We consider the following abstract version of the Moore–Gibson–Thompson equation with memory
depending on the parameters
α,β,γ>0, where
A is strictly positive selfadjoint linear operator and
g is a convex (nonnegative) memory kernel. In the subcritical case
αβ>γ, the related energy has been shown to decay exponentially in
[19]. Here we discuss the critical case
αβ=γ, and we prove that exponential stability occurs if and only if
A is a bounded operator. Nonetheless, the energy decays to zero when
A is unbounded as well.