Constraining Galaxy Mass Profiles with Strong Gravitational Lensing
  Measured time delays between the images of a gravitationally lensed source can lead to a determination of the Hubble constant ($H_o$), but only if the lensing mass distribution is well understood. The inability to sufficiently constrain galaxy mass models results in large uncertainties on the derived $H_o$, and severely hampers the cosmological application of this otherwise elegant method. At the same time, lensing must compete with new techniques that have the potential to measure the Hubble constant to within a few percent by the middle of the decade. In this letter we re-evaluate the role of strong gravitational lensing in the age of precision cosmology, and present a series of Monte Carlo simulations that demonstrate the effect of the galaxy mass distribution on Hubble constant determination. Though most gravitational lens systems are unlikely to contribute significantly to precision studies of the cosmological distance scale, the strong dependence of predicted time delays on the galaxy mass profile suggests that the most useful and interesting results may be obtained by running the traditional lensing problem in reverse -- namely, combining measured time delays with a well-constrained value of $H_o$ to study galaxy mass distributions. 
D. Rusin