An Etymological Dictionary of Astronomy and Astrophysics
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Describing a medium in which certain physical properties are different in different directions.

→ anisotropy; → -ic.

A solution to Einstein's theory of → general relativity that is spatially homogeneous but allows for rotation and/or shear. See also → Bianchi cosmological model.

→ anisotropic; → homogeneous; → cosmological; → model.

A variation in a property with direction, e.g. → cosmic microwave background anisotropy, → dipole anisotropy.

From → an-, privative prefix, + → isotropy.

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.

→ cosmic; → microwave; → background; → anisotropy.

A form of anistropy in the temperature of the → cosmic microwave background radiation, appearing as one hot pole and one cold pole, caused by our motion with respect to the cosmic background radiation. The temperature variations, amounting to 1 part in 1000, yield a velocity of about 600 km/sec for our Galaxy with respect to the background. → cosmic microwave background anisotropy.

→ dipole; → anisotropy.

The → anisotropy which is at the origin of the → cosmic microwave background polarization. The quadrupole anisotropy could arise from three types of perturbations: → scalar perturbation, → vector perturbation, and → tensor perturbation

→ quadrupole; → anisotropy.

Cosmology: Minute temperature variations of the cosmic microwave background radiation.

→ temperature; → anisotropy.

A space telescope launched by NASA in 2001 which measures the temperature fluctuations in the → cosmic microwave background (CMB) radiation. It creates a full-sky map of the CMB, with a 13 arcminute resolution via multi-frequency observations. WMAP is the first mission to use a → Lagrangian point L2 as its permanent observing station at a distance of 1.5 million km. WMAP completed its prime two years of mission operations in September 2003 and is continuing in 2009 its observations for still several years to come. WMAP's measurements have played a considerable role in establishing the current standard model of cosmology. They are consistent with a Universe that is dominated by → dark energy, with negative pressure or a → cosmological constant. In this model, the age of the Universe is 13.73 ± 0.12 billion years. The current expansion rate of the Universe measured by the Hubble constant, is 70.5 ± 1.3 km·s^-1 Mpc^-1. The content of the Universe consists of 4.56% ± 0.15% ordinary → baryonic matter, 22.8% ± 1.3% → cold dark matter, and 72.6% ± 1.5% of → dark energy, that accelerates the → expansion of the Universe.

WMAP, short for Wilkinson Microwave Anisotropy Probe, in honor of David Todd Wilkinson (1935-2002), who had been a member of the mission's science team.