This analysis explores scalar perturbations in a quasi-static regime, suggesting stability in linear scales.
We analyze RCD linear scalar perturbations in the quasi-static regime. The homogeneous cosmological background forces all EFT modification parameters to unity: mu = eta = Sigma = 1. RCD is indistinguishable from LCDM at linear scales, with Delta(f*sigma_8)/f*sigma_8 < 1e-6. Modifications activate only in the non-linear regime where coherence gradients become non-negligible. This linear safety is a structural consequence of the action, not a tuning.
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Arturo Cerezo (2026) studied this question.
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