The authors claim generalized Proca theory cannot produce a stable non-singular bounce, but the proof relies on an incorrect derivative and a flawed sound-speed argument.
Cosmological scenarios with bounce and Genesis in Horndeski theory and beyond: An essay in honor of I.M. Khalatnikov on the occasion of his 100th birthday
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abstract
This essay is a brief review of the recent studies of non-singular cosmological scenarios with bounce and Genesis and their stability in a subclass of scalar-tensor theories with higher derivatives -- beyond Horndeski theories. We discuss the general results of stability analysis of the non-singular cosmological solutions in beyond Horndeski theories, as well as other closely related topics: 1) the no-go theorem, which is valid in the general Horndeski theories but not in their extensions, 2) singularities in disformal transformations relating beyond Horndeski theories with general ones, 3) healthy behaviour of the scalar sector in the unitary gauge despite divergencies of coefficients in the quadratic action for perturbations ("$\gamma$-crossing"). We describe several specific examples of bouncing cosmologies and models with Genesis epoch which have neither ghosts nor gradient instabilities among the linearized perturbations about the homogeneous isotropic background during entire evolution.
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To bounce or not to bounce in generalized Proca theory and beyond
The authors claim generalized Proca theory cannot produce a stable non-singular bounce, but the proof relies on an incorrect derivative and a flawed sound-speed argument.