23 . Cosmic microwave background 1 23 . COSMIC MICROWAVE BACKGROUND

The energy content in radiation from beyond our Galaxy is dominated by the Cosmic Microwave Background (CMB), discovered in 1965 [1]. The spectrum of the CMB is well described by a blackbody function with T = 2.725 K. This spectral form is one of the main pillars of the hot Big Bang model for the early Universe. The lack of any observed deviations from a blackbody spectrum constrains physical processes over cosmic history at redshifts z ∼ < 10 7 (see earlier versions of this review). All viable cosmological models predict a very nearly Planckian spectrum inside the current observational limits. Another observable quantity inherent in the CMB is the variation in temperature (or intensity) from one part of the microwave sky to another [2]. Since the first detection of these anisotropies by the COBE satellite [3], there has been intense activity to map the sky at increasing levels of sensitivity and angular resolution by ground-based and balloon-borne measurements. These were joined in 2003 by the first results from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) [4], which were improved upon by analysis of the 3 year and 5 year WMAP data [5,6]. Together these observations have led to a stunning confirmation of the 'Standard Model of Cosmology.' In combination with other astrophysical data, the CMB anisotropy measurements place quite precise constraints on a number of cosmological parameters, and have launched us into an era of precision cosmology. This is expected to continue with the improved capabilities of the Planck satellite. Observations show that the CMB contains anisotropies at the 10 −5 level, over a wide range of angular scales. These anisotropies are usually expressed by using a spherical harmonic expansion of the CMB sky: T (θ, φ) = m a m Y m (θ, φ). The vast majority of the cosmological information is contained in the temperature 2-point function, i.e., the variance as a function only of angular separation, since we notice no preferred direction. Equivalently, the power per unit ln is m |a m | 2 /4π.