Environment Viewed from the Particle and Slowdown Estimates for Ballistic RWRE on $\mathbb{Z}^2$ and $\mathbb{Z}^3$

We consider a random walk in a random environment on $\mathbb{Z}^d$ under Sznitman's ballisticity condition $(T)$. Using techniques from arXiv:1405.6819, we show the existence of the invariant measure $Q$ with respect to the environment viewed from the particle for $d=2$ and $d=3$. This disproves a conjecture made in arXiv:1405.6819 that the invariant measure does not exist in dimension $2$. As a corollary, we prove sharp tail bounds for regeneration times for $d=3$. Finally, we also provide tail estimates for the Radon-Nikodym derivative $dQ/dP$, where $P$ is the original distribution on the environment.