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Gravitational waves provide strong evidence for inflation

Posted: MARCH 17, 2014

The first direct evidence for not only the inflation of the Universe when it was a mere fraction of a second old, but also the presence of gravitational waves rippling through space, has come to light following observations made by the BICEP2 telescope located at the South Pole.

The polarisation pattern in the CMB seen by BICEP2. The faint swirls are evidence of gravitational waves. Image: BICEP2 Collaboration.

Until now, inflation has been the big bang model's achilles heel. The big bang is a direct result of the observation, first made in the 1920s by Edwin Hubble, that we live in an expanding Universe and that once upon a time - 13.81 billion years ago to be precise - the Universe was much smaller, so small in fact that it was practically a single point containing all the mass and energy that is in the Universe today. With all that pent up energy it is little wonder the Universe instantly began to expand after it was born, but gradually astronomers in the twentieth century began to notice something odd. The Universe is huge and today it has expanded so that the farthest visible regions are around 96 billion light years apart. They are so far apart that it is impossible for light, or indeed any physical properties, to have had enough time to have been transmitted from one side to another given the age of the Universe and yet these distant regions all look pretty much the same, sharing the same characteristics.

In 1980, MIT physicist Alan Guth supplied a theoretical answer to this paradox, which he called inflation. He suggested that in the first instants after the big bang, the Universe underwent an extreme bout of expansion. It lasted less than a second but in this time the Universe inflated 100 trillion trillion times, far outpacing the speed of light (it could do this because it was space itself that was expanding - the speed of light is the speed limit for objects within space). This moment of inflation instantly took regions that had been in close proximity, sharing characteristics, and moved them far, far away from each other.

This was all well and good, but there was a problem: what caused inflation was a mystery and still is to a large extent. However, the new observations from BICEP2 promise to help narrow down the options.

"The issue with inflation is that it requires new fundamental physics beyond the four known forces of nature to explain," says Marc Kamionkowski, a cosmologist from Johns Hopkins University who was not part of the BICEP2 study. "The strength of the signal tells us how energetic inflation was, which will help us constrain the various theories. It is the smoking gun for inflation."

BICEP2 (Background Imaging of Cosmic Extragalactic Polarisation) studied polarised light in the cosmic microwave background (CMB). This microwave light was emitted around 380,000 years after the big bang when the Universe had sufficiently cooled to allow electrons to combine with atoms and light to travel unhindered throughout the cosmos without the electrons to scatter off.

The cosmic microwave background, as viewed by the Wilkinson Microwave Anisotropy Probe. The false colour indicate areas of higher and lower density in the early Universe. Image: NASA/WMAP Science Team.

However, prior to this epoch of 'recombination', photons of light had spent much of their time scattering off electrons. This left a polarisation signal in the CMB, just like the way light is polarised when it reflects off the surface of a pond. Using BICEP2 astronomers were able to measure this polarisation signal and found that its pattern was a particularly unique one, in technical jargon known as 'B-mode' polarisation. It describes how the polarisation can be either left handed or right handed and leaves an identifiable swirly pattern that is exactly what we would expect from gravitational waves that were expanded by inflation.

"We are convinced the signal is real and is not foreground emission," says Clem Pryke of the University of Minnesota. "The most reasonable interpretation is that these are gravitational waves dating from the inflationary epoch written in the CMB sky."

This observation of characteristics of the Universe dating back to the first tiny fraction of a second after the big bang was achieved by using a remarkable telescope. BICEP2 is composed of 512 superconducting detectors that, when chilled to a mere 0.25 degrees above absolute zero allow electrical currents to flow with zero resistance, vastly increasing the telescope's sensitivity. "This is the deepest measurement technologically of the polarisation that is possible," says Jamie Bock of Caltech, who co-led the BICEP2 experiment.

Although a truly remarkable observation in every way, Kamionkowski offers a note of caution, pointing out that the observations have to be verified first. The BICEP3 experiment is already underway at the South Pole, while in space the European Space Agency has its Planck telescope that has been studying the CMB and its polarisation. Nevertheless, Kamionkowski is enthused by the findings.

"Inflation has sent us a telegram encoded in the gravitational waves of the CMB," he says. "It will take decades to understand exactly what this telegram is telling us."