The Cosmic Microwave Background (CMB) is a relic radiation field that we observe in all directions at a uniform temperature of 3 Kelvin. This light last scattered during a hot and dense stage of the early universe, just as it was transitioning from ionized plasma to neutral gas. The discovery of the Cosmic Microwave Background by Penzias and Wilson in 1964 was a striking confirmation of the Big Bang theory, providing direct evidence of an early hot epoch.
Since its discovery, the CMB has been a crucial source of information about our universe. Numerous experiments have made increasingly detailed maps of the CMB temperature field, with accelerating progress over the last two decades since COBE reported the first detection of temperature anisotropies. Recently, the Planck satellite has produced extremely high signal-to-noise maps of the CMB temperature across the entire sky, while the South Pole Telescope and Atacama Cosmology Telescope have mapped smaller fields at arcminute resolution.
CMB Polarization and Inflation
The next frontier of CMB research involves measuring its polarization. Polarization describes the orientation of the light perpendicular to the direction of propagation (unpolarized light has no particular orientation) and the CMB is linearly polarized at the 10% level due to Thomson scattering of photons off free electrons in the surface of last scattering. CMB polarization was first detected by DASI from the South Pole in 2002 and has since been observed by many other experiments.
A key motivation for polarization measurements is the theory of inflation, which posits that the size of the universe expanded by an unimaginably large factor during a tiny fraction of a second at the time of the Big Bang. This theory is undeniably wild, and is not supported by any confirmed particle physics, yet it has been quite successful in explaining many observed features of our universe, including the origin of the CMB temperature anisotropies.
One as-yet-unobserved prediction of inflation is that it would produce a background of gravitational waves. These gravitational waves are too faint to directly detect today, but they can be observed through their signature on the CMB polarization. The dominant contribution to CMB polarization anisotropies is from density (or scalar) perturbations in the early universe, but an amazing fact is that these density perturbations only create polarization patterns of a particular type, known as E modes. Gravitational waves from inflation can source B-mode polarization, so a B-mode search allows us to target the signal of inflation at very high sensitivity without being swamped by the larger E-modes.
For a simple geometric explanation of E and B modes, see the diagram below. For both the E mode (left) and B mode (right), the polarization (indicated by the headless lines) varies along the horizontal direction indicated by the wave vector, k. For the case of a pure E mode, the polarization is parallel or perpendicular to k (i.e. horizontal or vertical). For a pure B mode, the polarization is rotated by 45° with respect to k.
To search for B modes, we make maps of the sky polarization and separate the E- and B-mode contributions for all possible wave vectors (the image at the top of this page shows the B-mode part of the BICEP2 map). Models of inflation predict that gravitational waves will source B modes at angular scales of a degree or larger, but they don’t predict the amplitude, which we parametrize with the tensor-to-scalar ratio, r. In March 2014, we reported a discovery of just such a signal with BICEP2, but subsequent results from the Planck satellite have shown that Galactic foregrounds (specifically dust) accounts for most, or perhaps all, of the B-modes. Our joint analysis with Planck was published in February 2015 and it represents the start of a new era in our field. Past instruments have strived to achieve the sensitivity to detect degree-scale B-modes, but now we face the challenge of characterizing them in detail to determine whether they originate in the early universe or else more locally.
- Wayne Hu and Martin White’s Polarization Primer
- BICEP2 Collaboration (2014), Detection of B-Mode Polarization at Degree Angular Scales by BICEP2, Phys. Rev. Lett. 112, 24, 241101 [ADS, arXiv:1403.3985]
- BICEP2/Keck and Planck Collaborations (2015), Joint Analysis of BICEP2/Keck Array and Planck Data, Phys. Rev. Lett. 114, 10, 101301 [ADS, arXiv:1502.00612]
- Abazajian et al. (2015), Inflation Physics from the Cosmic Microwave Background and Large Scale Structure (Snowmass white paper), Astropart. Phys. 63, 55 [ADS, arxiv:1309.5381]