Gravitational Waves, Doppler-Shift, Cosmic Microwave Background

Table of contents

Evidence for Big Bang Theory: Physical phenomena such as cosmological redshift, and the discovery of cosmic microwave background radiation and gravitational waves, have added weight to the Big Bang Theory.

Doppler-Shift or Redshift and Blueshift

Redshift and Blueshift describe how light changes as objects in space (such as stars or galaxies) move closer or farther away from us. American astronomer Edwin Hubble was the first to describe the redshift phenomenon (galactic redshift) and tie it to an expanding universe (galaxies are drifting apart).

Hubble’s law: the farther away galaxies are, the faster they are moving away from Earth ― also known as accelerating the expansion of the universe.

Cosmic Microwave Background (CMD)

With a traditional optical telescope, the space between stars and galaxies is completely dark. However, a sensitive radio telescope shows a faint background glow. This glow is strongest in the microwave region of the radio spectrum, and hence it is called a Cosmic Microwave Background (CMB).
CMD, also known as relic radiation (thermal radiation left over from the “Big Bang”), is fundamental to observational cosmology because it is the oldest light in the Universe and can be found in all directions. Its discovery is considered a landmark proof for the concept of “accelerating expansion of the universe” and the Big Bang Theory.

Gravitational Waves

Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Gravitational waves are ‘ripples’ in the fabric of spacetime caused by some of the most violent and energetic processes in the Universe. (Spacetime: https://www.youtube.com/watch?v=sryrZwYguRQ)
Massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt spacetime in such a way that ‘waves’ of distorted space would radiate from the source (like the movement of waves away from a stone thrown into a pond). These ripples travel at the speed of light through the Universe, carrying with them information about their origins.

Spacetime (NASA) & Gravitational Waves (NASA)

While the processes that generate gravitational waves can be extremely violent and destructive, by the time the waves reach Earth, they are billions of times smaller. In 2015, The Laser Interferometer Gravitational-Wave Observatory (LIGO), USA, physically sensed the distortions in spacetime caused by passing gravitational waves generated by two colliding black holes nearly 1.3 billion light-years away!

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.

Importance of Gravitational Waves

The gravitational waves can work as sirens to measure the expansion rate of the universe and to understand the origin and the future of the universe.

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?

Detection of microwaves in space
Observation of redshift phenomenon in space
Movement of asteroids in space
Occurrence of supernova explosions in space

Select the correct answer using the codes:

1 and 2
2 only
1, 3 and 4 only
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?

‘Higgs boson particles’ were detected.
‘Gravitational waves’ were detected.
Possibility of intergalactic space travel through ‘wormhole’ was confirmed.
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.