Cosmic Mirages: Unveiling the Mysteries of Gravitational Lensing
By Bhoomi Raiyani
Cosmofluencer (Season 04)
What if someone tells you, you can watch the invisible? Or that you can bend light? Of Course, you will find it impossible to do. However, what if I tell you that the beauty of physics provides that superpower to make it possible? Gravitational lensing is a very eccentric phenomenon that can do the impossible!
Lens
A lens is basically a piece of transparent material with a curved side used for concentrating or dispersing light rays.
Gravity
It is a fundamental interaction primarily observed as mutual attraction between all things that have mass according to Sir Issac Newton.
However, Einstein, in his Theory of General Relativity, a theory that is known as Geometric theory of gravity, gave a new face to it. According to him gravity arises from the fact that mass causes the curvature (deformation) in the space-time fabric. The greater the mass, the more the curvature.
This curvature affects the other matter passing over it. For instance, curvature in space-time fabric due to the mass of the Sun affects the orbit of Earth around it, making it elliptical.
The amazing thing is that it does not only affect matter but even light! When light passes through massive objects like clusters of galaxies, its path is changed slightly, giving rise to a phenomenon called Gravitational Lensing.
What is Gravitational Lensing?
Gravitational lensing is when light is deflected by objects with very strong gravity. This can cause the distant object to appear as multiple images, or to be stretched or magnified. If a curved piece of glass can become a lens when light passes through it then why can’t the curved space?
Einstein first predicted gravitational lensing in 1912, three years before he published his work on the Theory of Relativity. It occurred to him that if gravity bends the space, it would also bend the light travelling through that space.
When does it occur?
To observe gravitational lensing, you require three things: A source, an observer and a lensing medium between them.
The source can be some distant galaxy, quasars, black holes, or even regions of dark matter. Observer is obviously us but through a telescope and celestial bodies like clusters of galaxies or stars that are massive enough to act as a lens between us and the light source. When these three align perfectly and the source and lens are spherically symmetric, we will see the beautiful phenomenon of gravitational lensing.
A diagram showing how space time curvature leads to gravitational lensing.
(Image credit: NASA, ESA, CSA and STScI)
The Discovery of Gravitational Lensing
The first effective gravitational lensing was observed by Dennis Walsh, Robert F. Carswell, and Ray J. Weymann in 1979. The discovery was made using the Kitt Peak National Observatory’s 2.1 metre telescope. The gravitational lens was a quasar named Q0957+561, which appeared as two images separated by 6 arcseconds. It was a single quasar but appeared as two, which seems incredible but is not! Many times, when you look through the glass, you might see blurred double images of a single object. Here, between us and the quasar, there was an elliptical galaxy that served as a lens and created the double-vision effect.
(Image Credit: ESA, NASA/HUBBLE)
In 1985, another type of gravitational lensing was observed, the astronomers named it The Einstein Cross. The Einstein Cross (Q2237+030 or QSO 2237+0305) is a gravitational lensed quasar that appears as four images arranged perpendicular to each other, creating a cross. Here, the galaxy that created this distortion as a lens was named Huchra’s lens.
(Image Credit: ESA, NASA/HUBBLE)
In 1988, astronomers observed another case of gravitational lensing, known as the Einstein Ring. It is a beautiful effect in which you get this distorted blue galaxy in the halo (ring) form which is located behind a red galaxy.
(Image Credit: ESA, NASA/HUBBLE)
Types of Gravitational Lensing
There are three major types of gravitational lensing, according to the University of California.
1. Strong gravitational lensing: This is the most extreme type of gravitational lensing, which occurs when a massive galaxy or galaxy cluster is close to a distant galaxy. The lens magnifies the distant galaxy, making it visible. This type produces multiple images like those given in the above examples.
2. Weak gravitational lensing: Weak lensing does not produce multiple images of the same object. Instead, it produces a highly deformed or stretched image from an object far beyond the lens.
3. Microlensing: In cases of microlensing, there is no distortion in shape. However, the amount of light visible from an object changes periodically.
(Image Credit: ESA)
[First picture shows strong gravitational lensing, Second one portrays weak gravitational lensing and last one is microlensing.]
Applications of Gravitational Lensing
This extraordinary phenomenon has many useful applications too.
- Studying distant galaxies: As I mentioned before, what if we could see the invisible? Here, gravitational lensing provides us the why of studying the distant galaxy which otherwise would not have been possible.
- Mapping the distribution of galaxies: The understanding and interpretation of the gravitationally lensed images are enhanced using a matter map of the galaxy cluster. A history of matter distribution can identify multiple images of the galaxy or determine where missing galaxies will be seen in the image of a cluster.
- Detecting dark matter: Gravitational lensing can be used directly to detect dark matter. The distribution of lensed images reflects the distribution of all matter, both visible and dark. The results indicate that most of the matter in a galaxy cluster is not in the visible galaxies or hot gas around them and does not emit light, and is thus called dark matter.
Conclusion
Gravitational lensing revolutionised our understanding of the universe. The phenomenon exposed an incredible window into the cosmos, providing insights about dark matter, dark energy, galaxy evolution, and exoplanet detection. Future discoveries will continue to unravel, for technology will advance further to unlock mysteries hidden within gravitational lensing.
This bending light has revealed many mysteries and will continue to uncover them in the future.!
References
1. Different Types of Gravitational Lenses | ESA Hubble
2. Gravitational Lensing | NASA Hubblesite
3. Gravitational Lensing: What It Is And How It Is Helping Us Discover New Galaxies | Science ABC