  Experiments have demonstrated that the strong light-matter coupling in
polaritonic microcavities significantly enhances transport. Motivated by these
experiments, we have solved the disordered multimode Tavis-Cummings model in
the thermodynamic limit and used this solution to analyze its dispersion and
localization properties. The solution implies that wave-vector-resolved
spectroscopic quantities can be described by single-mode models, but spatially
resolved quantities require the multimode solution. Nondiagonal elements of the
Green's function decay exponentially with distance, which defines the coherence
length. The coherence length is strongly correlated with the photon weight and
exhibits inverse scaling with respect to the Rabi frequency and an unusual
dependence on disorder. For energies away from the average molecular energy
$E_{\text{M}}$ and above the confinement energy $E_C$, the coherence length
rapidly diverges such that it exceeds the photon resonance wavelength
$\lambda_0$. The rapid divergence allows us to differentiate the localized and
delocalized regimes and identify the transition from diffusive to ballistic
transport.
