Wednesday, June 6, 2012

gravitational lensing

what matter distribution generates a lensing effect where the lensing convergence is equal to the shear amplitude for any position?

bonus question: can the image of a lensed galaxy be rotated in gravitational lensing? if yes, under which circumstances?


  1. I know the answers to both, but seeing my PhD was in lensing, I'll hold my tongue for a bit :)

  2. The singular isothermal sphere (SIS: spherical symmetry, power-law density $\rho \sim r^{-2}$) has a constant deflection angle $\alpha = c$ and hence the convergence equals the shear, $\kappa = \gamma = \frac{c}{2 \theta}$ ($\theta$ is the angular position).

  3. yes, and for the bonus questions: there are situations where you can think of image rotation, such as multiple lensing along the line of sight, where the effective potential is not a scalar potential anymore (because of the nonlocality of the interaction) and can in fact rotate images. in cosmological lensing these effects are very weak.