Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Physics laws and expansion of universe?

  1. Jan 7, 2007 #1
    my interpretation of the expansion of the universe might be naive... correct me if I make any mistake.

    So, space in general is expanding, or being created all over the universe. If you have a photon traveling in free space, the frequency gets shifted. But Energy=h*frequency; the energy of the photon decreases as it travels. Where did the energy of that wave go?

    Similarly, what about momentum? p=E/c, is that not conserved as well?

    I think it has something to do with reference frame. How can one resolve the paradox?
    Last edited: Jan 7, 2007
  2. jcsd
  3. Jan 7, 2007 #2
    This question is not uncommon, and there is no good answer at this point.

    Conservation of energy and general relativity have not been reconciled.
  4. Jan 8, 2007 #3
    Yes, this is quite disturbing to me.
    Energy conservation has to do with time symmetry. Since there is an expansion, there is an arrow of time, so the symmetry is broken.
    If energy cannot be defined, then how about entropy?
    If energy and entropy cannot be defined, we cannot do much thermodynamics with the universe. There will be no 1st law and 2nd law of universal thermodynamics. And yet in thermodynamics, we often need to consider the entropy of the universe and things like that. Really strange and mind boggling.
    Yet we can define a temperature of the universe, flatness of the universe. So it seems at least we can come out with an equation of state of the universe.
  5. Jan 8, 2007 #4
    I would say perhaps that energy goes into the expansion? Though cause and effect kind of go out the window with that one
  6. Jan 8, 2007 #5
    Hey the apparent frequency of the photon changes.I don't think the real frequency of the photon changes.Please correct me if im wrong
  7. Jan 8, 2007 #6
    what i believe is that mass is a form of energy.Mass occurs between a specific range of energy density but the range is not clearly defined.Even if energy density is slightly different characteristics of mass may occur.All energy forms differ due to energy density.And when an object attains relative velocity the energy density tends to increase which gives an effect of mass
  8. Jan 8, 2007 #7
    The photon frequency does change really, not apparently.
    The universe expansion actually stretches the photon lengthening its wavelength.
  9. Jan 8, 2007 #8
    and since the rate of expansion of the universe is increasing, a photon's frequency will eventually be red-shifted to near zero... so its energy is gradually being drained...

    Also, if the space is expanding, then distance stars will move farther and farther away from us (accelerated frame of reference). yet, in that distance stars' frame, there will be no fictitious force due to that apparent acceleration... does it mean the equivalence principle is broken?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Physics laws and expansion of universe?
  1. Universe expansion (Replies: 6)

  2. Universal expansion (Replies: 37)