A measurement of the scale of homogeneity in the Early Universe

Abstract

We present the first measurement of the homogeneity index, $\mathcal{H}$, a fractal or Hausdorff dimension of the early Universe from the Planck CMB temperature variations $\delta T$ in the sky. This characterization of the isotropy scale is model-free and purely geometrical, independent of the amplitude of $\delta T$. We find evidence of homogeneity ($\mathcal{H}=0$) for scales larger than $\theta_{\mathcal{H}} = 65.9 \pm 9.2 \deg $ on the CMB sky. This finding is at odds with the $\Lambda$CDM prediction, which assumes a scale invariant infinite universe. Such anomaly is consistent with the well known low quadrupule amplitude in the angular $\delta T$ spectrum, but quantified in a direct and model independent way. We estimate the significance of our finding for $\mathcal{H}=0$ using a principal component analysis from the sampling variations of the observed sky. This analysis is validated with theoretical prediction of the covariance matrix \textcolor{black}{ and simulations, booth base purely on data or in the $\Lambda$CDM prediction.} Assuming translation invariance (and flat geometry) we can convert the isotropy scale $\theta_\mathcal{H}$ into a (comoving) homogeneity scale which is very close to the trapped surface generated by the observed cosmological constant $\Lambda$.