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What known laws of physics give evidences that supports the Big Bang Theory? Gravity is common, but what other laws of physics can support the theory?

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Good question. The idea of the Big Bang was originally predicted as an outcome of Einstein's Theory of General Relativity. Sparing you the complicated mathematics, the main idea is that the Universe is constantly increasing in the rate that it expands. But now, no one could really fathom this to be true at the time, as who would really trust some dawg's "theoretically" hypothesized ideas? So years later, this idea was confirmed by a guy named Edwin Hubble, who used some dope telescopes to see that the light from certain galaxies was redshifting (the light shining from them was becoming more and more red), which meant that the galaxies were moving further away from the Earth. The observation also found that the further away an object was from the Earth, the faster that object was moving away from the Earth (stronger redshift). Thus, the Universe is expanding.

Since the Universe has been predicted to always be expanding, we reach the result that it must have come from a singular point at some time. Thus, we have the evidence for the Big Bang.

Note: I simplified a lot of stuff that went down here in the interest of preserving understanding.

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The laws of thermodynamics, and those of particle physics, support the big bang, inasmuch as if their consequences are taken into account in a (simplified) big bang model, you can then predict a nearly correct value for the observed ratios of helium to hydrogen and to lithium in the visible universe, among other predictions which agree in large part with astrophysical observation. I recommend you read The First Three Minutes by Stephen Weinberg, which treats this subject in understandable detail.

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