The cosmic microwave background (CMB) has a perfect black-body spectrum with a temperature of 2.73 kelvin. But this temperature varies very slightly at different points in the sky, and this fluctuation - related to the microwave power spectrum - is thought to reflect the aggregation of matter in the early universe into galaxies and galaxy clusters. The recent Boomerang experiment, which measured the CMB by converting microwave signals into heat, detected a peak in the power spectrum predicted by the standard cosmological theory. But it failed to find a dip that was also thought to exist.

Unlike Boomerang, the Cosmic Background Imager (CBI) is a radio-interferometer - a type of instrument that has only recently become sufficiently refined to measure the CMB directly. The new telescope has imaged the CMB around three times more accurately than Boomerang, at an angular resolution of up to a twelfth of a degree - and it has found the trough in the data that Boomerang did not observe. This is the first time that a single experiment has measured this feature, which is a fundamental prediction of the standard model of cosmology.

According to Joseph Silk, an astrophysicist at the University of Oxford, this is good news for the 'inflation' theory, which links the echoes of the big bang to the power spectrum of the CMB. "These results support current theories, but most importantly they prove that the new technique is very promising", Silk told PhysicsWeb. Forthcoming radio-interferometer experiments include the Very Small Array (VSA) and the Degree Angular Scale Interferometer (DASI).