Gravitational Lensing

Gravitational lensing is the deflection of light by the curvature of spacetime around a massive object, such that the mass acts as a crude lens — magnifying, distorting, and often multiplying the image of a more distant source behind it. It is a direct consequence of general relativity: a light ray passing a mass M with impact parameter b is bent by an angle α = 4GM/c²b, twice the value Newtonian gravity predicts. When many rays from a single background source pass a foreground mass, they can reach the observer along several distinct paths, producing multiple images, luminous arcs, or, in the case of perfect alignment, a complete Einstein ring.

Lensing is conventionally divided into three regimes. Strong lensing, near massive galaxies and clusters, produces resolvable multiple images, giant arcs, and rings. Microlensing produces no resolvable images but a transient brightening of a background star as a compact foreground object drifts across it; the magnification spike traces a characteristic light-curve and has been used to detect exoplanets and to search for compact dark matter. Weak lensing is the small, statistical distortion of the shapes of millions of background galaxies by the large-scale distribution of matter, measurable only by averaging over many galaxies.

Because lensing responds to all mass — luminous or dark — it is one of the few ways to weigh the invisible. Mapping the gravitational shear across the sky yields a map of total mass that is dominated by dark matter, as in the celebrated Bullet Cluster, where the lensing mass is visibly offset from the X-ray-emitting gas. Time delays between the flickering images of a lensed quasar even provide an independent measurement of the expansion rate of the universe. First confirmed with the 1979 'Twin Quasar,' lensing has become a workhorse of twenty-first-century cosmology.

Predicted in outline by Einstein as early as 1912 and published in a 1936 note (prompted by the amateur engineer Rudi Mandl), gravitational lensing was long thought unobservable for stellar lenses. Fritz Zwicky pointed out in 1937 that galaxies and clusters would lens far more strongly. The first lens — the doubly imaged quasar Q0957+561 — was identified in 1979, and the first Einstein ring was imaged in 1988.