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Does the big bang theory conflict with the law of conservation?

  1. Aug 29, 2012 #1
    Perhaps I misunderstand the big bang theory but doesn't it basically hold that nothing existed prior to the big bang? If there truly was no energy or mass prior to the big bang then doesn't that conflict with the law of conservation of mass and energy?
     
  2. jcsd
  3. Aug 29, 2012 #2
    The big bang is about the early history of the universe, not the origin of the universe.

    The big bang explains how the universe expanded from a small, dense point.

    Where did that small, dense pointcome from? That's another thing entirely.

    Edit to add: This is explained in many public sources like this one http://en.wikipedia.org/wiki/Big_Bang
     
  4. Aug 29, 2012 #3
    Okay, thank you for the clarification.
     
  5. Aug 30, 2012 #4

    Chalnoth

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    Well, neither mass nor energy are perfectly conserved. Mass isn't a conserved quantity at all: we can and do generate new mass all the time in particle accelerators. Energy isn't conserved in curved space-times, such as an expanding universe.
     
  6. Aug 30, 2012 #5
    I wasn't aware of this. Could you elaborate?
     
  7. Aug 30, 2012 #6

    Chalnoth

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    Here's an in-depth analysis:
    http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

    A super-short way of saying it, though, is that energy can't be conserved because it is a coordinate-dependent quantity. And in General Relativity, coordinate-dependent quantities aren't "real". While you can define energies that are conserved in certain, specific situations, those same energy quantities tend to either not be conserved in other situations, or even fail to be conserved just from changing the coordinates.
     
  8. Aug 30, 2012 #7
  9. Aug 30, 2012 #8

    Chalnoth

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    Its fits with my previous statement. It is possible, under certain conditions, to define an energy such that it is conserved and also always zero for an expanding universe. It still isn't a general statement.
     
  10. Aug 30, 2012 #9
    Really? It may seem like particle accelerators create matter but isn't it really just the transfer of energy? Doesn't the law of conservation allow for the transfer of energy to matter and vice versa?
     
  11. Aug 30, 2012 #10

    Chalnoth

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    There are many conservation laws. I was pointing out that mass isn't one of them. Yes, particle accelerators obey the law of conservation of energy, because those experiments are performed in (approximately) flat space-time. And the small amount of curvature that is imposed by the Earth is easily handled by considering gravitational potential energy.
     
  12. Aug 30, 2012 #11
    To add to what Chalnoth said, you can in some cases define energy globally in GR, but this usually involves (but not always, there are ways of defining energy such as Bondi mass which are perfectly valid) using pseudo-tensors (as in the case of Berman).

    There is a FAQ about this:

    https://www.physicsforums.com/showthread.php?t=506985
     
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