General relativity will soon celebrate 110 years since its inception. Although such a time may seem very distant, it is still a valid theory of gravity. Testing of general relativity has been going on since the first years of its publication, which has greatly intensified in recent years. Recently, a number of tests of this beautiful and successful theory have been carried out, and general relativity has clearly triumphed every time. We talked about one such test here recently, today we will discuss another test conducted with the help of the Hubble Space Telescope.
Gravitational lensing
The principle of light bending in the gravitational field of a material object, in this case the Sun. For stars close to the Sun, the bending is greatest (top), for more distant stars the effect is smaller (middle), and for distant stars we can neglect it (bottom).
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After the publication of the new theory in 1915, a number of different solutions and proposals related to the theory appeared very soon. Albert Einstein himself proposed the so-called gravitational bending of light as one of the tests of his new theory. Light rays should be bent in the gravitational field of material bodies. As a result, we should then see the light source slightly shifted from its real position. In normal events, this phenomenon is relatively weak, yet measurable. But it was necessary to wait for a solar eclipse, when it was possible to observe distant stars near the Sun. The effect was actually observed in 1919 by two teams of British astronomers, and it made Einstein very famous.
The illustration shows the principle of gravitational lensing. A massive cluster of galaxies lying between Earth and a distant galaxy bends and brightens the light of the distant galaxy or quasar.
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The more general phenomenon of gravitational lensing is very closely related to the gravitational bending of light. The name is not accidental, because the gravitational lens really resembles a commonly known optical lens in its properties. The effect occurs when a distant object passes from our point of view directly behind a closer body with an intense gravitational field. The closer object then bends the rays of light or other electromagnetic radiation and thus behaves like a lens.
The rays passing through the intense gravitational field are amplified and also bent, so that various complex shapes are created, their typical representatives are, for example, the Einstein ring (the image of a lensed object folds into a perfect ring) or the Einstein cross (the image of a specific distant quasar, which is projected in four copies into cross-shaped). Multiple images of the same object are generally typical of gravitational lensing.
Roughly in the middle of the image, not far from the orange galaxy, you can see two bright bluish star-like objects. However, these are not real stars, but rather a quasar. It was originally thought to be two objects. But it was soon discovered that they were two images of the same object, the quasar Q0957+561. It is the first ever recorded gravitational lensing. This image is from the Hubble Space Telescope.
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The history of gravitational lensing goes back to 1912, when Einstein himself thought about it, but he believed that the effect does exist, but it is so weak that it will not be measurable in the foreseeable future. The idea was resurrected in 1924 by the Soviet physicist Orest Chvolson, but it did not receive more attention until the 1930s. Then the Czechoslovaks Rudi Mandl and František Link came up with the same idea, but mainly the legend of world astronomy, the American-Swiss scientist Fritz Zwicky, who determined that the effect could be measured very well. Nevertheless, the first gravitational lens was observed only in 1979.
Types of gravitational lensing
Demonstration of the principle of gravitational lensing and showing that we can map dark matter with this procedure.
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Today, we already observe gravitational lenses quite commonly, as evidenced by the results from the Webb Space Telescope, when gravitational lenses have already appeared several times. Gravitational lensing has an irreplaceable place in today’s physics, as it is widely used in astronomy. It is used in observing extremely distant objects, mapping dark matter, searching for exoplanets, etc. Moreover, it is one of the best-studied relativistic phenomena, which also offers a great proof in favor of general relativity.
Weak (left) and strong (right) gravitational lensing.
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Gravitational lenses are usually divided into three basic types – strong lenses, weak lenses and microlenses. Most often, we certainly encounter strong lenses. These are cases where the image distortion is large. Both the light source and the lens are usually billions or at least hundreds of millions of light years away. At the same time, it can be said that the gravitational lens in this case is some extremely massive object with a strong gravitational field, such as galaxy clusters or giant galaxies. All the spectacular photos of gravitational lenses are just examples of strong lenses.
The second type is weak lenses. Here the image distortion is already smaller and can almost never be detected for a single source, but only statistically from the observation of multiple sources. In this case, individual galaxies are often the gravitational lens. This type of gravitational lensing is quite prone to systematic errors, but it can be very useful for mapping dark matter, tracking dark energy or estimating cosmological parameters.
The principle of gravitational microlensing. In this way, for example, exoplanets can be discovered.
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Today, however, he will only deal with the third type, i.e. microlenses. These are cases where image distortion is no longer noticeable at all. However, we are able to detect a change in the amount of light received. The stars of our Galaxy, which lens more distant objects, usually serve as gravitational lenses in this case. In an extreme case, a star from a distant galaxy lenses another much more distant star. It was with this technique that astronomers detected the most distant known stars in the universe. Microchoking has also been used as a successful way to search for distant exoplanets.
Hubble and Gaia – an ideal cosmic collaboration
LAWD 37 in a Hubble image.
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The effect of gravitational microlensing was recently used by astronomers led by experts from the University of Cambridge. They focused on the nearby white dwarf LAWD 37. A white dwarf is a star that has, to put it bluntly, passed from adulthood to retirement. These are the remnants of less massive main sequence stars that have already used up their nuclear fuel and gone through the red giant phase, when they swelled up significantly. The red giant has shed most of the layers, leaving a star comparable in size to Earth, which is why they are called white dwarfs. This particular one is 15 light-years away in the direction of the constellation Ursa Major and is about 1.15 billion years old.
