Gravitational lens

As gravitational lensing, the deflection of light is denoted by heavy masses in astronomy. The name comes from the analogy to optical lenses and the acting force of gravity ago.

Light rays are deflected by a gravitational lens, the more distracted to ground, the closer they pass the deflecting mass. A gravitational lens focuses the light that passes the deflecting mass on the axis between the object and the observer. In various distances on the object passing light rays intersect the axis but at different distances. Consequently, a gravitational lens in the sense of the imaging optics produce no real image. The light distribution instead generated is a caustic.

Basically, the light from a distant source such as a star, a galaxy or other astronomical object is influenced by a lying before seen by the viewer object, the gravitational lens. In the gravitational field changes the direction of light propagation, so that the position of the source appears to be shifted in the sky. Her image can amplified, distorted, or even duplicated. After the odd -number theorem occurs always an odd number of images.

Depending on the mass and shape ( mass distribution) of the objects involved and their positions to each other can turn the effect varies widely, from spectacular strained multiple images to only slight changes in brightness, so that one speaks of the strong gravitational lensing, the weak gravitational lensing and from microlensing. A special case of gravitational lensing is the cosmic shear.

After it had been given in 1800 reflect on the gravitational deflection of light for the first time 1915/16 of Albert Einstein was correctly described with his general theory of relativity. After the first observations of the sun in 1919 and some theoretical work, however, reach only in 1979 due to improved observational techniques observations of other gravitational lenses. Since then, the gravitational lensing has developed into a diverse field of astronomy observer and also a tool for other fields such as cosmology.

History

The first combined experimental verification of the general theory of relativity (ART ), which caused a great sensation in public and made the general theory of relativity famous, in 1919 conducted and verified the prediction of general relativity that light, like any electromagnetic radiation is deflected in a gravitational field. In this case a solar eclipse was exploited to measure the apparent shift in the position of a star near the solar disk, since the effect should be strongest. The prediction of Einstein's theory that starlight grazing the edge of the sun on its way to Earth, should be deflected by 1.75 arc seconds, was confirmed in this original measurement with a deviation of 20%. The following from the Newtonian theory of gravitation deflection would be only 0.83 " was, as has been already calculated in March 1801 by Johann Georg von Soldner. Classical or calculated using the special theory of relativity, the effect would be half as large as only the time coordinate and not the spatial coordinate changes.

Similar measurements were carried out later with improved instruments. In the 1960s, the positions of quasars have been measured, so an accuracy of 1.5 % was achieved, while similar measurements with the VLBI ( Very Long Baseline Interferometry ) later, the accuracy increased to 0.2%. The positions of 100,000 stars were measured by the ESA Hipparcos satellite, which the predictions of ART could be verified accuracy of 0.1 %. The ESA's Gaia, which was launched on December 19, 2013, measure the position of over a billion stars and thus determine the curvature of space more precisely.

Phenomenology

Principle

Interstellar objects with a very large mass steer electromagnetic waves in a different direction. Accordingly, the image of the background object is displaced, distorted and possibly multiplied.

A special manifestation of the microlensing ( microlensing ). Here, the deflection is so small that it is not registered as a spatial shift, but makes itself as increase in brightness ( then drop ) noticeable.

The effect is based in each case on the described by Albert Einstein in his general theory of relativity as the effect of gravity on the spacetime curvature of space by massive objects containing or energy.

This effect can be detected in a total eclipse of stars that lie very close to the line of sight to the sun and are outshone by these otherwise: The position of these stars will appear slightly shifted away from the sun. The corresponding observation by Arthur Eddington in 1919 provided the first experimental confirmation of the theory of general relativity. Einstein thought it possible, but unlikely, that one can perceive multiple images of the same object under suitable conditions. However, he thought only of stars as a trigger of this effect; 1937 Fritz Zwicky examined the impact that can have a galaxy as a gravitational lens. 1963 saw Yu. G. Klimov, S. Love and Sjur Refsdal independent, then that quasars represent the ideal light sources for this effect.

To obtain a gravitational lens in the usual, so astronomical sense, usually the extremely intense gravitational fields of astronomical objects such as black holes, galaxies or clusters of galaxies are needed. With these it is possible that a light source located behind the lens, not only gravity will shift, but that the observer sees multiple images. The first such " strong gravitational lens " was discovered in 1979: the "Twin Quasar " Q0957 561. A well-known example is the 1985 Einstein discovered the cross in the constellation Pegasus, a four-fold image of the same object. In extreme cases (ie, an infinite number of pictures ) is observed even a line-shaped image of a point source of light, these are the so-called Einstein rings.

The first gravitational lens, which does not consist of a single galaxy, but a cluster of galaxies ( Abell 370), was recognized in 1987 independently by Genevieve Soucail, Yannick Mellier and others in Toulouse and Vahé Petrosian and Roger Lynds in the United States as such.

For weak distortions - due to a weak or distant gravitational field - the effects of gravitational lensing is not immediately apparent, since the actual shape of the objects is unknown behind the gravitational lens. In this case, the determination of the gravitational field is still possible by means of statistical methods, by shape and orientation of many existing in the background galaxies are examined. Here, it is assumed that the orientation of the galaxies in the background without a gravitational lens would be random. With gravitational lensing gives a shear of the background, so that galaxies appear more frequently aligned along a ring around regions with strong gravitational field. From this, the mass distribution can be determined, which causes the lens effect.

Since this effect is small, a large number of galaxies must be studied for a sufficient statistical significance. Furthermore, to take into account a number of possible systematic errors. These include the intrinsic shape of galaxies, the point spread function of the camera used, aberrations of the telescope and, possibly, the air turbulence of the atmosphere, which can also lead to a distortion of the image.

Microlensing

Unlike accepted by Einstein ( see above), can also the effects exerted by a single star on the radiation of a background object, can be observed. So you have a number of MACHOs detected because a single star bundled the light of underlying, much weaker object, and so (briefly) has increased. Also extrasolar planets could be detected with this effect.

The focal point of the lens effect of the Sun is at a distance of about 82.5 billion kilometers, or about 550 astronomical units and would provide a magnification by a factor of about 100 million.

Applications

If a gravitational lens focuses the light of the background object ( from the perspective of the terrestrial observer ), objects can be examined, which could otherwise not be registered because of their low apparent brightness. This makes it possible to observe galaxies at large distances and therefore in very early epochs of the cosmic evolution.

In addition, the distribution of the radiation in the image plane provides the opportunity to examine properties (mass and mass distribution) of the gravitational lens itself. The total mass is obtained directly, without having to resort to insinuations in the proportion of dark matter.

Statistical analysis of gravitational lens images can be used to limit parameters such as the cosmological constant or the matter density of the entire universe. The Hubble constant can be determined in more detail by means of gravitational lenses in circumstances such as Sjur Refsdal 1964 predicted. Researchers at the University of Zurich and the United States have thus determined the age of the universe with great accuracy to currently 13.5 billion years.

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