Astrophysics & Particle Physics-APPC 2026

Strong gravitational lensing is a phenomenon where a foreground massive object, e.g., a galaxy or a galaxy cluster, bends and magnifies the light of a more remote source, i.e., a quasar or galaxy. Foreseen by Einstein's general theory of relativity, this effect creates spectacular visual phenomena such as Einstein rings, giant arcs, and several images of the same celestial object.This is different from weak lensing, which involves statistical examination of minute distortions, since strong lensing can be easily observed and has high-precision measurements of cosmic structure.

The shape of these lensed images is a function of the matter distribution in the lensing object, and strong lensing is therefore a unique tool for charting the dark matter halos of galaxies and clusters.The most potent use of strong lensing is time-delay cosmography. Light from various lensed images of a quasar takes different paths and arrives on Earth with very small differences in time. Measuring such time delays enables astronomers to estimate the Hubble constant (H₀) and infer the expansion rate of the universe independent of other techniques. This has created interesting tensions among lensing-based measurements and CMB-based findings, giving rise to current discussions in cosmology.Strong gravitational lensing also amplifies background galaxies, allowing astronomers to investigate faint, ancient objects that are otherwise unreachable.

This “natural telescope” effect is particularly valuable for exploring reionization-era galaxies and the cosmic dawn.Observationally, strong lensing systems are discovered in large surveys such as the Sloan Digital Sky Survey (SDSS), Dark Energy Survey (DES), and with space telescopes like Hubble and JWST. Upcoming missions such as Euclid and the Nancy Grace Roman Space Telescope are anticipated to discover thousands of new strong lensing systems, offering unprecedented statistical power.Summarily, strong gravitational lensing is not merely a dramatic demonstration of general relativity but also a high-precision instrument for cosmology. It allows for careful investigations of dark matter distribution, cosmic expansion, and the early universe, transforming huge structures into natural laboratories for fundamental physics.