Measuring the Hubble constant with lens time delays in inhomogeneous universes

Phillip Helbig

The effects of a locally inhomogeneous universe on the uncertainty of the Hubble constant as determined from measured time delays in gravitational lens systems is discussed. The effect has been described adequately in the literature, but it is usually not taken into account when discussing measurements of the Hubble constant using gravitational lens time delays. Depending on the cosmological model and the redshifts of the particular lens system considered, the effect of local inhomogeneity can significantly increase the uncertainty in the determination of $\hnull$, and in `probable' cosmological models can be the dominant uncertainty.