In areas of the universe where distributions of matter have a high density, the curve in space-time becomes so strong that it even starts bending light. It is those areas that we now refer to as gravitational lenses as they are effectively able to bend the path of light so that even the light of the objects situated behind it (relative to the observer) may reach the observer’s eye.
However, referring to it as a lens is a slightly incorrect term. Unlike a conventional lens, a gravitational lens has no single focal point but rather a focal line where light is deflected from: the path closest to its center but where the light is still able to escape. An ideal lensing object is considered to be spherical. If the object lies perfectly between the observer and the object, he will observe a perfect ring of light around the lens, and if they are slightly misaligned, this ring (known as the Einstein ring) will become an arc.
However, the reality is usually more complex than that and involves massive objects like galaxy clusters with intricate shapes and thus do not have circular symmetry. The arc or ring of light will not be perfectly defined but rather scattered, distorted, and duplicated around the lens. These are the explanations that lie behind the intricate “smiley face” that the Hubble Space Telescope observed. Indeed, the two eyes constitute two extremely bright galaxies, and the smile, an arc of light that has been distorted through the effects of strong gravitational lensing. In the center of this image, taken with the NASA/ESA Hubble Space Telescope, is the galaxy cluster SDSS J1038+4849.
Hubble has provided astronomers with the tools to probe these massive galaxies and model their lensing effects, allowing us to peer further into the early Universe than ever before. This object was studied by Hubble’s Wide Field and Planetary Camera 2 (WFPC2) and Wide Field Camera 3 (WFC3) as part of a survey of strong lenses.
Therefore, a gravitational lens is a crucial cosmic object that has allowed us, as observers, to peer deeper into space thanks to its magnifying or bending properties and has therefore been significant to many space observations and discoveries.
How were gravitational lenses discovered?
The effects of gravitational lenses were indeed predicted by Albert Einstein’s theory of general relativity. However, he never issued any published prints on the subject, only some calculations that were kept secret until 1936 when physicists Khvolson and Link had already published articles on the matter. Besides, Swiss physicist Fritz Zwicky added a year later that gravitational lenses’ effects might also be observed in and around galaxy clusters, bringing some more specificity and detail to the matter.
After Einstein’s theory of general relativity had correctly calculated the value for the bending of light, the 1919 observation of a solar eclipse by Eddington and Dyson stated that the light from stars passing behind the Sun was slightly distorted that confirmed his calculations. However, it would not be until 1979 that the first gravitational lens was actually observed: Twin QSO.
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Types of gravitational lensing
Gravitational lenses are often considered in three different categories depending on their distortion strength. We distinguish between strong, weak, and micro lenses, which may all act upon conventional radiation (electromagnetic radiation under the form of light), but also gravitational waves or electromagnetic waves in the radio or x-ray spectrum.
Firstly, strong lenses create easily distinguishable arcs and rings. The lensing objects usually have the characteristic of being quite far from our own galaxy (hundreds of parsecs away). Despite their strong lensing effect, they do not have the strongest warp effect of space-time: the light must still reach the observer.
Secondly, weak gravitational lenses are only detectable when studying the light sources in a wider statistical manner, surveying them from different angles to find smaller distortions. By conducting such operations on wide fields of view, weak gravitational lensing can help determine the relative mass distribution over larger areas of the cosmos (including dark matter).
Finally, microlensing, causing the least obvious type of shape distortion but rather generating a boost in the intensity of light received, can magnify the luminosity of distant and otherwise imperceptible stars to allow us to receive its light. Thanks to microlensing, we observed the furthest star to date: the MACS J1149 Lensed Star 1, also known as Icarus.
As such, gravitational lensing is a crucial tool in our current observation of more distant and fainter objects and understanding of the effect of denser areas of space and their behavior concerning Einstein’s theory of general relativity. They allow us to extend our view deeper into the universe, distorting images or amplifying light, allowing our field of view to extend that of our telescopes and providing crucial information on matter distribution and the cosmic microwave background (CMB), amongst others.
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