How much do you know about Gravitational Waves?

Key Points:

• Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light.

• Gravitational waves were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as waves similar to electromagnetic waves but the gravitational equivalent.

• Gravitational waves were later predicted in 1916 by Albert Einstein on the basis of his general theory of relativity as ripples in spacetime.

• The first indirect evidence for the existence of gravitational waves came in 1974 from the observed orbital decay of the Hulse–Taylor binary pulsar.

• Direct observation of gravitational waves was not made until 2015, when a signal generated by the merger of two black holes was received by the LIGO gravitational wave detectors.

• Gravitational waves can penetrate regions of space that electromagnetic waves cannot, allowing the observation of the merger of black holes and other exotic objects in the distant Universe.

• Gravitational wave astronomy gives new insights into the workings of the Universe and could offer a possible way of observing the very early Universe.

• Scientists continue to demonstrate the existence of gravitational waves with continuously upgraded, highly-sensitive detectors used in joint observation runs.

• The most sensitive detector accomplished the task possessing a sensitivity measurement of about one part in 5×10^22 (as of 2012) provided by the LIGO and VIRGO observatories.

• A space-based observatory, the Laser Interferometer Space Antenna, is currently under development by ESA.

• Another European ground-based detector, the Einstein Telescope, is also being developed.

• Gravitational waves have a minuscule effect on Earth and are observable only with the most sophisticated detectors.Gravitational Waves: Properties and Sources

• Gravitational waves are waves in the fabric of spacetime that carry energy, momentum, and angular momentum away from their source at the speed of light.

• Gravitational waves are produced by objects with a changing quadrupole moment, such as a spinning dumbbell or two orbiting masses, and are polarized with a “plus” and “cross” polarization.

• The frequency, wavelength, and speed of a gravitational wave are related by the equation c = λf, where c is the speed of light. The wavelength of a gravitational wave can be up to 47 times the diameter of the Earth.

• Compact binaries such as neutron stars and white dwarfs emit gravitational waves as they spiral towards each other due to the loss of energy through gravitational radiation. Black hole binaries also emit gravitational waves during their in-spiral, merger, and ring-down phases.

• Supernovae explosions, spinning neutron stars with deformities, and the inflationary epoch in the early Universe are also sources of gravitational waves.

• Gravitational waves can carry off linear momentum and produce a “kick” that can eject a coalesced black hole from its host galaxy or carry a star cluster or gas with it.

• Gravitational waves exhibit redshifting due to the Doppler effect and distortions of spacetime, such as cosmic expansion.

• The first direct detection of gravitational waves, GW150914, came from the merger of two black holes, and the first binary neutron star inspiral was observed in GW170817.

• LIGO and Virgo are currently operational gravitational wave detectors, and more detectors are planned or under construction around the world.

• Gravitational waves provide a new way to study the Universe and its objects, such as black holes, neutron stars, and cosmology.

• Gravitational waves also confirm Einstein's theory of general relativity and open up new areas of research in physics and astronomy.

• The expected rate of gravitational wave detections is estimated to be around 40 events per year within a range of 200 megaparsecs.Gravitational waves are distinct from gravitational redshift, as they are ripples in the fabric of space-time caused by the acceleration of massive objects. The graviton, a hypothetical elementary particle, is speculated to be the force carrier that mediates gravity, but it is yet to be proven to exist. Gravitational waves have the potential to provide a wealth of observational data about the very early universe, as they are unaffected by the opacity of the early universe. Multiple detectors are used to distinguish signals from other "noise" and to determine the direction of the source. Gravitational wave astronomy has the potential to revolutionize the field, as gravitational waves can carry information about astronomical phenomena that have never been observed by humans. Ground-based detectors, such as Weber bars and interferometers, are used to detect gravitational waves, and the Laser Interferometer Gravitational Wave Observatory (LIGO) is currently the most sensitive detector. The amplitude of waves given off by the Hulse–Taylor binary at Earth would be roughly h ≈ 10−26, while astrophysicists expect that some gravitational waves passing the Earth may be as large as h ≈ 10−20.Gravitational Waves - Summary

• Interferometric detectors increase the rate of detectable signals from one every tens of years to tens per year.

• Shot noise limits interferometric detectors at high frequencies.

• Seismic noise and other forms of environmental vibration limit ground-based detectors.

• The Einstein@Home project uses distributed computing to detect simple gravitational waves.

• Space-based interferometers, such as LISA and DECIGO, are being developed for better detection.

• Pulsar timing arrays use millisecond pulsars to detect perturbations due to gravitational waves.

• The North American Nanohertz Observatory for Gravitational Waves, the Australian Parkes Pulsar Timing Array, and the European Pulsar Timing Array are active pulsar timing array projects.

• Primordial gravitational waves are gravitational waves observed in the cosmic microwave background.

• LIGO and Virgo have reported multiple gravitational wave observations.

• The detection of the first two neutron star-black hole binaries by the LIGO and VIRGO detectors was published in the Astrophysical Journal Letters.

• The possibility of GASER (a kind of laser emitting gravitational waves) was proposed by Giorgio Fontana.

• Gravitational waves have been featured in science-fiction novels.