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 zs ∼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 above1,2 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.
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Mandolesi, N., Calzolari, P., Cortiglioni, S. et al. Large-scale homogeneity of the Universe measured by the microwave background. Nature 319, 751–753 (1986). https://doi.org/10.1038/319751a0
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