BLACK HOLE
INTRODUCTION
A black hole is a region from which even light cannot escape. It is one of most intriguing, confusing and the most talked of object in the outer space by modern physicists and astronomers. Scientists believe that the information recieved regarding the black holes can be very helpful in determining the fate of the universe billions of years in future and the position of the universe billions of years in past. A black hole is a compressed dead star which has tremendous energy and gravitational power, so great that even light cannot escape from it. Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. A black hole has only three physical properties - mass, charge, and angular movement. Around a black hole there is an undetectable surface which marks the point of no return. This surface is called an event horizon.
STRUCTURE OF BLACK HOLE
EVENT HORIZON
An event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. A boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. This is one of the most important feature of a black hole which makes it so strange.
A body in space (away from a black hole) can travel in any direction - both away or towards the black hole. The closer the body gets to the black hole, the lesser can the body travel away from the black hole and the moment the body hits the even horizon of the black hole, it can no longer go in any direction but towards the centre of the black hole.
For the observer, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it. It is due to the Gravitation time dilation - the effect of time passing at different rates in regions of different gravitational potential. The lower the gravitational potential, the slower the time passes. At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift. Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.
SINGULARITY
At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite. For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution. The singular region can thus be thought of as having infinite density.
An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there. When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.
In the case of a charged or rotating black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility. It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox. It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.
ERGOSPHERE
Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still!! The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.
PHOTON SPHERE
A photon sphere is a spherical region of space where gravity is strong enough that photons are forced to travel in orbits. As photons travel near the event horizon of a black hole they can escape being pulled in by the gravity of a black hole by traveling at a nearly vertical direction known as an exit cone. A photon on the boundary of this cone will not completely escape the gravity of the black hole. Instead it orbits the black hole. These orbits are not stable.
The photon sphere is located farther from the center of a black hole than the event horizon and ergosphere. Within a photon sphere it is possible to imagine a photon that starts at the back of your head and orbits around a black hole only then be seen by your eyes.
An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there. When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.
In the case of a charged or rotating black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility. It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox. It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.
ERGOSPHERE
Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still!! The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.
PHOTON SPHERE
A photon sphere is a spherical region of space where gravity is strong enough that photons are forced to travel in orbits. As photons travel near the event horizon of a black hole they can escape being pulled in by the gravity of a black hole by traveling at a nearly vertical direction known as an exit cone. A photon on the boundary of this cone will not completely escape the gravity of the black hole. Instead it orbits the black hole. These orbits are not stable.
The photon sphere is located farther from the center of a black hole than the event horizon and ergosphere. Within a photon sphere it is possible to imagine a photon that starts at the back of your head and orbits around a black hole only then be seen by your eyes.
FORMATION
GRAVITATIONAL COLLAPSE
It is the inward fall of a massive body under the influence of gravity. It occurs when all other forces fail to supply a sufficiently high pressure to counterbalance gravity and keep the massive body in hydrostatic equilibrium. Gravitational collapse is at the heart of structure formation in the universe. An initial smooth distribution of matter will eventually collapse and cause the hierarchy of structures, such as clusters of galaxies, stellar groups, stars and planets. The compression caused by the collapse raises the temperature until nuclear fuel ignites in the center of the star and the collapse comes to a halt. The thermal pressure gradient (leading to expansion) compensates the gravity (leading to compression) and a star is in dynamical equilibrium between these two forces. Gravitational collapse of a star occurs at the end of its life time, also called the death of the star. When all stellar energy sources are exhausted, the star will undergo a gravitational collapse. In this sense a star is in a "temporary" equilibrium state between a gravitational collapse at stellar birth and a further gravitational collapse at stellar death. The end states are called compact stars. If the mass of the remnant exceeds about 3–4 solar masses — either because the original star was very heavy or because the remnant collected additional mass through accretion of matter— even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.
PRIMORDIAL BLACK HOLES
Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses. Primordial black holes could thus account for the creation of any type of black hole.
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 direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown. A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.
Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes.
CONCLUSION
Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes.
CONCLUSION
After knowing so many things regarding the black holes, I am sure that your thoughts will be swirling from admiration to philosophical to GOD KNOWS WHAT! Black Hole, for over 5 decades have been scientist's most interested object and now, I hope, you guys know why...
Knowing how black holes formed can be of a great deal of help to know how the universe was formed, how did the big bang take place, what are the chances of the Big Freeze or Heat Death, is it possible to know when they will take place, if yes, then when? What is the ULTIMATE FATE OF THE UNIVERSE?
The question is not expected to be answered in near future but the scientists are VERY curious to know more and more about this and with the technology of today and the fresh minds of the modern scientists, it is pretty sure that they WILL answer it.
Now you know why most people choose black as their favorite color, right? ;)
Now you know why most people choose black as their favorite color, right? ;)
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