What is big bang theory?
The big bang theory is the leading explanation for how the universe began, simply put, it says the universe as we know, started with an infinitely hot and dense single point that inflated and stretched first at unimaginable speeds. The universe began as some scientists believe with every speck of its energy jammed into a very tiny point, this was extremely dense and then exploded with unimaginable force, creating matter and propelling it outward to make the billions of galaxies of our vast universe.
Existing technology doesn’t yet allow astronomers to literally peer back at universe’s birth; much of what we understand about big bang comes from mathematical formulas and models. Astronomers can however see the “echo” of the expansion through a phenomenon known as the cosmic microwave background. While the majority of the astronomical community accepts the theory, there are some theorists who have alternative explanations besides the big bang such as external or an oscillating universe.
The big bang theory explains the evolution of the universe from a starting density and temperature that is well beyond humanity’s capability to replicate so extrapolations to the most extreme conditions and earliest times are necessarily more speculative, how the initial state of the universe originated is still an open question, but the big bang model does constrain some of its characteristics. For example specific laws of nature most likely came to existence in a random way, but as inflation models show, some combinations of these are far more probable. The big bang theory is built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time and such an infinite energy density may be a physical impossibility.
The big bang model is based on two assumptions; the first is that Albert Einstein’s general theory of relativity correctly describes the gravitational interaction of all matter. The second assumption, called the cosmological principle, states that an observer’s view of the universe depends neither on the direction in which he looks nor on his location. This principle applies only to the large-scale properties of the universe, but it does imply that the universe has no edge, so that the big-bang origin occurred not at a particular point in space at the same time. These two assumptions make it impossible to calculate the history of the cosmos after the Planck time; scientists have yet to determine what prevailed before Planck time.