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Gravitational Waves

  Gravitational Waves 

In physics, in terms of a metric theory of gravitation, a gravitational wave is a fluctuation in the curvature of space-time which propagates as a wave, traveling outward from a moving object or system of objects.

Gravitational radiation is the energy transported by these waves.

Important examples of systems which emit gravitational waves are binary star systems, where the two stars in the binary are white dwarfs, neutron stars, or black holes.

Although gravitational radiation has not yet been directly detected, it has been indirectly shown to exist.

This was the basis for the 1993 Nobel Prize in Physics, awarded for measurements of the Hulse-Taylor binary system.


The sighting came from a small telescope on the roof of a laboratory sat on the ice sheet three quarters of a mile from the geographic South Pole. First came the rumours. But then researchers at the Harvard-Smithsonian Centre for Astrophysics went public. Their telescope had spotted indirect evidence of gravitational waves, or ripples in space-time, from the earliest moments of the universe.

The scientists have not yet published their work, and no other team has confirmed the finding. Yet even without these mainstays of scientific rigour, excitement has swept through the community and into the world beyond. If confirmed, the observation will rank among the greatest scientific discoveries of the past 20 years. A Nobel prize is all but guaranteed.

Albert Einstein predicted gravitational waves in his 1916 theory of general relativity. His equations married space and time, and showed how the product, space-time, was warped by matter and energy. The warping of space-time gives rise to the force of gravity.

Gravitational waves are tremors in space-time caused by intense gravitational forces. The Harvard team found evidence for primordial gravitational waves – those set in motion during the first trillionth of a second of the universe.

Primordial gravitational waves are seen as the smoking gun for a theory called cosmic inflation. Conceived in its original form more than 30 years ago by Alan Guth at MIT, inflation says that the early universe experienced a terrific burst of expansion. The growth spurt lasted a mere fraction of second, but smoothed out irregularities in space, and made the cosmos look almost the same in every direction.

The violent expansion had another effect too. It amplified primordial gravitational waves, making them large enough for researchers to detect. Without inflation, the effects of these ripples in space-time would be too minuscule for today's technology to spot.

Telescopes cannot see gravity, but they can see the effects of gravity. What the Harvard team spotted was the telltale signature that primordial gravitational waves imprinted on the faint light left over from the big bang. This ancient afterglow fills the universe, and is known as the cosmic microwave background.

Because gravitational waves squeeze space as they propagate, they make some patches slightly warmer than others. These warm spots polarise light waves that pass through, meaning the light waves vibrate in one direction more than others. In this case, the vibrations of light waves from the big bang are twisted, producing the distinctive pattern detected by Harvard's Background Imaging of Cosmic Extragalactic Polarisation telescope (Bicep2).

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