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According to the first law of thermodynamics, law of conservation of energy states,

The total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.

Question is where was the energy that triggered the big bang at the first place? What are the popular hypotheses?

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    $\begingroup$ Big bang is surely out of the coverage of thermodynamics. Your question is lecit but you don't need the law of thermodynamics to arrive at it. Also, not sure if energy conservation holds at cosmological scale. Just think of dark energy. $\endgroup$
    – Alchimista
    Oct 5 '20 at 6:07
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    $\begingroup$ Energy conservation is only true for a system which is time symmetric, as formulated through Noether's theorem. The Universe is not time symmetric, so we can't expect its energy to be conserved. In fact, it isn't — due to the expansion of the Universe, radiation is constantly redshifted and hence loses energy. On the other hand, as @Alchimista says energy is also created though dark energy (and no, these two processes don't equal out, as they have nothing to do with each other). $\endgroup$
    – pela
    Oct 5 '20 at 21:21
  • $\begingroup$ @pela so we can say that the law is not applicable for our universe. Is it because the precondition of the law, an isolated system? $\endgroup$ Oct 5 '20 at 21:29
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    $\begingroup$ Yes, the law is only applicable for an isolated system, i.e. energy doesn't leave or enter the system. But that's not a sufficient condition: it is also important that the system is time symmetric, i.e. if you repeat an experiment tomorrow, you should get the same result as today. This is of course true for the Universe in most cases, on sufficiently small time scales, but only approximately. $\endgroup$
    – pela
    Oct 5 '20 at 21:37
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This is a difficult question to answer. Physics really starts after the big bang. Scientist don't know about the laws, if any, before the first instance after the big bang. The time before and during the big bang is really the province of philosophy.

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There are theories which suggest that the universe is like a giant spring, constantly expanding and collapsing. Its possible that pre-big-bang, there was another universe which collapsed, only to re-expand into the universe we know now.

But in reality we have no idea

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