caused by these types of gravitational waves passing Earth.
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In principle, gravitational waves come in a variety of wavelengths, which lie along a spectrum – just as light waves run a
spectrum from long-wavelength microwaves through visible light to short-wavelength gamma rays.
And there are other places to look for the expected different wavelengths of gravitational waves. Cosmologists
suggested that a growth spurt in the baby universe called inflation would let us see traces of shorter waves in maps of
the cosmic microwave background, or CMB, the first light emitted in the universe, roughly 380,000 years after the big
bang.
Guth and his colleagues first proposed the idea of inflation in the 1980s to explain a wrinkle (ride) in the CMB: the
temperature variations we see are too uniform for matter to have expanded slowly from a tiny point. Instead, they say,
space-time ballooned in size by more than 20 orders of magnitude in a fraction of a second after the big bang. Then the
expansion slowed to a more sedate pace.
Getting polar
Inflation should have stretched the very first gravitational waves created during the big bang, taking them from
imperceptible wavelengths to a size we can detect in the CMB. This is via something called its polarisation, which is the
orientation of its light waves. In the same way that sunlight is polarised as it scatters off molecules in Earth's
atmosphere, the CMB is polarised as it scatters off electrons in the cosmos. Rippling gravitational waves would subtly
change the polarisation pattern, twisting the CMB into distinctive swirls called B-modes (see diagram, below).
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Previous temperature maps of the CMB suggested that the signal from primordial gravitational waves should be very
weak. That seemed to rule out many of the simplest proposed explanations for inflation. Theorists have since bickered
(chamailler) about whether inflation really happened, with some suggesting that we scrap (laisser tomber) the idea
entirely for a new model of cosmic birth. But most agreed that finding primordial gravitational waves would clinch
(conclure) the concept.
The BICEP2 result not only says that inflation is real, it also shows that the signal is strong enough to put the simplest
models of the theory back in the game.
"We see a big excess of power, and it looks exactly like the gravitational wave signal that we had been seeking," says
Pryke. "There's a huge zoo of inflationary models, but if we look at the simplest ones, they would predict values in the
ballpark that we're seeing."
Beginning to end
In the wake of these results, scientists will be eagerly (impatient) anticipating polarisation maps from projects such as
the POLARBEAR experiment in Chile or the South Pole Telescope. Updated CMB maps from the Planck space
telescope, due out later this year, are also expected to include polarisation data. Seeing a similar signal from one or
more of these experiments would go a long way towards shoring up the BICEP2 results. But the team will still need to
explain why they see a much stronger signal than the one predicted by temperature maps.
With primordial gravitational waves firmly in hand, scientists could then start teasing (taquiner) out details of the
universe in the very first moments of creation. For instance, the frequency and power of the waves seen by BICEP2
show that they were rippling through a particle soup that had an energy of about 1016 gigaelectronvolts. By contrast, the
Large Hadron Collider is expected to reach a record-setting energy peak of just 13 gigaelectronvolts.
"If you imagine a particle accelerator that could reach these scales, it would be as big as the distance between here
and the nearest star," says Pryke.
Muscular result
Intriguingly, the BICEP2 result matches predictions for what physicists call grand unification theory. At the very high
energy suggested by BICEP2, three fundamental forces in physics – the strong, weak and electromagnetic forces –
should have been merged into one force. As the universe cooled and energy scales dropped, the strong force was the
first to peel off (décroche) from this merger, and previous theories suggested that event could have triggered
(déclencher) inflation.
The new observation doesn't prove that theory is right, but it is suggestive, says Guth. The detection is also the first
whiff (bouffée) of quantum gravity, one of the thorniest puzzles in modern physics. Right now, theories of quantum
mechanics can explain the behaviour of elementary particles, but the equations fall apart when gravity is added to the
mix. Seeing gravitational waves in the CMB means that gravity and quantum theory must work together somehow.
"If gravity were not quantised, inflation would not produce gravitational waves," says Guth. "So we really are seeing a
direct effect caused by the quantisation of gravity, and it is the first time we've seen anything like that."
Pinning down inflation may shed some light on the end of the universe, too. Guth and his colleagues favour a theory
called eternal inflation, which says that the universe is constantly giving birth to smaller "pocket" universes within an
ever-expanding multiverse. We live in one of these pockets, and our cosmos will continue expanding forever until
everything is diffuse, dark and cold.
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But other pockets will continue to be born, inflate and grow to produce stars, planets and maybe life at a rapid and
ever-increasing rate. "Life as a whole has a very great prognosis," says Guth.
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