A Bit of History

In 1968, Rees (1968) pointed out that in an anisotropic model Universe, the microwave background radiation emerging from the recombination period would be polarised. The observed polarisation would reflect the last few scatterings of the photons just prior to the recombination. The first attempt to detect polarisation was reported by Nanos (1979)onthe basis of his thesis work in 1973–1974 under the supervision of David Wilkinson. Perhaps not surprisingly, he could only put upper limits on any anisotropy, but it was an important first step. Subsequently, Caderni et al. (1978); Lubin and Smoot (1981) attempted to improve on the limits set by Nanos. However, it was not until 2002 when Kovac et al. (2002) reported the first positive detection of polarisation by the DASI ground-based experiment team. Shortly afterwards a positive detection was reported from the WMAP experiment (Kogut et al., 2003; Page, 2003).

The 2003 flight of the BOOMERANG long-duration balloon-borne experiment, launched from the South Pole, was one of the great CMB mapping experiments. The 1998 flight had already revealed, for the first time, the first two peaks of the temperature power spectrum (de Bernardis, 2000; Ruhl, 2003), while the 2003 flight yielded the first polarisation power spectrum. The data and analysis papers of the 2003 flight did not appear until 2006 (Jones, 2006; MacTavish, 2006).

The Background Physics

Here we present the physics of how electromagnetic waves in a vacuum scatter off electrons. The main process of interest in the cosmological context is Thomson scattering, the scattering of light off a free electron.