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B How do physicist determine the energy of the universe?

  1. Apr 23, 2016 #1
    I have heard popularized physic claims that the total energy of the universe may be zero. Supposedly positive mass energy plug negative field energy eg gravity cancel.

    How is this claim supported by science?
  2. jcsd
  3. Apr 23, 2016 #2


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    Science Advisor

    It stems from a particular way of writing down the equations for General Relativity.

    Normally, in General Relativity, gravitational potential energy isn't used, which means that energy isn't conserved in General Relativity. It is possible, in certain specific circumstances, to add a gravitational potential energy back in to balance the books and keep total energy conserved*. Specifically, if you have a closed universe. If you use this formulation (called the Hamiltonian formulation), then the total energy is zero.

    * It's not always possible in General Relativity to even write down the total energy of a system, which is part of the reason why energy isn't conserved.
  4. Apr 23, 2016 #3


    Staff: Mentor

    Which are not acceptable sources here on PF.

    It depends on what you mean by "the total energy of the universe". Strictly speaking, there is no such thing in General Relativity; the spacetime model used to describe the universe as a whole does not have the properties it would have to have for a meaningful "total energy" to be defined. Sean Carroll discusses the reasons why in this blog post:

  5. Apr 24, 2016 #4
    Peter, a quick questions regarding popular science as an acceptable source. I totally appreciate that trying to establish some science that isn't supported in the literature via a popular science source is not okay. However it seems to the OP was trying to ask about the view expressed in popular science and see if they are correct. In order to do this they must refer to popular science and hence use it as a source for that purpose, is that not okay? Furthermore I notice you referred to Sean Carroll's blog which I think is excellent but I note is a popular science source.
  6. Apr 24, 2016 #5


    Staff: Mentor

    And the usual answer is, try to find a better source. Note that the Carroll blog post I linked to, while it is a blog post, links to actual peer-reviewed papers. Pop science sources can certainly lead you to good information, and in such cases, the pop science source itself might give a good heuristic summary of the information (as Carroll's blog post does). Also, note that Carroll gives an actual equation, ##\nabla_\mu T^{\mu \nu} = 0##, and explains what it means in layman's terms. You can look in any GR textbook and find that equation, along with all the detailed math connected to it; so you don't have to take Carroll's word for it, you can check his explanation for yourself against the actual science. So if someone started a PF thread about Carroll's blog post, we would know what actual science it was based on.

    But many pop science sources--even books and articles by scientists--don't do that. They just present a pop science claim, like "the total energy of the universe is zero", without giving the reader any way to check the actual science that underlies the claim, or even check whether or not there is any. So if someone starts a PF thread based on such a source--like this one--we have no way of knowing what, if any, actual science that source is even using. We can try to guess (and note that, even though I discouraged using pop science sources, I also tried to guess, as did Chalnoth); but that's going to be a lot less productive than having the actual science already there to be checked at the outset.
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