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Not only is it difficult to figure out how a whole universe could appear from nothing, but also no plausible explanation for the mechanism of the explosion itself springs to mind. The only known force in nature capable of affecting the universe as a whole is gravity. Gravity is, however, an attractive force, whereas the explosion requires a repulsive force. The mystery of the explosion lies in the first fraction of a second of the life of the universe. At such early times, the universe was in a state of incredibly high temperature and density, a situation so foreign to laboratory conditions that it is impossible to extrapolate known physics to that realm.
The cause of the explosion is shrouded in the extreme conditions of the very early universe. For a while, cosmology treated the existence of the universe as a given and concentrated on the consequences of the explosion rather than its cause. In the course of the scientific step-by-step reconstruction of the chronology of the universe, a number of puzzling paradoxes surfaced, of which two are of interest here. The first deals with the strength of the explosion. The push of the Bang was exquisitely well fine-tuned, allowing for the existence of a universe with galaxies, stars, planets, and life.
Any other push would have meant either a structureless universe or a universe collapsing back onto itself after a brief existence. Why was the Bang so well engineered? The second paradox deals with the large-scale properties of the universe. The extreme large-scale smoothness of the temperature and of the density of matter cannot be explained by the standard Big Bang model. No known physical process can account for it. What is the origin of this conspiracy? Is our universe very peculiar, that is, did it just start this way by sheer luck -- in the scientific jargon, with very special initial conditions -- or are there mechanisms that can account for the fine-tuned push of the explosion and for the smoothness of the universe?
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