Strong gravitational lensing

Topologically, multiple image production is governed by the odd number theorem.

[1] Strong lensing was predicted by Albert Einstein's general theory of relativity and observationally discovered by Dennis Walsh, Bob Carswell, and Ray Weymann in 1979.

[3] The upcoming Vera C. Rubin Observatory and Euclid surveys are expected to discover more than 100,000 such objects.

With the constraints from multiple images or arcs, a proposed mass model can be optimized to fit to the observables.

The subgalactic structures that currently interest lensing astronomers are the central mass distribution and dark matter halos.

A light source passes behind a gravitational lens (point mass placed in the center of the image). The aqua circle is a source as it would be seen if there was no lens, white spots are the multiple images of the source.