  Due to the importance of the Cox-Ingersoll-Ross process in different areas of
finance, a broad spectrum of studies and investigations on this model have been
carried out. In case of ambiguity, we characterize it by applying the
$G$-expectation theory and the associated $G$-Brownian motion. In this paper,
we provide the existence and uniqueness of the solution of the
Cox-Ingersoll-Ross process in the presence of volatility uncertainty. In
addition, some properties of the solution are indicated, such as the regularity
and strong Markov property. Besides, we calculate some moments of the CIR
process using a generalization of the nonlinear Feynman-Kac theorem.
