Strong‐Lensing Time Delay: A New Way of Measuring Cosmic Shear

The phenomenon of cosmic shear, or distortion of images of distant sourcesunaccompanied by magnification, is an effective way of probing the content andstate of the foreground Universe, because light rays do not have to passthrough mass structures in order to be sheared. It is shown that the delay inthe arrival times between two simultaneously emitted photons that appear to bearriving from a pair of images of a strongly lensed cosmological sourcecontains not only information about the Hubble constant, but also the longrange gravitational effect of galactic scale mass clumps located away from thelight paths in question. This is therefore also a method of detecting shear.Data on time delays among a sample of strongly lensed sources can providecrucial information about whether extra dynamics beyond gravity and dark energyare responsible for the global flatness of space. If the standard $\Lambda CDM$model is correct, there should be a large dispersion in the value of $H_0$ asinferred from the delay data by (the usual procedure of) ignoring the effect ofall other mass clumps except the strong lens itself. The fact that there hasnot been any report of a significant deviation from the $h =$ 0.7 mark duringany of the $H_0$ determinations by this technique may already be pointing tothe absence of the random effect discussed here.Comment: ApJ in pres