The Big Bang Theory of Cosmic Origins (excerpt)
Many ancient thinkers, like Aristotle, believed that the universe had always existed. The celestial dimensions of its heavens were fixed and unchanging as its objects eternally revolved around the earth. In contrast to this Greek view, Christians harked back to their Judaic root and affirmed that, even though we may lack philosophical evidence for the finitude of the universe, scripture teaches that it had a definite beginning. Whereas some like Augustine believed the beginning lay in the distant past, the "days" of Genesis signifying long eras, others during the Enlightenment calculated that the universe originated relatively recently. Astronomers now estimate that approximately 13.7 billion years ago the universe, in an extremely hot and powerful fireball, exploded into existence. The relevant contemporary scientific theory is termed the Big Bang Theory.
In the mid-nineteenth century physicists formulated the second law of thermodynamics, according to which heat can flow only from hot to cold bodies. Applied to the universe, the second law implied that without the input of additional energy, the universe was moving gradually to a state of thermodynamic equilibrium. The universe will end in an entropic freeze. But since the big freeze has not yet occurred - as would be the case were the universe infinite in time - the universe could not be infinite in past duration. Only the postulation that matter was continuously created, a suggestion once proposed but no longer held, could save one from having to affirm a beginning to the universe.
In 1927 the Belgian astronomer and Catholic priest Georges Lemaître proposed that the universe began with the explosion of a single atom. He based his conjecture on the fact that distant nebulas give off light that shifts to the red on the light spectrum. Edwin Hubble in 1929 argued that this red shift provided evidence that the galaxies did not maintain a constant distance from each other but were receding at a speed proportional to their distance from us (the Hubble Constant). Though the movement of any one galaxy might be relatively small, the movement of galaxies with respect to each other would be quite significant over time as the universe itself expanded. Galaxies twice the distance from us move away twice as fast. Thus, it has taken the same amount of time for each galaxy to move from its initial point to its present location.
If the galaxies are receding from each other, then by looking back in time, we might arrive at a point when the universe actually began. The farther one moves back in time, the smaller the universe becomes. Cosmic matter becomes denser and hotter, until the universe is only a fraction (one ...
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