What is BIG BANG theory ? Please explain .

The Big Bang Theory is the prevailing cosmological model that explains the early development of our Universe. Basically, it states that at one point the Universe existed in an extremely hot and dense state known as a singularity, which was composed entirely of energy. Then, around 13.7 billion years ago, this tiny singularity expanded very rapidly in an event known as cosmic inflation. As the Universe expanded, it cooled and became less dense. Energy was converted into subatomic particles, which would later come together to form atoms. The first elements were hydrogen and helium; heavier elements would be synthesized within stars or during supernovae. The Big Bang is considered to be the origin of space and time, and everything in the Universe was formed from the energy released by it.

The Big Bang Theory does not fully explain the true origin of the Universe, it is only based on the fact that a singularity (which was the Universe) was already there and that it expanded to form the modern Universe. What caused the singularity to expand, how it came into being, and what was before it is unknown. Speculation abounds, but ultimately the cause of the Big Bang may never be fully realized because this is outside our realm of observation.

The idea that would become the Big Bang Theory was first proposed by a Catholic priest named Georges Lemaitre, who called it his "hypothesis of the primeval atom." Since then, scientists have built on his ideas and much evidence has arisen. The Big Bang is a very well tested and widely accepted model and is the best explanation for the full range of phenomena astronomers observe. Since its conception, many scientists have proposed alternative theories, but none of them could hold as much weight. The two most commonly mentioned pieces of evidence for the Big Bang are the expansion of the Universe and the cosmic microwave background, although much more exists.

It has been consistently observed that galaxy clusters are moving away from each other and the universe is expanding. Expansion is observed through a phenomenon called redshift, in which the lightwaves from objects that are moving further away are stretched towards the red side of the spectrum. Almost all the galaxies outside our Local Group are moving away from us in every direction. If the Universe is expanding, it must have been smaller in the past. The Big Bang Theory makes more sense when we know this.

Furthermore, the discovery of the cosmic microwave background radiation in the 1960's helped validate the Big Bang. This leftover thermal radiation permeates the Universe almost uniformly, and is from an early stage in its development. When the Universe was still young, it was very hot and filled with an opaque fog of hydrogen plasma. As it expanded and cooled, stable atoms could form. These atoms could no longer hold onto the radiation, and it was released into space, making the Universe transparent. That same radiation which was released long ago remains today as a weak afterglow of the Big Bang, and a confirmation of the event.

There are some widely held misconceptions about the Big Bang. This is due to the difficulty in clearly and accurately defining what the event was and what the theory actually says to the general, nonscientific population.

1. The Big Bang does not explain the true origin of the Universe. Its primary focus is the development of the Universe over time.
2. It was not an explosion of matter into existing space. Rather, it was the expansion of space itself. Picture an inflating balloon, or a crumpled map being unfolded.
3. It doesn't say that "something" was produced from "nothing."
4. The fact that we do not know for certain what caused the Big Bang is not a weakness in the theory; we know it happened regardless of what caused it

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The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it talks about the universe as we know it starting with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.
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