Electric charges oscillating up & down produce electromagnetic radiation. In a similar way, general theory of relativity predicts that oscillating massive objects should produce gravitational radiation, or gravitational waves.
Gravitational waves are ripples in overall geometry of space and time produced by moving masses. (similar to ripples in water pond)
Gravitational radiation is exceedingly difficult to detect because gravity by nature is much weaker than electromagnetic radiation. (1050 times weaker)
How are Gravitational-Wave produced?
Cosmic events that produce such ripples are exploding stars, collisions between ultra-dense neutron stars or merging black holes or supernovae.
While Einstein predicted the presence of gravitational radiation it was only in 2016, that for the 1st time gravitational waves were detected.
These radiation came from merger of two neutron stars.
Gravitational Wave Astronomy
Significance
Discovery of gravitational waves is beginning of new era in astronomy. So far, all observations of universe are made through electromagnetic radiation emitted from objects from visible light to ‘gamma rays.
Gravitational waves are a new way of “seeing” what happens in space.
We can now detect events that would otherwise leave little to no observable light, like black hole collisions.
Gravitational Wave Observatories
LIGO
Laser Interferometer Gravitational-wave Observatory (LIGO) is designed to detect gravitational waves.
LIGO detectors use laser interferometry to measure minute ripples in space-time caused by gravitational wave.
LIGO has detected the gravitational twice now, the second time witnessing the merging of a second black hole pair.
INDIGO and LIGO-INDIA
IndIGO (Indian Initiative in Gravitational-wave Observations) is a consortium of Indian physicists to set up experimental gravitational-wave observatory facilities in India.
LIGO, India is a planned gravitational-wave observatory as part of worldwide network to be established at Hingoli District, Maharashtra.
LISA Pathfinder
LISA stands for Laser Interferometer Space Antenna.
It is project led by European Space Agency to a build a space-based observatory for detecting gravitational waves.
It will consist of three spacecrafts separated by 2.5 million km in an equilateral triangular formation.
It is expected to be launched in 2037.
Gravitational Waves from Cosmic Inflation Phase (Isotropic)
We have seen how according to General theory of relativity whenever massive objects move or change shape, they create ripples in the fabric of space-time called gravitational waves.
Only that these gravitational waves from objects of low mass is very feeble to detect.
While we have found gravitational waves from neutron mergers and blackhole mergers, there is another source of gravitational waves. This is from the cosmic inflation phase you have read above.
During cosmic inflation we saw how the universe expanded by 1050 times in a span of 10-43-10-35 seconds. In other words, the universe expanded incredibly fast in a tiny fraction of a second to grow from a size much smaller than an atom to a size bigger than a galaxy.
Before this period everything in the universe was confined in a very small space called singularity. (refer to the story of big bang)
When such rapid expansion happened at such a small fraction of time, all the matter in universe was shaking.
Since all the matter that ever was shaking it should have sent ripples in spacetime which expanded 1050 times.
And because universe is ever expanding since then, these ripples should be present everywhere in the universe. (just like cosmic microwave background radiation)
We still haven’t detected these. The moment we would do it, a Nobel prize will be due. (and a question in your prelims paper)
These gravitational waves are called isotropic because they engulf the entire universe and will look the same whatever direction you will look.
On the other hand, gravitational waves from neutron stars or blackhole merger are called anisotropic as they have a specific source and do not engulf the universe.
