Gravitational Waves, Doppler-Shift, Cosmic Microwave Background

Einstein’s Theory of General Relativity

• In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers and that the speed of light in a vacuum was independent of the motion of all observers. As a result, he found that space and time were interwoven into a single continuum known as spacetime.
• Events that occur at the same time for one observer could occur at different times for another. This was the theory of special relativity. In 1915, Einstein published his theory of general relativity. In it, he determined that massive objects distort spacetime, which is felt as gravity. Gravitational lensing and gravitational waves are strong evidence for Einstein’s theory of general relativity.

Gravitational Lensing

• Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lie behind it.

• The Hubble constant is a unit of measurement that describes the rate at which the universe is expanding. Two parameters that are essential to estimating the Hubble constant are the distance of the stars from Earth and how fast they are moving away from us (their velocity). But to date, the most precise efforts have landed on very different values of the Hubble constant.
• Scientists have proposed a more accurate and independent way to measure the Hubble constant, using gravitational waves. A flash of light would give an estimate of the system’s velocity (system: neutron stars or black holes orbiting each other), or how fast it is moving away from the Earth. The emitted gravitational waves, if detected on Earth, should provide a precise measurement of the system’s distance. By knowing the system’s velocity and distance, a precise calculation of the Hubble constant is possible (which will describe the rate at which the universe is expanding).

[UPSC Prelims 2012] Which of the following is/are cited by the scientists as evidence(s) for the continued expansion of the universe?

1. Detection of microwaves in space
2. Observation of redshift phenomenon in space
3. Movement of asteroids in space
4. Occurrence of supernova explosions in space

Select the correct answer using the codes:

1. 1 and 2
2. 2 only
3. 1, 3 and 4 only
4. None of the above

Explanation:

• Supernova explosions can also cause gravitational waves. So, points 1, 2 and 4 are correct. So, the answer is d) None of the above.

[UPSC Prelims 2019] Recently, scientists observed the merger of giant ‘blackholes’ billions of light-years away from the earth. What is the significance of this observation?

1. ‘Higgs boson particles’ were detected.
2. ‘Gravitational waves’ were detected.
3. Possibility of intergalactic space travel through ‘wormhole’ was confirmed.
4. It enabled the scientists to understand ‘singularity’.

Explanation:

• Merger of giant blackholes causes gravitational waves. Answer: b) ‘Gravitational waves’ were detected.

• A wormhole can act as a bridge or a shortcut between two points in curved spacetime which are well separated in practical terms to the inhabitants of the universe. The existence of wormholes has been predicted by Ludwig Flamm, in 1916, soon after Einstein proposed his General Theory of Relativity.

• A singularity (gravitational singularity or (spacetime singularity) is a condition in which gravity is so intense that spacetime ceases to exist and our laws of physics become invalid. Singularities were first predicated as a result of Einstein’s Theory of General Relativity, which resulted in the theoretical existence of black holes.
• In essence, the theory also predicted that any star reaching beyond a certain point in its mass (aka. the Schwarzschild Radius) would exert a gravitational force so intense that it would collapse. At this point, nothing would be capable of escaping its surface, including light. This phenomenon is known as the Chandrasekhar Limit, named after the Indian astrophysicist Subrahmanyan Chandrasekhar, who proposed it in 1930.