Tech UPTechnologyTelescopes bigger than galaxies

Telescopes bigger than galaxies

 

Although they are more myth than reality, planetary alignment stories promise fantastic events . Thousands of years from now, when all the planets align, either the day of reckoning or the promised hero will come or homeopathy will no longer be sold in pharmacies. Planetary alignments do not usually give us much more than excuses to go out and observe the night sky , but other types of alignments, which occur on much larger scales, do allow us to deepen our study of the universe. I’m talking about galaxy alignments or more specifically gravitational lensing .

These lenses are the result of one of the most famous predictions of General Relativity : that light (and the photons that make it up) are affected by gravity . In Albert Einstein’s theory, the quantity that is affected by gravity (which here takes the form of the curvature of spacetime ) is energy , rather than mass. This causes any particle to be affected by gravity and its trajectory changed when it passes near a very massive object .

Indeed General Relativity was not the first theory of gravity to predict that light should be affected by this force. Isaac Newton ‘s theory of Universal Gravitation already predicted something like this. For Newton, light was made up of particles . And although he had no experimental way of finding out the mass of these particles, he assumed that they must have mass . But to calculate how the trajectory of light was modified in the presence of a gravitational field, he did not need to know it , after all this would be a problem of free fall or orbits. What will determine the angle by which the trajectory is deflected will be the speed of the object and not its mass (similar to the fact that, in the absence of air, a feather and a cannonball fall at the same speed).

However, the prediction made by both theories was not the same. Specifically , Einstein predicted an angle that would be double the one predicted by Newton . In 1919 this prediction was put to the test, from which the German and his theory emerged victorious. There, the properties of gravitational lensing were used for the first time to observe a distant star. Since then we have found lenses of this type formed by a multitude of stars scattered throughout the universe. Here are some of the most spectacular cases:

cosmic serpent

The first of the stars affected by this phenomenon is called the Cosmic Serpent , for more than obvious reasons. This galaxy lies behind a cluster of galaxies with a considerably less romantic name: MACSJ1206.2-0847. This cluster is located about 4.5 billion light-years away and contains enough mass to redirect light from objects much further away. The image of the Cosmic Serpent, which is actually made up of four superimposed partial images , is not only modified by the cluster as a whole, but some of its galaxies manage to create that sinuous shape that can be seen. Thanks to this image, it has been possible to identify regions of intense star formation in this galaxy.

Einstein cross

In the following image we can observe a case in which the distortion of the gravitational lens has multiplied the original image by four . The Einstein Cross , which is how this image is known, consists of the light from a distant quasar being amplified by a relatively close galaxy and which coincidentally is located on the line that joins the quasar with the Earth. Quasars are especially luminous active nuclei of galaxies surrounded by an accretion disk. The black hole at its center can have a mass from several million solar masses to tens of billions of times the mass of the Sun. The gas that rushes into the black hole is heated by the incredible frictional forces, emitting light in the process. These objects are among the most luminous and violent known , with some quasars reaching luminosities thousands of times that of the Milky Way as a whole.

einstein ring

Actually this name is given to the type of structure, which has been observed repeatedly. This one in particular is called LRG 3-757 and shows a distant galaxy being distorted by a closer one. When the alignment between the three objects (the two galaxies and the Earth) is exact, a perfectly circular image is formed , which is called the Einstein Ring , in honor of the German physicist’s predictions. In this case the alignment was not exact, but it was enough to form this almost circular structure, reminiscent of a horse’s shoe .

All of these images are cases of what is known as strong gravitational lensing , in which the image is completely distorted and altered by the lens. However, there are also, and are much more common, weak gravitational lenses , which have the effect of changing the apparent position of the affected distant star. These lenses, which is how the Sun could be considered during that 1919 eclipse that confirmed Einstein’s predictions, are very useful in astrophysics. To detect these lenses it is usually necessary to compare several images of the same region of the sky , observing the change of position of the different objects. With this process, for example, the presence of objects that do not emit light but do accumulate great mass , such as naked black holes or dark matter , can be identified.

References:

V. Belokurov et al, 2007, The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy, The Astrophysical Journal. 671 (1), doi:10.1086/524948

Bernard F. Schutz, 1985, A First Course in General Relativity, Cambridge University Press

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