A physically realistic boundary condition for cosmological models at the Planck time is proposed. It predicts that the present amplitude of the microwave background temperature anisotropy on large angular scales should lie close to ∼${10}^{\mathrm{-}6}$. It is shown that homogeneous cosmologies display no ‘‘isotropy problem’’ for inflation or quantum cosmology to solve.

• Received 7 February 1994

DOI:https://doi.org/10.1103/PhysRevD.51.3113