Gravitational Waves: Why is the Discovery of Gravitational Waves Important?

Universe’s ‘Noisy’ Gravitational Wave Background

Context (IE): Scientists have found evidence that low-frequency gravitational waves background exists. These waves create a cosmic background hum that permeates through outer space.

Gravitational Waves

Albert Einstein predicted the existence of gravitational waves in his General Theory of Relativity.
Gravitational waves are ‘ripples’ in the fabric of spacetime caused by some of the Universe‘s most violent and energetic processes.

Space-time continuum: Space and time don’t exist as independent entities. Combining the three dimensions (height, width, and depth) of space and one dimension of time into a single four-dimensional continuum is known as spacetime. (Spacetime: https://www.youtube.com/watch?v=sryrZwYguRQ)

What causes Gravitational Waves?

Gravitational waves are created when massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt spacetime so that ‘waves’ of distorted space would radiate from the source.

Some examples of events that could cause a gravitational wave are:
- when a star explodes asymmetrically (called a supernova)
- when two big neutron stars orbit each other
- when two black holes orbit each other and merge
- These ripples travel at the speed of light through the Universe, carrying information about their origins.

Spacetime (NASA) & Gravitational Waves (NASA)

When were Gravitational Waves First Detected?

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!
But those waves were high-frequency gravitational waves, believed to be produced by the merger of two small black holes about 1.3 billion years ago.

How LIGO Detects Gravitational Waves?

While the processes that generate gravitational waves can be extremely violent and destructive, they are billions of times smaller when they reach Earth.
When a gravitational wave passes by Earth, it squeezes and stretches space by a minuscule amount.
LIGO can detect this squeezing and stretching. Each LIGO observatory has two “arms”. A passing gravitational wave causes the length of the arms to change slightly.

How were Low-Frequency Gravitational Waves Detected?

The low-frequency gravitational waves were detected by studying pulsars using radio telescopes worldwide. Indian Pulsar Timing Array (InPTA) was also involved in this.
Pulsars are distant rapidly-rotating neutron stars that emit pulses of radiation, observed from the Earth as bright flashes of light.
These bursts take place at extremely precise intervals, making them useful as cosmic clocks.
It was noted that some of the signals from these neutron stars arrived a little early while a few others were late. These inconsistencies were due to deformities caused in spacetime by gravitational waves.

Neutron stars: They are remnants of stars after a supernova explosion (when a massive star runs out of fuel and collapses). The star’s core collapses under gravity during the explosion, causing protons and electrons to combine and form neutrons.
Neutron stars are the densest objects observed in the universe.

Why is the Discovery of Gravitational Waves Important?

Like the spectrum of electromagnetic waves, from microwaves to radio waves, there can be a wide range of gravitational waves of different wavelengths, frequencies, and energies.
- Faraway cosmic objects which cannot be detected visually (like black holes) can be detected and studied with the help of gravitational waves.
- The gravitational waves can work as sirens to measure the universe’s expansion rate and understand the universe’s origin and future.

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.

ESA - Gravitational lensing

How exactly do gravitational waves help measure the universe’s expansion rate?

The Hubble Constant is a unit of measurement that describes the universe’s expansion rate.
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).
To date, the most precise efforts have landed on very different values of the Hubble constant.
Scientists have proposed a more accurate 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 precisely measure 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.

Wormhole

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.