In astronomy, a gravitational lens is a distribution of matter, such as a galaxy or black hole, capable of curving the trajectory of transiting light in a manner analogous to an optical lens.
Gravitational lenses are predicted by the theory of general relativity, according to which the trajectory of electromagnetic radiation, such as light, is determined by the curvature of space-time produced by celestial bodies. The first experimental evidence of this effect were collected in 1919 by observing during a total eclipse the deflection of light rays of stars produced by the Sun, since then a large number of gravitational lenses has been discovered thanks to technological developments of astronomical instrumentation.
The effect of a gravitational lens is the apparent deformation of the image of celestial bodies whose emitted light is found to pass near the masses that produce the curvature of space-time. It may happen that the deflection of the rays makes their source appear displaced from its true position. For the same reasons, more or less sharp distortions of the source image can occur due to the effect of a gravitational lens, such as Einstein’s ring.
Gravitational lensing can also act on galactic scales or galaxy clusters, and gravitational lensing effects attributable to dark matter in the universe have also been detected.
Imagine that space is like an extended sheet that is tightly stretched at the ends. If we put a very heavy lead ball on the sheet, it tends to deform at the contact point. Similarly, in the Universe, space curves in the presence of very heavy objects. When we observe luminous objects in space that are far away from us, the images of these objects can be deflected and deformed if an object of very high mass, such as a galaxy or a cluster of galaxies, stands between them and us.
This effect, called “gravitational lensing”, occurs because the curvature of space due to the galaxy or cluster (the same that occurs at the lead ball) can cause the deviation of the trajectory of light. If we observe a light source and a massive object (MACHO, massive astrophysical compact halo object) comes between us and the source, the phenomenon we witness is called microlensing, because the mass of the MACHO is not large enough to create a gravitational lens.
The phenomenon is very similar to that of a gravitational lens, only that the various split images are not detectable because they are too close. It follows that not being able to observe several separate images, we will see them all together, with a consequent increase in brightness of the object we are observing. This increase in brightness is related to the mass of MACHO.
Theoretical origin and first observations
In 1913 Albert Einstein, in the content of a short page addressed to George Hale, hypothesized the possibility to prove the bending of electromagnetic radiation when crossing gravitational fields by examining the light coming from fixed stars apparently located near the Sun. But in that writing the quantity calculated for the angle of the light rays results of only 0.84″ (seconds of arc); subsequently this proposed measure was rectified, by the same author, with a quantity equivalent to a little more than its double: that is in 1.75″, value validated by the astronomical detection realized after about six years from the sending of the above mentioned letter. In it (traced by Einstein) is noted the stylized sketch where the line of a radiation, started in the left point of the diagram, bends near the edge of a circumference, the solar disk, and continues so just declined towards the opposite point.
The first observation of the deflection of light rays consisted in simply measuring the variation of the position of stars due to the curvature of space-time around the Sun. On May 29, 1919, during an eclipse of the Sun, the group led by Arthur Eddington and Frank Watson Dyson observed that the light of the stars near the solar disk was slightly deflected, as the stars appeared in positions shifted compared to the case in which they were usually observed, in accordance with Einstein’s theory. The eclipse therefore allowed to clearly observe the displacement of the stars, which otherwise would have been impossible to detect given the intense brightness of the Sun itself.
The result of the experiment was announced on November 6, 1919 in London before the Royal Society and the Royal Astronomical Society, gathered in the press conference for the exceptional event that sanctioned the superiority of general relativity theory over classical mechanics. The news was soon spread by newspapers around the world. In fact, although the corpuscular theory of light, together with Newton’s law of universal gravitation, predicted a deviation of light rays, this was only half of that predicted by Einstein and observed by Arthur Eddington and Frank Watson Dyson.
Specific uses of gravitational lenses
This phenomenon is used to study the most remote parts of the universe or to detect less obvious astronomical bodies (gravitational optics) such as the properties of more distant stars and quasars or other smaller objects or those occulted by gaseous cosmic matter.
By observing this phenomenon, the distance of an object can be more accurately measured. Knowing that the double images that reach us travel along different lengths, it is possible, by evaluating the delay with which their possible variation of brightness appears, to obtain an estimate of the distance of the emitting object (if it has a temporal and constant intrinsic variability depending on its nature and physical dynamics, already known by the observer in similar objects).
This method was largely applied to improve and refine the factor calculated for the Hubble constant and therefore the rate of global expansion of the universe, as well as to understand the overall distribution of energy density (radiant, kinetic, mass of solid bodies) in cosmic regions close to our event horizon. If the observed radiative variations occur with the specific modalities of the models related to reference samples (Cepheid variables, class Ia supernovae, galactic rotations by Tully-Fisher relation, super red giant stars), knowing the shape and concentration that causes the curvature of the gravitational field, it is possible to deduce empirically (by spectroscopy) the original property of the emitted radiations, therefore identifying also the speed of receding of the observed bodies with respect to the terrestrial control point.
Among the main huge galactic clusters with qualities of powerful lenses, two in particular are scrutinized: catalogued as Abell-2218 and the other Abell-1689, whose estimated distances from us are about 3 billion light-years for one and 2.2 billion light-years for the other. In the latter case, evaluating dynamics and intensity of deflected light, most of its matter is considered dark (i.e. invisible to our receptive and artificial apparatus) and peripheral, therefore mainly non-baryonic. The images of astronomical objects behind them and making them perceptible to us date back more than 10 billion years (in light-time).
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.
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