Gravitational Lensing [UPSC Notes]

A team of scientists, including Indians, used the effect of gravitational lensing to map the dark matter in the universe. It also showed that it is distributed around galaxies, in multiple tomographic pieces of distances away, similar to a medical CT scan. Know more about this development in this article. This topic is relevant for the IAS exam science and technology segment.

What is Gravitational Lensing?

The phenomenon occurs when a huge amount of matter, such as a massive galaxy/cluster of galaxies or a black hole, generates a gravitational field which distorts and magnifies the light from objects behind it.

• It is based on Einstein’s theory of general relativity (Mass bend light).
• It helps astronomers understand black holes, dark matter, etc.
• Common camera lenses in a magnifying glass work by bending light rays that pass through them in a process known as refraction, in order to focus the light somewhere else.
• Currently, there are three gravitational lensing experiments underway by scientists worldwide; the deepest amongst which is the Hyper Suprime-Cam (HSC) Survey carried out by this team.
• The observations utilized one of the most powerful astronomical cameras in the world, the Hyper Suprime-Cam (HSC) present on top of the 8.2m diameter Subaru Telescope on Mauna Kea in Hawai’i, in the middle of the Pacific Ocean. 

Details of the experiment:

• Dark energy matter makes up 95 per cent of the universe.
• Still very little is known about what they actually are or their evolution. 
• A team of astrophysicists and cosmologists, including Dr Surhud More from the Inter-University Center for Astronomy and Astrophysics (IUCAA), was able to characterize the structure of dark matter.
• They used images taken from the Subaru telescope in Hawai’i.
• By this development, the team was able to show that the ‘clumpiness’ of matter is smaller than that expected from observations of the Cosmic Microwave Background (CMB) in the universe.

Know more about dark matter in the linked article.

The methods and findings:

• They measured a value for the ‘clumpiness’, known to scientists as S8, of 0.76, which is in line with values that other gravitational lensing surveys have found.
• However, it does not match with the value of 0.83 predicted by the Cosmic Microwave Background, when it was about 380,000 years old, which dates back to the origin of the universe.

Significance:

• The density of dark matter and the amount of ‘clumpiness’ are quite significant to fathom to know how structure in the universe evolves with time.
• By mapping out dark matter in a tomographic manner ( in 3D), we are now able to carry out a tough test of the cosmological model.

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