Loops abound in the cosmic microwave background: A $4\sigma$ anomaly on super-horizon scales

We present a topological analysis of the temperature fluctuation maps from the \emph{Planck 2020} Data release 4 (DR4) based on the \texttt{NPIPE} data processing pipeline. For comparison, we also present the topological characteristics of the maps from \emph{Planck 2018} Data release 3 (DR3). We perform our analysis in terms of the homology characteristics of the maps, invoking relative homology to account for analysis in the presence of masks. We perform our analysis for a range of smoothing scales spanning sub- and super-horizon scales corresponding to $FWHM = 5', 10', 20', 40', 80', 160', 320', 640'$. Our main result indicates a significantly anomalous behavior of the loops in the observed maps compared to simulations that are modeled as isotopic and homogeneous Gaussian random fields. Specifically, we observe a $4\sigma$ deviation between the observation and simulations in the number of loops at $FWHM = 320'$ and $FWHM = 640'$, corresponding to super-horizon scales of $5$ degrees and larger. In addition, we also notice a mildly significant deviation at $2\sigma$ for all the topological descriptors for almost all the scales analyzed. Our results show a consistency across different data releases, and therefore, the anomalous behavior deserves a careful consideration regarding its origin and ramifications. Disregarding the unlikely source of the anomaly being instrumental systematics, the origin of the anomaly may be genuinely astrophysical -- perhaps due to a yet unresolved foreground, or truly primordial in nature. Given the nature of the topological descriptors, that potentially encodes information of all orders, non-Gaussianities, of either primordial or late-type nature, may be potential candidates. Alternate possibilities include the Universe admitting a non-trivial global topology, including effects induced by large-scale topological defects.