gravitational lensingeffect
Massive bodies, such as galaxies, curve the space around them, like a heavy ball lying on a stretched sheet. Light rays passing nearby bend around this "dent," changing direction. As a result, we see distorted or even multiple images of distant objects — for example, a galaxy may appear as a luminous arc. This effect helps astronomers "weigh" galaxy clusters and find dark matter.
How it works
There are three types of lensing: strong (multiple images, arcs, and rings from massive clusters), weak (barely perceptible distortion of the shapes of distant galaxies, allowing mapping of dark matter), and microlensing (brightening of a star when a massive, compact object passes in front of it). Lensing has become a powerful tool of cosmology.
💡 Sometimes a gravitational lens creates a perfect ring — an Einstein ring. Imagine looking into a huge cosmic eye! Such rings can be almost perfectly circular if the lens, source, and observer are aligned exactly in a line.
\alpha = \frac{4GM}{c^2 r}
where α is the deflection angle, G is the gravitational constant, M is the lens mass, c is the speed of light, r is the impact parameter
\theta_E = \sqrt{\frac{4GM}{c^2}\frac{D_{ls}}{D_l D_s}}
where θ_E is the angular radius of the Einstein ring, D_ls, D_l, D_s are angular diameter distances between lens and source, lens and us, source and us