  There have been various varying speed of light (VSL) models with one free
parameter, $b$, to characterize the time variation of the speed of light as a
function of a scale factor, $c = c_0a^{b/4}$, based on the expanding universe.
One needs to induce cosmological evolutions of other physical constants and
quantities having different powers of scale factor as a function of $b$ to
satisfy all known local physics laws, including special relativity,
thermodynamics, and electromagnetic force. These models should be based on the
Friedmann-Lema\^{i}tre-Robertson-Walker metric satisfying the isotropic and
homogeneous three-space known as the cosmological principle. Adiabaticity is a
necessary condition to keep homogeneity and isotropy because a net energy flux
would falsify the isotropy if there is a preferential energy flow direction. It
also might forge homogeneity if the outward (inward) flow is isotropic. Thus,
any VSL model based on the expanding universe should preserve an adiabatic
expansion condition to be a viable model. We show that this condition specifies
the cosmological evolution of the Planck constant as $\hbar = \hbar_0
a^{-b/4}$.
