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History, evolution, observational evidence of black holes best definition | Gurugrah






Black Hole | Gurugrah

Black hole -

A black hole or black hole is an object with a powerful gravitational field. Anything that falls inside the black hole’s boundary, called the event horizon, cannot escape and eventually collapses forever. This is why black holes are often called “black”. Because black holes suck up all the light that falls on them, they look like dark objects. However, scientists have found that black holes also have a temperature and can emit Hawking radiation.


Black holes are places in space where gravity is so strong that nothing can escape it. The theory of general relativity predicts that if you have a really small mass, space can start rotating around it, and that’s when you get a black hole. The limit of no escape is called the event horizon. However, this doesn’t really have much effect on the fate of the object or the circumstances of its passage.


Black holes are like black bodies, which do not reflect light. They also have a temperature, which is very low. This means that we cannot see black holes directly, but we can see their effects.


In the 18th century, two scientists thought that it might be impossible for light to escape from objects with a strong gravitational field. In 1916, a scientist discovered the first modern solution of general relativity that described black holes. In 1958, a scientist published the first explanation of a black hole as a region of space in which some matter cannot survive.


Black holes are mysterious objects that are predicted by the laws of physics. For a long time, people thought they were just a mathematical curiosity; But theoretical work in the 1960s showed that they do indeed exist in the universe. Cygnus X-1, the first known black hole, was discovered in 1971 by several different researchers.


In April 2019, the EHT published the first direct image of a black hole and its surrounding region. This was a great achievement! This was possible only because of the gravitational lensing effect. Black holes are very distant, so they can only be detected by this method. On 11 February 2016, the LIGO Scientific Collaboration and the Virgo Collaboration announced that they had detected gravitational waves. This was the first time scientists had seen evidence of black hole mergers.


History of the black holes –

The idea of ​​a body so massive that even light cannot escape it was first proposed by the English astronomer and clergyman John Mitchell in a paper published in November 1784. This mass is formed when the diameter of a star exceeds 500 times the diameter of the Sun, and the escape velocity of its surface exceeds the average speed of light. Michel called these bodies “black stars”.


He correctly noted that these supermassive but non-radiating bodies could be detected through their gravitational effects on nearby visible bodies. In the 1800s some people got really excited about the possibility that there were giant, invisible “dark stars”. But when people started to figure out that light is a wave, it became clear that gravity can have a big effect on how many lights escape.


Development of black holes: Given the strange character of black holes, it has long been questioned whether such objects can actually exist in nature or are merely pathological solutions to Einstein’s equations. Einstein himself thought wrongly that black holes would not form, because he believed that the angular momentum of the collapsing particles would stabilize their motion in some radius.


This prompted the general relativity community to reject all results to the contrary for many years. However, a minority of relativists continued to argue that black holes are physical objects, and by the late 1960s, they had convinced most researchers in the field that there was no obstacle to the formation of an event horizon.


Gravitational Fall -

Gravitational collapse occurs when the internal pressure of an object is insufficient to oppose the object’s own gravity. For stars, this is usually either because a star has too little “fuel” left to maintain its temperature via stellar nucleosynthesis, or because a star that is stable cannot process excess matter in such a way that Receives doesn’t raise its core temperature.


In any case, the temperature of the star is no longer high enough to prevent it from collapsing under its own weight. The collapse can be prevented by the degeneracy pressure of the star’s constituents, allowing the condensation of matter into an exotic dense state.


The primordial black hole and the big bang –

Gravitational collapse requires a very high density. These high densities are only found in stars in the present age of the universe, but in the early universe, the densities were much higher immediately after the Big Bang, possibly allowing for the formation of black holes. High density alone is not sufficient to allow the formation of black holes because a uniform mass distribution will not allow the mass to clump.


For a primordial black hole to form in such a dense medium, there must have been an initial density perturbation that could then grow under its own gravity. Different models of the early universe differ widely in their predictions of the scale of these fluctuations. Various models range from one Planck mass to hundreds of thousands of solar masses.


Despite the early universe being extremely dense—about as dense as it usually is to form a black hole—it did not collapse back into a black hole during the Big Bang. The models for the gravitational collapse of objects of relatively constant size, such as stars, do not necessarily apply in the same way as those for rapidly expanding space such as the Big Bang.


High-energy collisions –

Gravitational collapse isn’t the only process that can create a black hole. In principle, black holes could form in high-energy collisions that attain sufficient density. As of 2002, no such phenomenon has been found, directly or indirectly, as a loss of mass balance in particle accelerator experiments.


