Solution (source code)

= Solution

Since a Poisson count has mean and <variance> $\lambda t$, substitute the centering and scaling into its moment generating function:
$$
\boxed{M_{U_t}(\theta)=\exp\left\{\lambda t\left(e^{\theta/\sqrt{\lambda t}}-1-\frac\theta{\sqrt{\lambda t}}\right)\right\}.}
$$
This is finite for every real $\theta$ when $t>0$.