When the Harvard-Smithsonian Center for Astrophysics announced a press conference for a "Major Discovery" (capital letters in the original e-mail) involving an unspecified experiment, rumors began to fly immediately. By Friday afternoon, the rumors had coalesced around one particular observatory: the BICEP microwave telescope located at the South Pole. Over the weekend, the chatter focused on a specific issue: polarization in the Cosmic Microwave Background left over from the Big Bang. With the start of the press conference, it's now clear that we've detected the first direct evidence of the inflationary phase of the Big Bang, in which the Universe expanded rapidly in size.

BICEP, the Background Imaging of Cosmic Extragalactic Polarization experiment, was built specifically to measure the polarization of light left over from the early Universe. This light, known as the cosmic microwave background (CMB), encodes a lot of information about the physical state of the cosmos from its earliest moments. Most observatories (such as Planck and WMAP) have mapped temperature fluctuations in the CMB, which are essential for determining the contents of the Universe.

Polarization is the orientation of the electric field of light, which conveys additional information not available from the temperature fluctuations. While much of CMB polarization is due to later density fluctuations that gave rise to galaxies, theory predicts that some of it came from primordial gravitational waves. Those waves are ripples in space-time left over from quantum fluctuations in the Universe's earliest moments.

Primordial gravitational waves remain one of the outstanding untested hypotheses of inflation, the most popular model that explains the incredible uniformity of the CMB. According to inflationary theory, the Universe expanded very rapidly in the first fraction of a second, filling the cosmos with gravitational ripples. While inflation so far seems to explain a lot about the Universe, we have no direct evidence for it. BICEP, as a dedicated CMB polarization observatory, could provide some hints about primordial gravitational waves—and by extension, inflation.

One press conference later...

Update (1:25pm CT): The earliest moments of the Universe's history are hidden from us: the Universe was opaque to all forms of light from the Big Bang until about 380,000 years afterward. However, we can still reconstruct much of what happened in the interim, thanks to radiation emitted when the cosmos became transparent. That light (the cosmic microwave background) encodes much of what happened before it formed, much as earthquakes reveal information about what's going on deep beneath the surface.

Now researchers with the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) observatory have announced the first measurement of distortions in the cosmic microwave background light that could reveal what happened in the first tiny fraction of a second after the Big Bang. Those distortions take the form of twisting of the light's polarization created by gravitational disturbances from inflation: the hypothetical rapid expansion of the earliest moments of the cosmos. If these results can be squared with other observations, they would become the best evidence for inflation, providing us the best picture of the Universe in the split-second after the Big Bang.

Inflation was originally predicted in 1981 and has undergone a number of refinements since. The idea, if it is borne out by observations, would help explain a huge number of observed phenomena in the Universe. However, all evidence for inflation is circumstantial, which is why the BICEP2 results are so important. While they do not constitute a direct detection of either primordial gravitational waves (the distortions causing the light polarization) or inflation, the BICEP2 results could provide the best evidence for both—evidence that could not be easily explained away by other theories.

This observation cannot be the end of the story, however. The measurement of polarization is significantly larger than prior observations have come up with, a difference that cannot be immediately dismissed. It's not clear whether the problems are with the interpretation and analysis of the BICEP2 data or if something more subtle is at work.

Inflation, in theory

Let's flush out this idea of inflation. According to the consensus theory in cosmology, the Universe expanded exponentially during its first moments. That inflation explains a lot of observed features of the cosmos, such as the remarkable uniformity of the CMB. However, inflation is less a theory than a set of models with differences in details; worse still, other theories produce similar predictions and match the observations we have so far.

One possible way to distinguish between models is the presence or absence of primordial gravitational radiation. Inflation would have created substantial fluctuations in the structure of space-time, with their strength and properties depending on the details of the particular inflationary model. However, though astronomers have known for decades that gravitational waves exist, the evidence for them is indirect, so few expect to measure primordial gravitational radiation directly in the foreseeable future.

A Polarizing Topic Light is a wave of electromagnetic energy. The polarization of light is the orientation of the electric field of that wave. While many light sources intrinsically produce unpolarized light—meaning it is a mixture of all possible polarization angles—a variety of things can twist the polarity or block certain polarization angles. Gravitational waves can rotate the polarization of light, providing an indirect means of detection.

Nevertheless, this gravitational radiation would affect any light passing through, curling its polarization in a unique way. Because the effect resembles the mathematical twistings of magnetic fields, which physicists perversely assign the letter "B," it is known as B-mode polarization. The distinction is important, because other phenomena can also polarize light, but without the telltale curling.

Because it bears a mathematical resemblance to electric fields, the non-twisting version is known as E-mode polarization. Gravitational radiation contributes to E-mode as well, but it is the only source of B-mode polarization. Only a small fraction of CMB photons are polarized, and of those, most are E-mode in any reasonable prediction. The goal of polarization observations involves measuring the ratio of the two modes, which would allow cosmologists to separate the contributions of gravitational waves from other effects. That ratio (written as "r") is known prosaically as the "tensor-to-scalar" ratio, where "tensor" refers to the B-mode and "scalar" to the E-mode. A ratio of 0 means no primordial gravitational radiation exists—a result that would potentially rule inflation out.

To B-mode or not to B-mode

Up until BICEP2, the tensor-to-scalar ratio could only be inferred indirectly, through measuring other CMB properties. Particularly, it could only be given an upper bound, meaning that the value must be between zero and a certain maximum value. BICEP2 measured the polarization spectrum directly, in principle allowing researchers to separate the E- and B-modes, yielding the ratio directly.

The BICEP2 research team confidently stated that they can exclude the possibility that the ratio is zero. That in itself is a significant find: if the minimum possible value of the ratio is greater than zero, then primordial gravitational waves—and inflation—are very likely. However, the most likely value of the ratio the team estimated was about twice as high as the maximum value estimated via indirect methods. (The earlier data came from the orbiting WMAP CMB observatory, along with the South Pole Telescope and Atacama Cosmology Telescope.)

That discrepancy is not easily explained away. The BICEP2 researchers proposed a relatively simple modification to the behavior of inflation to reconcile the two numbers and show why they don't agree, but we'll have to see if the cosmology community finds this modification acceptable.

Could aspects of the analysis of the BICEP2 data be in error instead? Polarization is challenging to observe, and there are many possible sources of confusion. Some of those include material in the foreground (dust and gas), Earth's atmosphere, and another type of gravitational distortion: the bending of light known as gravitational lensing. While the researchers are confident they have controlled for these phenomena adequately, the fit between BICEP2 data and the gravitational lensing signature in particular seems poor. If there is more lensing than their analysis provided, then gravitational radiation could contribute less than the researchers claimed—perhaps bringing the tensor-scalar ratio into line with other observations.

While it's premature to say the BICEP2 team has provided the "First Direct Evidence of Cosmic Inflation" (as the press release declared), these results shouldn't be dismissed. You can be sure that other researchers will consider many possibilities; the story of BICEP2, inflation, and primordial gravitational radiation is just beginning.