Top: likelihood as a function of the length of an edge of the fundamental domain L for a cubic-torus topology. In this figure, χ_rec gives the distance to the surface of recombination. The data are component-separated CMB temperature maps degraded to HEALPix N_side = 16 resolution and smoothed with an FWHM = 660′ Gaussian filter. The common mask of f_sky = 0.78 is used. The likelihood is marginalized over the amplitude of fluctuations, but maximized over the orientation of the fundamental domain. Lines for different estimates of the CMB temperature from Planck data are black: SMICA; magenta: SEVEM; green: Commander-Ruler; blue: NILC. The red line is for a simulated isotropic sky from a fiducial flat simply-connected model. Noise has been accounted for but is negligible at N_side = 16. The likelihoods are normalized to match the likelihood obtained with the common mask in the R₀ → ∞ isotropic flat limit. The vertical lines mark the positions where χ_rec is equal to the characteristic sizes of the fundamental domain, from left to right, ℛ_i = L/ 2, and . Dots, superimposed onto the SMICA curve, designate the discrete set of models studied. Bottom: zoom into the transitional region near χ_rec ≈ R_i. Black PlanckSMICA and red fiducial curves are the same as in the top panel. The grey curve (open circles) is the likelihood marginalized over the orientations for the PlanckSMICA map. Only ℛ_i and ℛ_m are within the scale range shown.