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Is energy really conserved?
Why can’t energy be created or destroyed?

One of the laws of the universe that dazzles me the most is the law of conservation of energy. I however have a couple of questions regarding this law. Since Einstein's equivalence tells us that matter and energy are different manifestations of the same thing:

  • Does this mean that the amount of energy + matter has been the same since the beginning of the universe?

If this is true, then another question pops up. Since the universe is expanding at an accelerating rate, and the law of conservation of energy tells us that the amount of energy can't increase:

  • does this mean that the universe is getting emptier?

Also, an intertwined question:

  • Is there some sort of average 'energy density' in the universe? If so, can we notice the effects of the accelerating expansion of the universe by looking at this 'energy density'? (this might sound like/ be a stupid question).
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marked as duplicate by Sklivvz, Qmechanic Dec 23 '12 at 17:16


To calculate average energy density, you have to account for the radiation as well. I presume that this would be a difficult calculation. From an experimental point of view, there have be no reports of violation of energy conservation that I know of. Reasonable question, +1. –  Antillar Maximus Dec 23 '12 at 15:17
Possible duplicates: physics.stackexchange.com/q/19216/2451 and links therein. –  Qmechanic Dec 23 '12 at 17:16
@Qmechanic That was totally useless. –  user14445 Dec 23 '12 at 17:19
Well, a good strategy to not get a question closed as a duplicate is to mention potential Phys.SE duplicates in the question formulation, and emphasize why they do not answer your question in a satisfying manner. If OP (or someone else?) does so, I will be happy to re-open the question. –  Qmechanic Dec 23 '12 at 17:31