CMB Observational Techniques and Recent Results

The Cosmic Microwave Background (CMB) consists of photons that were last created about 2 months after the Big Bang, and last scattered about 380,000 years after the Big Bang. The spectrum of the CMB is very close to a blackbody at 2.725 K, and upper limits on any deviations from of the CMB from a blackbody place strong constraints on energy transfer between the CMB and matter at all redshifts less than 2 million. The CMB is very nearly isotropic, but a dipole anisotropy of ±3.346(17) mK shows that the Solar System barycenter is moving at 368 ±2 km/sec relative to the observable Universe. The dipole corresponds to a spherical harmonic index l = 1. The higher indices l≥ 2 indicate intrinsic inhomogeneities in the Universe that existed at the time of last scattering. While the photons have traveled freely only since the time of last scattering, the inhomogeneities traced by the CMB photons have been in place since the in- flationary epoch only 10−35 sec after the Big Bang. These intrinsic anisotropies are much smaller in amplitude than the dipole anisotropy, with ΔT ≤ 100 µK. Electron scattering of the anisotropic radiation field produces an anisotropic linear polarization in the CMB with amplitudes ≤ 5 µK. Detailed studies of the angular power spectrum of the temperature and linear polarization anisotropies have yielded precise values for many cosmological parameters. This paper will discuss the techniques necessary to measure signals that are 100 million times smaller than the emission from the instrument and briefly describe results from experiments up to WMAP.