BLACK HOLES

BLACK HOLES

If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse. The stellar remnant thus becomes a black hole. The mass at which this occurs is not known with certainty, but is currently estimated at between 2 and 3 solar masses.

A black hole is a region of space-time from which nothing, matter or even light, can escape. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.

The theory of general relativity predicts that a sufficiently compact mass will deform space-time to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return.

Black holes of stellar mass are expected to form when massive stars collapse in a supernova at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, super massive black holes of millions of solar masses may be formed.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter. There is growing consensus that super massive black holes exist in the centres of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the centre of our Milky Way.

It was not understood earlier how a mass-less wave such as light could be influenced by gravity. Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion.

A white dwarf slightly more massive than the sun will collapse into a neutron star, which is itself stable. Neutron stars above approximately three solar masses would collapse into black holes.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers.

The simplest black holes have mass but neither electric charge nor angular momentum. There is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near a black hole's horizon; far away, the external gravitational field is identical to that of any other body of the same mass.

Far away from the black hole a particle can move in any direction, only restricted by the speed of light. Closer to the black hole space-time starts to deform. Inside of the event horizon all paths bring the particle closer to the centre of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon - a boundary in space-time through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.

As predicted by general relativity, the presence of a mass deforms space-time in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole. Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it. At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational red shift. Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations. The shape of the event horizon of a black hole is always approximately spherical.

At the centre of a black hole as described by general relativity lies a gravitational singularity, a region where the space-time curvature becomes infinite. 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.

Observers falling into a stationary black hole cannot avoid being carried into the singularity, once they cross the event horizon. They can prolong the experience by accelerating away to slow their descent, but only up to a point; after attaining a certain ideal velocity, it is best to free fall the rest of the way. 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 !

A pan-sized black hole may be useful in Chinese kitchen to make noodles!

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 space-time with the black hole acting as a wormhole. It also appears to be possible to follow closed time like curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight. The ideal gas law explains the connection between pressure, temperature, and volume.

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. The type of compact star formed depends on the mass of the remnant—the matter left over after the outer layers have been blown away, such from a supernova explosion or by pulsations leading to a planetary nebula. The resultant mass can be substantially less than the original star (about 25%).

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 even the degeneracy pressure of neutrons is insufficient to stop the collapse. No known mechanism is powerful enough to stop the implosion and the object will inevitably collapse to form a black hole.

The gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the early universe may have resulted in very massive stars, which upon their collapse would have produced black holes of up to 103 solar masses. These black holes could be the seeds of the super massive black holes found in the centres of most galaxies.

Gravitational collapse requires great density. 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. In principle, black holes could be formed in high-energy collisions also that achieve sufficient density.

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 super massive black holes seem to have grown.

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 super massive black holes, which could have formed from the coagulation of many smaller objects.

If a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to human weight would evaporate in an instant. A black hole the weight of a car would take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Imagine a car sized star 200 times brighter than the whole sun !

The deformation of space-time around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole.

SUMMARY OF THE UNIVERSE

The universe started off as a Big bang from extremely condensed state (named singularity) of matter and energy of nearly infinite mass and nearly nil volume; develop into the universe of the present and then end up into a Big crunch once the stars die out to become black holes and all black holes merge into the same singularity just before Big bang. This cycle may be repeating repeat without any apparent beginning or end.

Interestingly or intriguingly isn’t this what religions said ?!

Modern science shakes hands with spirituality here.