I do have a small question, which may seem a bit stupid but we are all here to learn. Anyways my question is: Is the universe expanding using the energy it had from the Big Bang? if so what would happen if the universe stops expanding?

  • $\begingroup$ The most widely accepted cosmological theory holds that the expansion of the universe is currently accelerating, and this is primarily due to dark energy. What do you think could cause this acceleration to be reversed? $\endgroup$ – Chappo Hasn't Forgotten Monica Aug 12 '19 at 4:56

Yes, everything that happens in the universe depends on the energy delivered by the Big Bang. The very substance of the stars and galaxies was formed from it. Never forget the 1st Law of Thermodynamics: mass/energy can neither be created nor destroyed. If the universe stops expanding it will undergo gravitational collapse, slowly at first but ever faster, to become a blue-shifted, contracting universe. The same energy that caused the expansion will be recaptured in the collapse, to turn the universe into a fireball similar to the one from which it was created. This is called the Big Crunch. One version of what follows says it then disappears into a singularity, only to emerge again as a white hole and continue the cycle ad infinitum.

At present, dark energy has thrown a spanner in the works, and we can't be sure the universe will collapse, but if dark energy turns out to be a reality it must have been there all the time in some form, from the very birth of the universe.

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    $\begingroup$ Conservation of energy cannot be applied to an entire universe governed by General Relativity. $\endgroup$ – Rob Jeffries Aug 7 '19 at 8:23
  • $\begingroup$ How do you know? $\endgroup$ – Michael Walsby Aug 7 '19 at 8:55
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    $\begingroup$ As space expands, the dark energy density does not decrease, that is, more dark energy is created. See medium.com/starts-with-a-bang/… $\endgroup$ – PM 2Ring Aug 7 '19 at 13:29
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    $\begingroup$ A physical law is simply an empirical statement that seems to hold for a given data set. Theories are usually developed to support the law in its context, and make predictions beyond that data set. There's no reason to expect the thermodynamic laws hold at the cosmological scale when the observations don't support them. $\endgroup$ – Sir Cumference Aug 7 '19 at 23:53
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    $\begingroup$ I don't think there is a problem if the facts are well referenced. $\endgroup$ – Keith McClary Aug 8 '19 at 17:48

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