Black Holes: A Chapter in Cosmology

Understanding Black Holes

A Black hole is a region in spacetime exhibiting extremely strong gravitational strength that forbids anything escaping from its sphere of influence including all matter and radiation. The boundary that limits its sphere of influence is termed as the event horizon. The central region of a black hole is termed as a singularity.

Let’s understand space-time first. As conceptualized by Einstein, it can be imagined as a fabric. The interwoven fabric consists of fibers which are called geodesics. Near a heavy body, these geodesics are curved. A black hole affects it such that any object near the hole, close enough to feel the pull, ultimately falls into the hole. The slope becomes infinite and even light cannot escape from it. Since light cannot come out of it, we cannot see into a black hole (thus the epithet black). However, they can be detected due to their effect on the surroundings.

How are Black Holes formed?

Collapse of Stars

Stars are the heavenly bodies that are formed by virtue of the nuclear fusion reactions that involve the formation of helium. When the gases become dense enough (mainly hydrogen and helium) they form stars. The star has immense gravitational pull. In order to oppose its own gravity, hydrogen and helium serve as the necessary nuclear fuels that cause expansion (The example of a balloon seems very apt. The air pressure serves to compress the surface whereas the air molecules in the balloon cause it to expand outwards nullifying the effect). Eventually, the stars consume all their fuel with the passage of time. Strangely enough the more massive the body, the earlier it consumes it fuel. This is because of the greater gravitational pull that the more massive star has to utilize a greater portion of the energy resources.

When the star is running on empty, there are two possibilities. Either the star exists or it collapses under its own gravity. This outcome is determined by the Chandrasekhar limit (named after Subramanian Chandrasekhar). It is approximately 1.5 times the mass of the sun. Masses smaller can exist as white dwarfs and others continue collapsing until it becomes so dense that it forms a black hole.

Inside a Black Hole

No one can be sure of that as it is impossible to observe it from the outside. What has been deduced from theories is that there is a singularity deep inside the black hole where the curvature of space-time is infinite and where the laws of physics breakdown because of their inability to deal with infinities. It is at this point that the concept of space-time is no more. However, it remains impossible to observe a black hole singularity. If an astronaut manages to enter a black hole, the difference of gravity at his upper and lower end would shred him apart. Even if he manages to enter it, he cannot see it until he collides with it. Thus what it contains remains a mystery.

Reality behind the myth

It is a universally accepted fact that black holes do not emit anything but that might not be true in its entirety. The fact that the event horizon of a black hole resulting from the merger of two or more black holes remains the same or increases gave rise to the idea that black holes have entropy. It was proposed that the area of the event horizon is proportional to the entropy of the black hole system. But the theory was considered dubious as a body possessing entropy must also possess heat thus it must be a source of radiations. But how can it be possible when even light cannot escape from it and nothing is faster than light.

The answer lies in the space beyond the black which in fact is not all that empty as it contains fluctuating fields and the presence of the fields is responsible for the creation of antiparticle/particle pairs (reverse of the annihilation process it seems). The law of conservations of energy is still conserved in this situation.

It is known that the energy of the particle under the influence of gravity is negative and in the strong gravitational field of the black hole, it is possible for even antiparticles to become positive particles having negative energy. In accordance with the mass-energy equivalence

E=m*(c*c)

The negative energy leads to a decrease in mass of the black hole and in turn the size but the temperature increases so should the entropy. This is explained by the fact that as the size of the black hole decreases, the less distance anti-particle travels before becoming a positive particle and greater is the rate of emission. Therefore, even though the size of the black hole decreases, the energy balance is more than compensated for by the emission that lies in the EM spectrum of radiation.

Conclusion

Why are they important

Black holes hide the matter but still contributes to the total mass of the universe. Moreover, black holes that are very small, called primordial black holes, having a very much smaller radius despite the fact that Chandrasekhar limit prevent such stars from collapsing might give us a general idea of the temperature and pressure conditions during the early stages of the big bang expansion.

Our universe is thought to have been and still being expanded from a singularity the opposite process of the formation of a black hole). Understanding the black hole singularity shall enable us to understand the big bang similarity, beyond which the realm of physical science seems to come to a halt. It may enable understand the initial boundary conditions that led to the present day universe.

And finally, it shall enable us to understand gravity in a way that shall enable us to devise the Full Unified Theory (till now we have failed in unifying gravity with other forces). This will enable us to harness gravity even though it may seem fictional. Thus, from a hypothetical thought to the realm of existence, the concept of black holes has sure come a long way.

By Mazhar Abbas

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