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Conservation of energy entails that all energy in the present has existed for all time. Therefore, this implies that the big bang was initiated with preexisting energy. Where did this energy come from?

Qmechanic
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mcchucklezz
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2 Answers2

-1

There is no energy conservation in General Relativity which can be seen for instance at the big bang (as OP mentions). But also the redshift / blueshift of a photon due to expanding or shrinking of spacetime violates energy conservation.

The Noether theorem tells us that a conserved quantity (energy in your question) is connected to a symmetry (time translation symmetry). Since this symmetry is not generally true in GR (e.g. at the big bang, the creation of spacetime itself) energy conservation does not hold.

Sean Carroll has a nice explanation on his blog. http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Noldig
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  • Doesn't this imply something troubling? If laws of physics can be violated, do we have any way to understand the universe? – mcchucklezz Mar 14 '18 at 15:02
  • The laws of physics are not violated, energy conservation is just a very useful concept on most of the scales we are working at, especially in effectively flat space. On cosmological scales energy conservation is not a law of physics, and we understand why not and can exactly calculate the "violation". There is more information in the link – Noldig Mar 14 '18 at 15:05
  • Did you mean to say no energy conservation in GR? – Daniel Underwood Mar 14 '18 at 15:23
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    especially the big bang violates conservation of energy This doesn't make sense. The reason conservation of energy doesn't apply to relativity is that there is no way to define the total energy of a system in GR (except in certain special cases). Since you can't define it, it doesn't make sense to say that the big bang violates its conservation. This statement also doesn't make sense because in classical GR the big bang is singularity, and only times after the singularity exist. Therefore we can't compare conditions before the big bang with conditions after. –  Mar 14 '18 at 15:28
  • Since this symmetry is "broken" by the big bang (the creation of spacetime itself) energy conservation does not hold. This also doesn't make sense. Noether's theorem gives us conservation laws because of symmetries of the laws of physics. Solutions to the equations can be asymmetric, and that has nothing to do with Noether's theorem. –  Mar 14 '18 at 15:29
  • this is why I put broken into "". I know that it is not broken in a sense like SSB. I wanted to give a slightly more intuitive answer with some examples where one can see how energy conservation is broken. – Noldig Mar 14 '18 at 15:37
  • I edited my answer, I don't know enough about GR in order to incorporate the part on the definition of total energy so I left it out. – Noldig Mar 14 '18 at 15:55
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The answer to this is the same answer to the question: 'Where did all the material in the universe actually come from in the first place?'

And that answer is: At some point, all of it must have literally popped into existence from absolutely nothing.

EDIT: Some may try to claim it's ALWAYS been there. But that doesn't change the fact that, at some point, all energy and material literally must have popped into existence from nothing. Because before the alleged Big Bang, there probably existed only infinite space. Infinite space has ALWAYS existed (why literally infinite space exists in the first place is another brain-melting debate), but all material and energy has not always existed.

At the very LEAST, the primordial singularity, that alleged point smaller than an atom which contained all the material and energy in the universe, must have popped into existence from nothing.

White Prime
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