A note on viability of nonminimally coupled $f(R)$ theory
Tomi Koivisto, Nicola Tamanini
TL;DR
This paper examines the stability conditions of nonminimally coupled $f(R)$ gravity theories, confirming previous results and clarifying issues related to sound speed definitions to ensure physical viability.
Contribution
It verifies the consistency conditions for nonminimally coupled $f(R)$ theories and clarifies a subtlety in the definition of sound speeds.
Findings
Confirmed absence of tachyons and instabilities under certain conditions
Clarified the distinction in sound speed definitions in these theories
Supported the theoretical viability of specific nonminimally coupled $f(R)$ models
Abstract
Consistency conditions for nonminimally coupled theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds.
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