The Universe’s Hum and an Opportunity to Explore

Why in News?

• Astronomers have heard the subtle murmur made by gravitational waves echoing throughout the universe for the first time.
• According to research findings, scientists have discovered a "background hum" permeating the universe at low frequencies, pointing towards the existence of gravitational waves.

 

Gravitational Waves

• These waves are created during events such as supermassive black hole mergers, or collisions between two black holes that are billion times bigger than our Sun.
• These collisions are so powerful that they create distortions in spacetime, known as gravitational waves.

 

Background of Detection of Gravitational Waves

• The first detection of gravitational waves was announced on February 11, 2016.But those waves were of high frequency.
• These had been predicted, almost exactly a century ago by Albert Einstein as a natural consequence of his theory of gravity; the Theory of General Relativity.
  • General Relativity implies that under certain circumstances, space itself would be stretched and compressed resulting in the production of gravitational waves much like throwing a stone in a placid pool of water.

 

LIGO and Detection of Gravitational Waves

• LIGO (Laser Interferometry Gravitational-wave Observatory) is based on the principle of interference.
• A laser beam is split into two, each of which is sent down a pair of arms, each several kilometres long which are oriented perpendicular to each other. The beams are reflected back and then made to interfere.
• If there has been no disturbance, the beams cancel each other out exactly.
• However, occasionally when a gravitational wave passes through the interferometer arms, it would stretch and compress the arms by an incredibly small amount — a million trillion times smaller than the proton.
• Gravitational waves were first detected by LIGO, its detectors have observed dozens of short high-frequency gravitational wave bursts. So have detectors in Italy and Japan.

 

Limitations of LIGO

• The detections at LIGO and other detectors have all been of high-frequency waves, typically a few kilohertz.
• The sensitivity is determined by the length of the arms of the detector; the lower the frequency of the waves, the longer the arms that are required to detect them. 

 

The Concept of LISA (Laser Interferometer Space Antenna)

• Limitation of LIGO is one of the motivations for LISA, the planned space-based detector by the European Space Agency.
• This detector would have arms which would be several million kilometres long. 
• For the nanohertz (a billionth of a hertz) waves which have now been reported, one would have needed a galaxy-sized detector — clearly not something that can be built.
• The gravitational wave spectrum covers a broad span of frequencies. LISA operates in the low frequency range, between 0.1 mHz and 1 Hz (compared to LIGO's frequency of 10 Hz to 1000 Hz).
• Because of this, a wider variety of gravitational wave sources are now within the detection range of LISA because the waves it seeks have a much longer wavelength.
• The basic idea is to use radio pulses from objects called millisecond pulsars to detect the elusive waves.
• Millisecond pulsars are rapidly spinning neutron stars which beam radio waves in regular pulses. These pulses arrive on the Earth with extremely regularity.
• If an ultra-low frequency gravitational wave distorts the intervening space between a pulsar and us, it can change the arrival time of these pulses.

 

 Origin and Detection of Nanohertz Waves

• The origin of nanohertz waves is not yet determined, though the most likely scenario is of supermassive black holes orbiting each other.
• Blackhole each with a mass millions of times of our Sun, are typically found at the centre of galaxies.
• When galaxies collide or merge, these could pair off and produce the waves which are detected.
• The combined effect of mergers and collisions produce the constant background of these disturbances of space-time.

 

The Process of Data Collection by LISA

• As of now the concept of LISA is theory. What is required is humongous and tiring data collection of many millisecond pulsars over several years.
• For this, five multinational teams have been collecting data for over two decades on pulsar timings.
• These are the North American Nanohertz Observatory for Gravitational Waves, the European Pulsar Timing Array (PTA), Indo-Japanese PTA, Parkes PTA from Australia, and the Chinese PTA.
• Each of these groups is a collaboration of several scientists from many institutions.

 

India-Japan Cooperation on the recent Detection of “Low-Frequency Gravitational Waves”:

• The Indo-Japan PTA has researchers from the National Centre for Radio Astrophysics (NCRA), Raman Research Institute and several other institutes.
• The upgraded Giant Meter wave Radio Telescope (µGMRT) at Narayangaon near Pune was used to collect data.
• This array of 30 radio antennae, each with a diameter of 45 metres, separated by a maximum distance of 25 kilometres, is amongst the most advanced radio telescopes in the world for low-frequency observations.
• Data from a particular class of millisecond pulsars were collected for over a decade to detect the effect of gravitational waves.

 

Why Detection of Low-Frequency Gravitational Waves is a Significant Achievement?

• Detection and study of gravitational waves has opened a new window into our universe at the largest scales.
• Now astronomers, part of different collaborative initiatives, have announced the detection of ultra-low frequency gravitational waves which could expand this window to explore unexplored regions of the universe.
• The detection is an accomplishment since even without the gravitational waves, there are several factors which might cause the pulsar timings to vary.
• All of these need to be individually accounted for and compensated for.
• The only way that such a tiny signal can be gleaned from the background is by doing a statistical analysis of dozens of pulsars over many years.
  • This is one of the reasons why none of the collaborations is yet claiming a fool-proof discovery — the five-sigma level in scientific jargon, where the chance of it being a random event is one part in 3.5 million.
• Nonetheless, no one doubts that as more data is collected and analysed, the gold standard would be achieved.

 

Conclusion

• Scientists have been looking for low-frequency gravitational waves for decades.
• Whatever the ultimate origin of what is being called the “hum” of the universe by the press, it is clear that these nanohertz waves would, in future, allow us to explore the earliest universe.

 

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Q1) What is spacetime?

In his Special Theory of Relativity, Einstein proposed that 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, known as spacetime.

 

Q2) When does blackhole merger occur?

A black-hole merger occurs when two black holes start to spiral towards each other, radiating energy as gravitational waves. These waves should have a characteristic sound called a chirp, which can be used to measure the masses of the two objects.

 

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Source: The Indian Express