First glimpse of big bang ripples from universe’s birth
Waves in the very fabric of the cosmos are allowing us to peer further back in time than anyone thought possible, showing us what was happening in the first slivers of a second after the big bang. If confirmed, the discovery of these primordial waves will have rippling effects throughout science. It backs up key predictions for how the universe began and operates, and offers a glimmer 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 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 of seeing this signal by chance are less than 1 in 3.5 million.
«It is absolutely mind-boggling 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 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.»
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.
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 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.
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).
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 about whether inflation really happened, with some suggesting that we scrap the idea entirely for a new model of cosmic birth. But most agreed that finding primordial gravitational waves would clinch the concept.