Phase Transition for Stochastic Block Model with more than $\sqrt{n}$ Communities

Predictions from statistical physics postulate that recovery of the communities in Stochastic Block Model (SBM) is possible in polynomial time above, and only above, the Kesten-Stigum (KS) threshold. This conjecture has given rise to a rich literature, proving that non-trivial community recovery is indeed possible in SBM above the KS threshold, as long as the number $K$ of communities remains smaller than $\sqrt{n}$, where $n$ is the number of nodes in the observed graph. Failure of low-degree polynomials below the KS threshold was also proven when $K=o(\sqrt{n})$. When $K\ge \sqrt{n}$, Chin et al.(2025) recently prove that, in a sparse regime, community recovery in polynomial time is possible below the KS threshold by counting non-backtracking paths. This breakthrough result lead them to postulate a new threshold for the many communities regime $K\ge \sqrt{n}$. In this work, we provide evidences that confirm their conjecture for $K\ge \sqrt{n}$: 1- We prove that, for any density of the graph, low-degree polynomials fail to recover communities below the threshold postulated by Chin et al.(2025); 2- We prove that community recovery is possible in polynomial time above the postulated threshold, not only in the sparse regime of~Chin et al., but also in some (but not all) moderately sparse regimes by essentially by counting occurrences of cliques or self-avoiding paths of suitable size in the observed graph. In addition, we propose a detailed conjecture regarding the structure of motifs that are optimal in sparsity regimes not covered by cliques or self-avoiding paths counting. In particular, counting self-avoiding paths of length $\log (n)$–which is closely related to spectral algorithms based on the Non-Backtracking operator–is optimal only in the sparse regime. Other motif counts–unrelated to spectral properties–should be considered in denser regimes.