Hearing the Universe Hum

Why in NEWS

Astronomers from part of different collaborative initiatives have announced the detection of ultra-low frequency gravitational waves which could expand this window to explore hitherto unexplored regions of the universe.

Historical Background

• The first detection of gravitational waves was announced on February 11, 2016.
• 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.

What is 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.

What are Gravitational Waves?

• Gravitational waves were first theorized by Albert Einstein.
• They 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 space-time, known as gravitational waves.
• Detection and study of gravitational waves has since opened a new window into our universe at the largest scales.
• Since gravitational waves were first detected by LIGO(Laser Interferometry Gravitational-wave Observatory), its detectors have observed dozens of short high- frequency gravitational wave bursts.

What is LIGO?

• 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. This will result in the beams not cancelling each other.

• The signal is analysed to rule out other possibilities like seismic tremors, is then taken as a gravitational wave detection.
• 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.

What is LISA?

• LISA stands for Laser Interferometer Space Antenna.
• It is a space based gravitational wave observatory building on the success of LISA Pathfinder and LIGO.ser

Applying LIGO Principles into LISA

• LISA is the planned space-based detector by the European Space Agency.
• This detector would have arms which would be several million kilometres long.
• For the nano-hertz (a billionth of a hertz) waves which have now been reported, one would need a galaxy-sized detector – clearly not something that can be built.
• The scientists instead decided to use nature itself and used our Milky Way as the detector in an ingenious manner.
• The basic idea is to use radio pulses from objects called millisecond pulsars to detect the elusive waves.

What is Millisecond Pulsar?

• Millisecond pulsars are rapidly spinning neutron stars which beam radio waves in regular pulses.
• These pulses arrive on the Earth with extremely regularity.

How Millisecond pulsar helps in detecting gravitational waves?

• 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.
• Data from a particular class of millisecond pulsars were collected for over a decade to detect the effect of gravitational waves.

Future ahead

• The origin of these nanohertz waves is not yet determined, though the most likely scenario is of supermassive black holes orbiting each other.
• These gargantuan objects, 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.
• Although such mergers and collisions may seem unlikely, given the vast scale of the universe and the 13 billion years since galaxies first formed, there could be many such events.
• The combined effect of these mergers is to produce the constant background of these disturbances of space-time.
• Other possibilities are also being considered, including exotic objects like cosmic strings and even inflation – an event at the very beginning of the universe which resulted in an exponential increase in its size.