De-Higgsing In Eleven-Dimensional Supergravity On The Squashed $S^7$

In this paper we construct the subset of modes on $S^7$ that are relevant in the compactification of eleven-dimensional supergravity on a squashed $S^7$ when restricted to the sector that comprises singlets under the $Sp(1)\times Sp(2)$ isometry of the squashed sphere. Some of the properties of these modes, connected to the transition from the round $S^7$ to the squashed $S^7$, are analysed in detail. Special features of the Rarita-Schwinger operator, described in earlier work by Buchdahl, are explained and related to properties of the squashed $S^7$ operator spectrum obtained in previous work by the authors. We then discuss how the singlet modes give rise to supermultiplets in the left-squashed case, the phenomenon of de-Higgsing, and what happens to the AdS$_4$ fields in these supermultiplets under an orientation reversal (``skew-whiffing'') of the squashed $S^7$. Finally, we consider the possible choices of boundary conditions that appear for some of these fields in AdS$_4$ in the case of the right-squashed non-supersymmetric compactification, and how these choices may affect the stability of the gravity theory.