Although the phrase "big bang" sounds like a huge noise, it actually refers to the birth of the universe. Some scientists believe that at the beginning of time, all matter and energy were contained in an infinitesimally small point. The big bang theory concerns itself with the expansion of that matter and energy, and the emergence of our universe. It suggests that the universe is continually expanding -- that space is growing, and the bodies in it are getting farther apart.
At the time of the big bang, four major forces -- strong and weak nuclear forces, electromagnetism and gravity -- were united in one, and they only became differentiated as the universe continued its development. Scientists still can't explain how the forces operated as one or how they became separate. Some scientists are skeptical of the theory, holding that it contradicts the first law of thermodynamics, which is that matter cannot be created or destroyed; for them, the theory describes the evolution of the universe, not its creation. Critics say that according to the big bang, in the early development of the universe, there was movement that was faster than the speed of light. Yet those who support the big bang theory claim that the laws of physics didn't apply in the very early stages of the universe's growth.
Many scientists today subscribe to inflation theory. According to inflation theory, the universe expanded extremely rapidly immediately following the big bang. This event explains the temperature and density fluctuations in the cosmic microwave background, the radiation signature left behind by the big bang. Scientists developed this line of thought in the 1980s to address problems with the standard big bang theory.
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