Let
- $(E,\mathcal E)$ be a measurable space;
- $(\kappa_t)_{t\ge0}$ be a Markov semigroup on $(E,\mathcal E)$;
- $\mu$ be a finite measure on $(E,\mathcal E)$ which is subinvariant with respect to $(\kappa_t)_{t\ge0}$;
- $p\ge1$.
Under this assumptions, $(\kappa_t)_{t\ge0}$ has a unique extension to a contraction semigroup on $L^p(\mu)$.
If $(\kappa_t)_{t\ge0}$ is strongly continuous at $f\in L^p(\mu)$, the orbit $$\operatorname{orb}f:[0,\infty)\to L^p(\mu)\,,\;\;\;t\mapsto\kappa_tf$$ is continuous and hence Borel measurable.
Is there a suitable assumption under which every orbit is Borel measurable? For example, is it sufficient to assume that $$[0,\infty)\times E\;,\;\;\;(t,x)\mapsto(\kappa_tf)(x)\tag1$$ is Borel measurable for all bounded $\mathcal E$-measurable $f:E\to\mathbb R$?