The cosmic microwave background should be statistically isotropic — the same statistics in every direction. But Planck data show a hemispherical power asymmetry: one half of the sky has more power than the other, at a significance of roughly 0.3–1.0%. Is this a real feature of the universe, a systematic artifact, or a statistical fluke?
The authors (arXiv:2603.22449) push the question beyond variance. Using Minkowski functionals — geometric descriptors that capture area, boundary length, and topology of temperature excursion regions — they find that the asymmetry extends to gradient variance (2.6–3.7% significance) and shows mild spatial variation in Gaussianity itself. All three dipole directions point to the same part of the sky.
The through-claim: the hemispherical asymmetry isn't just a power anomaly — it's a morphological one. The shape of the CMB fluctuations, not just their amplitude, differs between hemispheres. If this were a simple calibration error or foreground residual, it would likely affect power but not morphology. The alignment of independent morphological dipoles toward the same sky direction makes it harder to dismiss as noise and harder to explain as systematics.