A gravitational lens is a distribution of matter (for example, a cluster of galaxies) between a distant light source, which is able, by bending the radiance from a satellite, to pass towards the contemplator and the observer. This effect is known as gravitational lensing, and the amount of flexion is one of Albert Einstein's predictions in the general theory of relativity. Classical physics also talks about the bending of light, but this is only half of what GR says.
Creator
Although Einstein made unpublished calculations on this subject in 1912, Orest Holson (1924) and Frantisek Link (1936), as a rule, believe that they were the first to voice the effect of a gravitational lens. However, he is still more often associated with Einstein, who published an article in 1936.
Theory confirmation
Fritz Zwicki in 1937 suggested that this effect could allow galaxy clusters to act as a gravitational lens. Only in 1979 this phenomenon was confirmed by observation of the quasar Twin QSO SBS 0957 + 561.
Description
Unlike optical, a gravitational lens produces the maximum deflection of light, which passes closest to its center. And the minimum of the one that extends further. Therefore, the gravitational lens does not have a single focal point, but has a line. This term in the context of light deflection was first used by O.J. Lodge. He noted that "it is unacceptable to say that the gravitational lens of the sun acts in this way, since the star has no focal length."
If the source, the massive object, and the observer are in a straight line, the source light will look like a ring around matter. If there is any displacement, only a segment can be seen instead. This gravitational lens was first mentioned in 1924 in St. Petersburg by the physicist Orest Khvolson and quantitatively worked out by Albert Einstein in 1936. As a rule, it is referred to in the literature as Albert rings, since the former did not deal with the flow or radius of the image.
Most often, when the mass of the lens is complex (for example, a group of galaxies or a cluster) and does not cause a spherical distortion of space-time, the source will resemble partial arcs scattered around the lens. Then the observer can see several altered images of the same object. Their number and shape depend on the relative position, as well as on the simulation of gravitational lenses.
Three classes
1. Strong lensing.
Where there are easily visible distortions, such as the formation of Einstein rings, arcs and multiple images.
2. Weak lensing.
Where the change in background sources is much smaller and can only be detected by statistical analysis of a large number of objects to find coherent data of only a few percent. The lens shows statistically how the preferred stretching of the background materials is perpendicular to the direction toward the center. When measuring the shape and orientation of a large number of distant galaxies, their locations can be averaged to measure the shift of the lens field in any region. This, in turn, can be used to restore the distribution of mass: in particular, the background separation of dark matter can be reconstructed. Since galaxies are elliptical in nature and a weak gravitational lens signal is small, a very large number of galaxies must be used in these studies. The data of the study of weak lenses should carefully avoid a number of important sources of systematic error: the internal shape, the tendency of the scattering function of the camera point to distort, and also the possibility of atmospheric vision to change images.
The results of these studies are important for evaluating gravitational lenses in space in order to better understand and improve the Lambda-CDM model and to ensure consistency of other observations. They can also provide an important future limitation of dark energy.
3. Microlensing.
Where no form distortion is visible, but the amount of light received from the background object changes over time. The subject of lensing can be stars in the Milky Way, and the source of the background is balls in a distant galaxy or, in another case, an even more sent quasar. The effect is small, so even a galaxy with a mass exceeding the mass of the Sun by 100 billion times will create several images separated by just a couple of arcseconds. Galactic clusters can produce spacing for minutes. In both cases, the sources are quite far, many hundreds of megaparsec from our universe.
Time delays
Gravity lenses act equally on all types of electromagnetic radiation, and not just on visible light. Weak effects are studied both for the cosmic microwave background and for galactic research. Strong lenses were also observed in radio and x-ray modes. If such an object creates several images, there will be a relative time delay between the two paths. That is, the description will be observed on one lens earlier than on the other.
Three types of objects
1. Stars, remnants, brown dwarfs and planets.
When an object in the Milky Way passes between the Earth and a distant body, it will focus and amplify the background light. Several events of this type have been observed in the Large Magellanic Cloud, a small Universe near the Milky Way.
2. The galaxies.
Massive planets can also act as gravitational lenses. Light from a source behind the universe bends and focuses to create images.
3. Clusters of galaxies.
A massive object can create images of a distant object lying behind it, usually in the form of stretched arcs - a sector of the Einstein ring. Cluster gravitational lenses allow you to observe luminaries that are too far away or too weak to be seen. And since looking at long distances means looking into the past, humanity gains access to information about the early Universe.
Solar gravitational lens
Albert Einstein predicted in 1936 that light rays in the same direction as the edges of the main star would converge to a focus at about 542 AU. Thus, a probe located at such a distance (or more) from the Sun can use it as a gravitational lens to magnify distant objects on the opposite side. The location of the probe can be shifted as needed to select different targets.
Drake's probe
This distance is far beyond the progress and capabilities of the equipment of space probes, such as Voyager 1, and beyond the borders of famous planets, although for thousands of years Sedna will move further in its highly elliptical orbit. The high gain for potential detection of signals through this lens, such as microwaves on the 21 cm hydrogen line, led Frank Drake to speculate in the early days of SETI that the probe could be sent this distance. Multipurpose SETISAIL and then FOCAL were proposed by ESA in 1993.
But, as expected, this is a difficult task. If the probe passes 542 AU, the possibilities of increasing the lens will continue to operate at farther distances, since the rays that fall into focus at large pass further from the distortions of the solar corona. Criticism of this concept was given by Landis, who discussed issues such as interference, a large increase in the target, which would complicate the design of the focal plane of the mission, and the analysis of the spherical aberration of the lens.