Gaia satellites
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Because LAWD 37 is one of the closest white dwarfs, astronomers have been studying it in great detail for a long time. For this, they use a whole range of instruments, including the European Gaia satellite focused on astrometry, i.e. measuring the positions and movements of celestial objects. Gaia, which creates a detailed map of the Milky Way with billions of stars, is ideal for these purposes.
Although scientists knew a lot about LAWD 37 because they could make spectroscopic measurements, until now they lacked information about the mass of the white dwarf LAWD 37. The mass is fairly easy to determine for binary systems, but difficult to determine for single objects. But since the mass of individual objects is well measured through gravitational lensing, the astronomers thought they could apply the measurements from the Gaia satellite and see if LAWD 37 will be in a suitable position for microlensing anytime soon.
The image nicely shows how the white dwarf LAWD 37 (marked with a blue circle) is approaching the background star (marked with a red circle). Left image from 1998, right from 2016. The blue arrow shows the direction of movement and the frame at the top shows the moment of closest approach.
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So they used data from the Gaia satellite to predict the white dwarf’s motions and were delighted to discover that LAWD 37 would soon come close enough to one distant star to take advantage of the microlensing effect. Therefore, they were able to gain observation time on the famous Hubble Space Telescope. Then, in November 2019, all that was needed was to direct the telescope to the target area and patiently wait for the results.
The discovery of the Hubble telescope
Measurements of the white dwarf Stein 2051 b, located in a binary system with a main sequence star.
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And the measurement really succeeded, the results speak for themselves. Indeed, the astronomy team was able to detect gravitational microlensing in LAWD 37. It was already possible to capture a microlens in the white dwarf Stein 2051 b, but it is located in a binary system with a red dwarf. This is the first observed case of microlensing in a single white dwarf.
In addition, it was possible to measure the mass of this star quite accurately, which is 0.56 times the mass of the Sun, which is fully consistent with previous theoretical assumptions and models of the development of white dwarfs. For the first time, it was also possible to test the relationship between the radius and the mass of a white dwarf.
The white dwarf passes in front of the brighter star in the background.
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We also got a unique opportunity to study the properties of matter in the extreme conditions of a white dwarf. In these objects, there is no ordinary matter, but a so-called electron degenerate gas, which prevents further collapse into a neutron star or a black hole. The study of white dwarfs is very important for understanding the evolution of stars, especially since even our Sun will turn into a white dwarf in a few billion years. We (mankind) probably won’t live to see it, but it’s still essential knowledge.
A test of general relativity
Hubble image. The bright star roughly in the center of the image is the white dwarf LAWD 37. In the cutaway, we see a blue wavy curve that shows the motion of the white dwarf as seen from Earth. And to the upper left of the white dwarf, also a more distant star of the main sequence, in front of which the white dwarf passed.
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But it was not a significant milestone for astronomers investigating general relativity. We don’t measure gravitational microlenses in white dwarfs very often, so it also meant the possibility to test the predictions of Einstein’s theory again in detail. It probably won’t surprise you too much that general relativity has triumphed once again. Both the change in the intensity of the incoming light due to the microlens and the bending of the light near the lensing white dwarf were in perfect agreement with the theory. In other words, the distant star temporarily changed its apparent position in the sky exactly in accordance with the theoretical prediction.
It is not without interest that the measurement was made by a group of astronomers almost exactly 100 years after British astronomers led by Frank Dyson, Arthur Eddington and Andrew Crommelin first measured the bending of the light of distant stars in the Sun’s gravitational field. They succeeded in the eclipse of the Sun in May 1919, and they had observation posts in Sobral, Brazil, and on Prince Island off the coast of Africa. It is incredible how far the possibilities of astronomy have advanced in a century. Today, we can already detect the same effect in stars outside the Solar System several light-years away.
Similar image once more. On the right, however, we see a series of boxes that show the position of the white dwarf (highlighted red square) and the more distant star (blue circle) in various phases of 2019 and 2020.
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By the way, in this case it was quite a challenge for astronomers to distinguish the real signal from the noise. The light of the distant star in the background was quite weak, moreover, we had seen only a few similar events in the past, so they could not rely too much on past experience. For that reason, it was not at all easy to detect gravitational microlensing. In this case, the signal was 625 times weaker than the signal that was detected in 1919 during the solar eclipse by the British group.
Conclusion
New research conducted by the collaboration of the Hubble and Gaia telescopes opens up completely new possibilities for future astronomical observations. Given the ability of the Gaia satellite to accurately measure the movements of stars, it is easy to imagine that the proven scenario could be repeated in the near future. Experts are already thinking about other similar cases where Gaia could predict an eclipse or approach of two objects and other telescopes could then monitor the given event. In the coming years, we can therefore expect measurements of additional gravitational microlenses, which will help us to reveal many secrets of various types of stars and other objects.
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2023-10-13 22:11:02
#Hubble #test #general #relativity