Growth –

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its surroundings. This growth process is one possible way through which some supermassive black holes may form, although supermassive black hole formation is still an open area of ​​research.


Evaporation –

In 1974, Hawking predicted that black holes are not completely black, but emit small amounts of thermal radiation at temperatures ℏ c 3 /(8π GM kB); This effect is known as Hawking radiation. By applying quantum field theory to the stationary black hole background, they determined that a black hole should emit particles that exhibit a full black body spectrum.


Since Hawking’s publication, many others have confirmed the result in various ways. If Hawking’s theory of black hole radiation is correct, black holes are expected to shrink and evaporate over time as they lose mass by emitting photons and other particles.


Observational Evidence –

Naturally, black holes themselves do not emit any electromagnetic radiation other than the hypothetical Hawking radiation, so astrophysicists searching for black holes generally must rely on indirect observations. For example, the existence of a black hole can sometimes be inferred by observing the gravitational effects of its surroundings.


On 10 April 2019, an image was released of a black hole, seen magnified because the light path is highly bent near the event horizon. The dark shadow in the middle arises from light paths absorbed by the black hole. The image is in false colour because the light halo detected in this image is not in the visible spectrum, but rather radio waves.


Detection of gravitational waves from merging black holes –

On 14 September 2015, the LIGO Gravitational Wave Observatory made the first successful direct observation of gravitational waves. [164] This signal was consistent with theoretical predictions for gravitational waves produced by the merger of two black holes: one with a mass of about 36 solar masses, and the other with a mass of about 29 solar masses. This observation provides the most convincing evidence to date for the existence of black holes. For example, the gravitational wave signal suggests that the separation of the two objects before the merger was just 350 km (or roughly four times the Schwarzschild radius estimated mass). The objects must therefore have been extremely compact, leaving black holes as the most plausible explanation.


Proper speed of stars orbiting –

Sagittarius A* – The proper motion of stars near the centre of our own Milky Way provides strong evidence that these stars are orbiting a supermassive black hole. Since 1995, astronomers have tracked the motion of 90 stars orbiting an unseen object with the radio source Sagittarius A*.


By fitting their motions to Keplerian orbits, astronomers were able to estimate in 1998 that it would take a 2.6 × 10 M ☉ object to cause the motion of those stars in a volume with a radius of 0.02 light-years. Must be included. Since then, one of the stars – called S2 – has completed one complete orbit. From the orbital data, astronomers could refine a calculation of a mass of 4.3 × 10 M ☉ and a radius of fewer than 0.002 light-years for the object due to the orbital motion of those stars.


The upper limit of the object’s size is still too large to test whether it is smaller than its Schwarzschild radius; Nevertheless, these observations strongly suggest that the central object is a supermassive black hole because there are no other plausible scenarios for confining so much invisible mass into such a small volume. Additionally, there is some observational evidence that this object may have an event horizon, a characteristic unique to black holes.


Accumulation of matter –

Due to the conservation of angular momentum, gas falling into a gravitational well created by a massive object typically forms a disk-like structure around the object. Artists’ impressions such as representations of a black hole with a corona usually depict the black hole as if it were a flat-space body that hides part of the disk behind it, but is actually images of gravitational lensing black holes. This will greatly distort the Growth Disk.


X-ray binaries –

X-ray binaries are binary star systems that emit most of their radiation in the X-ray part of the spectrum. These X-ray emissions are typically produced when one star (a compact object) accredits material from another (regular) star. The presence of an ordinary star in such a system provides an opportunity to study the central object and determine whether it may be a black hole.


Galactic nucleus –

Astronomers use the term “active galaxy” to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission. Theoretical and observational studies have shown that the activity in these active galactic nuclei (AGN) can be explained by the presence of supermassive black holes, which can be millions of times more massive than stellar ones. Models of these AGN have a central black hole that may be millions or even billions of times more massive than the Sun; a disk of interstellar gas and dust called the accretion disk; and two jets perpendicular to the accretion disk.


Microlensing –

Another way to test the black hole nature of an object is by observing the effects caused by a strong gravitational field in its vicinity. One such effect is gravitational lensing: the distortion of spacetime around a massive object causes rays of light to deflect, like light passing through an optic lens. Weak gravitational lensing has been observed, in which light rays are deflected by only a few arcseconds.


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By Chanchal Sailani | December 29, 2022, | Editor at Gurugrah_Blogs.

 

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