voids

images compare WMAP with radio source map

Rudnick et al., NRAO, AUI,NSF,NASA

As of 2007, the largest known void in the universe is a hole devoid of galaxies about 6 to 10 billion light-years from Earth in the direction of the constellation Eridanus. It is estimated to be 913 million light-years in diameter. About 9300 Milky Way galaxies, fitted edge-to-edge, would be needed to bridge it.

In 2004 an analysis of the cosmic microwave background (CMB) revealed a “cold spot” in this direction.¹ Other researchers then used the results of a sky survey by the Very Large Array of radio telescopes to look for radio sources in the same direction.² An analysis based on the lack of such sources led them to conclude that the cooling was due to the CMB photons passing through a region filled with dark energy but free of matter. Such photons lose a small amount of energy, and so are "cooler." 

1. P. Vielva, E. Martinez-Gonzalez, R. B. Varreiro, J. L. Sanz and L. Cayon,
Detection of Non-Gaussianity in the Wilkinson Microwave Anisotropy Probe First-Year Data Using Spherical Wavelets
Astrophysical Journal, vol. 609, pages 22-34, 1 July 2004.

2. Lawrence Rudnick, Shea Brown and Liliya R. Williams.
Extragalactic Sources and the WMAP Cold Spot.
Astrophysical Journal, arXiv:0704.0908v2

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