Large-scale homogeneity of the Universe measured by the microwave background

The isotropy of the cosmic-microwave background is a measure of the homogeneity of the Universe at the epoch of last scattering, conventionally taken to be the epoch of recombination of the primeval plasma at a redshift z_s ~1,000. Here we report measurements of the isotropy at a wavelength of 3 cm. At the 95% confidence level, our results restrict departures from isotropy to < 5-7×10^-4 on angular scales of ~2-10°. The angular scale of our observations is of particular cosmological significance because regions of size ~1° and above^1,2 were not causally connected at the epoch of last scattering, at least in conventional big-bang models. Thus in these models, isotropy on scales ≳1° an be stipulated only as an initial condition. In the new inflationary models³, on the other hand, a causal process to produce large-scale isotropy is included.