An Etymological Dictionary of Astronomy and Astrophysics

Tiny fluctuations in the intensity of the → cosmic microwave background radiation (CMBR) as a function of angular position over the sky, first discovered in the → Cosmic Background Explorer (COBE) observations. At a level of 1 part in 100,000, these temperature variations trace the distribution of matter and energy when the Universe was very
young, about 380,000 years old. Since the CMB spectrum is described to a high precision by a → blackbody law with temperature T₀, it is usual to express the anisotropies in terms of temperature fluctuations ΔT/T₀ and expand them on the sky in → spherical harmonic series

ΔT/T₀ (θ,φ) = Σ a_lmY_lm(θ,φ), where θ and φ are the → spherical polar coordinates, Y_lm is the spherical harmonic functions with → multipole index l, and the sum runs over l = 1, 2, …, ∞, m = -l, …, l, giving 2l + 1 values of m for each l, and a_lm is the multipole moment of the decomposition. The power spectrum of the anisotropies is defined as C_l≡ mean | a_lm |² = 1/(2l + 1) Σ mean | a_lm |². See also → CMB angular power spectrum.

See also: → cosmic; → microwave; → background; → anisotropy.