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We consider the physical implications of very light axiodilatons motivated bya novel mechanism to substantially reduce the vacuum energy proposed inarXiv:2110.10352. We address the two main problems concerning the lightaxiodilaton that appears in the low-energy limit, namely that the axion has avery low decay constant $f_a \sim $ eV (as read from its kinetic term) and thatthe dilaton is subject to bounds that are relevant to tests of GR once$\rho_{\rm vac} \leq 10^{-80} M_p^4$. We show that eV scale axion decayconstants need not be a problem by showing how supersymmetric extra dimensionsprovide a sample unitarization for axion physics above eV scales for whichnon-anomalous matter/axiodilaton couplings can really have gravitationalstrength, showing how naive EFT reasoning can mistakenly overestimates axioninteractions at eV. When axions really do couple strongly at eV scales weidentify the dimensionless interaction in the UV completion that is also O(1),and how axion energy-loss bounds map onto known extra-dimensional constraints.We find a broad new class of exact exterior solutions to the vacuum axiodilatonequations and knowledge of axiodilaton-matter couplings also allows us tonumerically search for interior solutions that match to known exteriorsolutions that can evade solar-system tests. We find no examples that do so,but also identify potential new candidate mechanisms for reducing the effectivedilaton-matter coupling to gravitating objects without also undermining theunderlying suppression of $\rho_{\rm vac}$.

Light Axiodilatons: Matter Couplings, Weak-Scale Completions and Long-Distance Tests of Gravity