In March 2014 a team of scientists analyzing data from the BICEP2 telescope at the South Pole found B-mode polarization—a specific imprint in the cosmic microwave background (CMB) that could have been created by the gravitational waves resulting from cosmic inflation. (See Physics Today, May 2014, page 11.) The scientific paper describing the results was published in June.1 Inflation is a stupendously rapid expansion that the universe is thought to have undergone within the first 10−36 seconds after the Big Bang. During the expansion, quantum fluctuations in the gravitational field grew in size, eventually turning into long-wavelength gravitational waves that stretched space in one transverse direction and compressed it in the perpendicular one.
As a consequence of the orthogonal stretchings and compressions of space, the CMB is expected to have become polarized—that is, having a larger amplitude in one direction than in the perpendicular direction. The important point is that the polarization induced by gravitational waves exhibits a distinct twisting pattern, known as a B mode, that distinguishes it from the polarization produced by, for example, density fluctuations,2 known as E modes, which exhibit a circular or radial pattern.
The above description explains why the BICEP2 team’s announcement, if confirmed, will mark one of the most dramatic discoveries in decades. Not only would it confirm a process that occurred when the universe was about 10−36 seconds old, it would also be the first detection of the direct effect of gravitational waves and a clear indication that the gravitational field obeys the laws of quantum mechanics.
Extraordinary claims require extraordinary scrutiny, and a few scientists were quick to point out that dust particles in our own Milky Way galaxy could, in principle, produce a polarization signal similar to the one observed by the BICEP2 team.3 Although little is known about the precise polarization pattern produced by dust, any generic polarization pattern is expected to have roughly equal contributions of E modes and B modes. For the dust polarization to be entirely free of B modes would require special circumstances.
As shown in the figure, though, the CMB and dust contributions to the B-mode maps can be distinguished by their frequency dependences. Whereas the CMB has a blackbody spectrum that peaks near a frequency ν of 160 GHz, the thermal-emission spectrum of dust rises roughly as the power law ν1.6 over the frequencies at which CMB measurements are made. Also, at relatively low frequencies, synchrotron radiation from electrons spiraling in the galactic magnetic field makes a significant contribution to the B modes imprinted on the CMB.
Sources of B-mode polarization as a function of frequency. The B-mode spectrum of the cosmic microwave background (CMB) is a blackbody function with a temperature of 2.73 K, while models of synchrotron and dust emission from our galazy predict steeply falling and rising spectra, respectively, over the frequencies at which CMB measurements are made. The amplitudes of all three contributions can be determined through measurements at three or more frequencies. Measurements at 22 GHz from the Wilkinson Microwave Anisotropy Probe indicate that the synchrotron contribution, while significant at the lowest frequencies, is relatively small at BICEP2’s 150 GHz. On the other hand, extrapolation of Planck B-mode measurements at 353 GHz indicate that the dust contribution at 150 GHz may be appreciable in the region of the sky that BICEP2 surveyed.
Sources of B-mode polarization as a function of frequency. The B-mode spectrum of the cosmic microwave background (CMB) is a blackbody function with a temperature of 2.73 K, while models of synchrotron and dust emission from our galazy predict steeply falling and rising spectra, respectively, over the frequencies at which CMB measurements are made. The amplitudes of all three contributions can be determined through measurements at three or more frequencies. Measurements at 22 GHz from the Wilkinson Microwave Anisotropy Probe indicate that the synchrotron contribution, while significant at the lowest frequencies, is relatively small at BICEP2’s 150 GHz. On the other hand, extrapolation of Planck B-mode measurements at 353 GHz indicate that the dust contribution at 150 GHz may be appreciable in the region of the sky that BICEP2 surveyed.
In an effort to distinguish CMB from dust, the BICEP2 team correlated its 150-GHz map with a lower signal-to-noise map at 100 GHz. If their 150-GHz signal were due entirely to dust, that correlation would be far weaker than if it were entirely CMB. The amplitude of that correlation favors CMB over dust, but at a level of only 1.6 standard deviations.
The researchers then used a correlation analysis to look for similarities between the spatial distribution of their B modes on the sky and those expected from models of the dust distribution in our galaxy. That analysis showed no correlation. The amplitude of their signal was, moreover, larger than that expected from only dust. The models, however, were based solely on measurements of the spatial distribution of dust on the sky, and the polarization orientation and amplitude were largely a matter of guesswork.
In September 2014 the Planck satellite team released its results on polarized dust emission at intermediate and high galactic latitudes,4 and at face value those results are not encouraging for attributing the BICEP2 signal to cosmic inflation. The new Planck data suggest that the contribution of dust to the B-mode polarization may be larger than that assumed in the dust models used by BICEP2. More precisely, the Planck researchers extrapolated the B-mode amplitude they measured at 353 GHz on the BICEP2 patch of sky down to the 150-GHz frequency of BICEP2 and found that polarization by Milky Way dust might explain the signal detected by BICEP2.
The Planck team was careful to note, though, that because of uncertainties in the extrapolation and statistical errors, its results do not rule out the possibility that the B-mode spectrum measured by BICEP2 contains a component produced by inflation. A far better way to distinguish CMB from dust will be to correlate the Planck 353-GHz maps with the BICEP2 150-GHz maps. If the BICEP2 signal is due entirely or largely to dust, the spatial distribution of that signal should resemble the spatial distribution of the 353-GHz map. The BICEP2 and Planck teams are now working together to perform that correlation analysis, and they are also including new data from the Keck Array in the South Pole at 100 GHz and 150 GHz.
Several other experiments will also expand the observed frequency range and make measurements on different and broader regions of the sky. In particular, the Cosmology Large Angular Scale Surveyor experiment, scheduled to be deployed next year, will observe at 40, 90, 150, and 220 GHz and will separate polarization components in situ.5 If there are indeed detectable cosmic B modes, they should be seen with similar amplitudes by those other experiments, not only on the BICEP2 patch of sky but everywhere else as well. Furthermore, a true inflationary gravitational-wave signal should exhibit additional hallmarks, such as a characteristic angular power spectrum and statistics.
Overall, the B-modes story demonstrates how progress in science is truly achieved. Rather than through a direct march to the truth, science advances in a zigzag path that involves many false starts, detours, and blind alleys. Crucially, the scientific method requires that theories should make falsifiable predictions that can be tested through subsequent experiments or observations. Science therefore allows for self-corrections. Still, there may be some lessons to be learned here about the importance of communicating exciting and promising new scientific results to the public in a way that stresses the process, the uncertainties involved in measurements and interpretation of data, and their possible implications.
