Effect of the Great Attractor on the cosmic microwave background radiation

Abstract

ANISOTROPY in the cosmic microwave background radiation (CMB) is expected as a result of fluctuations in gravitational potential caused by large-scale structure in the Universe. The background radiation is redshifted as it climbs out of gravitational wells (the Sachs–Wolfe effect1). Here we present a map of the anisotropy in CMB temperature δT/T of our region of the Universe as viewed by a distant observer, predicted on the basis of the gravitational potential field. We calculate this field in the vicinity of the Local Group of galaxies from the observed peculiar (non-Hubble) velocities of galaxies, under the assumption that the peculiar motions are induced by gravity2–4. If the cosmological density parameter Ω is 1, the gravitational potential field of the Great Attractor and surrounding regions produces a maximum Sachs–Wolfe anisotropy of δT/T = (1.7 ± 0.3) x 10–5 on an angular scale of 1°. Doppler and adiabatic contributions to this anisotropy are expected to be somewhat larger. If similar fluctuations in the gravitational potential are present elsewhere in the Universe, the anisotropy present when the CMB was last scattered should be visible from the Earth, and should be detectable in current experiments5. A fundamental test of whether gravity is responsible for the generation of structure in the Universe can be made by looking for the imprint in the CMB of deep potential wells similar to those found in our neighbourhood6.