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Gravitational waves

What is a gravitational wave?

   Updated March 23, 2014

To understand gravitational waves must glimpse the structure of space-time defined by Albert Einstein in his theory of general relativity (1916). Einstein has linked three dimensions of space and one dimension of time, in a same tissue of space-time. This four-dimensional tissue "looks" on a trampoline area distended by the mass of planets, stars and galaxies. This distortion, compression or curvature of space-time in three dimensions is that we feel like gravity. In other words, a planet like the Earth is in orbit simply because it follows the curves of hummocky spatial tissue and distorted by the presence of the Sun and other planets of solar system.
Thus gravitational waves (OG) are deformations of the structure of space-time that propagate as waves at the speed of light. They reflect the dynamics of space-time under the effect of rapid movements of ordinary matter while electromagnetic waves (photons) are produced by the movement of electric charges.
In summary, gravitational waves or waves of curvature are produced by the acceleration of masses. Only the most relativistic objects in the universe, those who are extremely massive as black holes or neutron stars, can "shake" slightly space-time if they are accelerated. For example, two black holes 2 or neutron stars of the order of a few solar masses which rotate around each other, generate gravitational waves. But contrary to electromagnetic waves, gravitational waves interact very little with matter, they travel the cosmos without being absorbed, making them invisible in the electromagnetic pictures of our telescopes.

NB: electromagnetic waves (radio, IR, optical, UV, X and gamma) are perturbations in the electromagnetic field, which propagate in space-time, while the gravitational waves are waves of space-time itself.


In addition to detecting the variation (distance between peaks and troughs) is extremely low, the frequency to be detected, is a very low frequency. If we want to measure 10 000 km (radius of the wave), the variation of the gravitational wave generated by a black hole in our galaxy, the detectors must be able to observe a change in the wavelength of the size of an atom, that is 10-10 meter. Moreover, these variations are extremely rare in our Galaxy, we must substantially improve our current detectors and get those variations in other galaxies.
Although not detected, scientists know they exist in the Universe. In 1975, radio astronomers Russell Hulse and Joseph Hooton Alan Taylor (Nobel Prize in Physics 1993) discovered, PSR B1913 +16 in the constellation Aquila, a binary pulsar (two neutron stars) with exceptional orbital characteristics. Indeed the two star orbit around one another in 7.75 hours in an extremely small volume of the order of 1.1 (periastron) to 4.8 times (apoastre) the radius of the sun. The small acceleration of the orbital period of this massive system (2 objects rotate faster and faster) and the shortening of the radius of the orbit (loss of 3 mm per orbit) has demonstrated the existence of gravitational waves. According to the theory of general relativity the orbit of a binary system is slowly modified by the emission of gravitational waves.
Over thirty years Taylor and his colleagues made ​​measurements which correspond exactly to the theory. For several other binary pulsars have confirmed the results of Taylor. The measures do not capable of detecting the energy of gravitational waves but are indirect evidence of the effects of gravitational waves emitted by a system.


Video: representation of gravitational waves or bending waves generated by two black holes or two pulsars (neutron stars). A pulsar is a neutron star, extremely dense, density of an atomic nucleus, hence its name. Its giant magnetic field rotates about the axis of rotation to the rotation frequency of the star, some rotate in a millisecond, magnetic beam ejects particles which generate radio waves. They are cosmic beacons. When two black holes or two neutron stars rotate around each other, objects distort space-time and this deformation causes small gravitational waves like on video.

Detection supposed of a gravitational wave


A South Pole Telescope called BICEP-2 (Background Imaging of Cosmic Extragalactic Polarization 2) has enabled scientists to analyze the polarization of the light emitted by the early universe. This detection confirms Einstein's general relativity predicted the existence of gravitational waves, as a shiver of space-time caused by a large displacement of masses.
How gravitational waves can be detected when they do not interact with matter?
The telescope has detected a subtle property of the cosmic microwave background discovered in 1964, the famous primitive radiation Big Bang old 13,800,000,000 years. BICEP measured the polarization to large-scale microwave radiation. Only primordial gravitational waves can print such a model only if they were amplified by inflation.
What is inflation?
The distribution of matter in space is too uniform to be due solely Big bang. In the 1970s, cosmologists have imagined a sudden expansion of the universe, they called inflation. This inflation took place from the first second after Big Bang. Only inflation can amplify sufficiently primordial gravitational wave signal to make it detectable. Scientists with BICEP-2 specifically were looking to measure the polarization of the cosmic microwave background, i.e. the orientation of the electric field in the sky.


They were looking for a specific type of polarization called B-modes, a model of vortex in the direction to polarized light from the ancient universe.
In theory, this swirling pattern of polarization (see picture opposite) can only be created from gravitational waves. This is what BICEP-2 was found. " This is a very clean signature of these gravity waves," said Stanford physicist Kent Irwin.
"But because of the importance of these results, they should be viewed with skepticism... there is, in this condition, oddities in the results that may be disturbing... I'm looking forward to seeing these results confirmed or reversed by other experiences, " said David Spergel, a professor of astrophysics at Princeton University. Indeed, the measurement is so difficult to do, it could easily be contaminated. Collaboration with other space telescopes such as Planck, should publish the results soon on the polarization of the cosmic microwave background. Further experiments are working on similar goals, which may support or go against BICEP-2. The June 5, 2014, at the Congress of the American Astronomical Society, David Spergel announced that the polarization B-Mode detected by BICEP2 could instead be the result of light scattering in the dust between the stars of the Milky Way. If the primordial gravitational waves are so anxiously hoped, is that they can confirm that inflation has occurred.

 Ondes gravitationnelles crées par l'inflation cosmique

Image: Gravitational waves of cosmic inflation, interpreted in the radiation of the cosmic microwave background image collected by a telescope experience BICEP-2 (Background Imaging of Cosmic  Extragalactic Polarization) at the South Pole. Scientists estimate that the polarization or orientation wave, light is proof in the form of a signature called B-mode polarization or swirling pattern of polarization. This wave is represented on the image by small black lines guided by the whirlwind. The color indicates small temperature fluctuations in the cosmic microwave background that correspond to density fluctuations in the early universe.

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