What is Gravitational Lensing?
Gravitational lensing is a phenomenon predicted by Einstein's theory of General Relativity, where massive objects like galaxies or galaxy clusters bend light from more distant objects as it passes by. This bending occurs because the mass warps the fabric of spacetime around it, altering the path of light rays.
How Does it Work?
Similar to how an optical lens focuses or distorts light, a massive celestial body acts as a 'gravitational lens.' When light from a background source (like a distant quasar or galaxy) travels towards us and passes near a massive foreground object, its path is deflected. This deflection can result in magnified, distorted, or even multiple images of the background source.
A Practical Example: Einstein Rings and Arcs
A classic example of gravitational lensing is an 'Einstein Ring,' where the background source, foreground lens, and observer are perfectly aligned, causing the light to form a complete ring around the foreground object. More commonly, partial rings or arcs are observed when the alignment is not perfect, showcasing stretched and duplicated images of distant galaxies.
Importance in Astronomy and Cosmology
Gravitational lensing is a powerful tool for astronomers. It allows us to detect and study extremely distant and faint objects that would otherwise be invisible. Crucially, it provides a unique way to map the distribution of dark matter, as the lensing effect is primarily caused by the total mass, visible and invisible, within the foreground lens.