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

Expansion and Redshift

  1. May 28, 2004 #1
    In my last Cosmology class, the professor explained how the expansion of the universe increase the wavelength of the light. But his explanation was a non-local one, I mean, you need to compare the time beween two wavecrests to determine that the light was redshifted. The explanation was that the expansion of the universe stretch the space between the two wavecrests. But what is the local explanation. What if I observe just one photon? I supposed that it should be redshifted two, but then the explanation given by him does not apply. What's the QM explanation in terms of the energy of the photon?
  2. jcsd
  3. May 31, 2004 #2


    User Avatar
    Science Advisor

    I think, strictly speaking, there is no local explanation, because expansion depends on distance (Hubble law). Thus, there is no measurable expansion if the considered (spatial) distance between two events is zero.

  4. Jun 1, 2004 #3
    But then, how can a unique photon be redshifted?
  5. Jun 1, 2004 #4
    Well if distances are increasing, then the distance between crests of wave peeks are increasing as well. So the further a photon travels, the more its peeks are extended. I suppose that the distances between atoms will remain the same even though space in general increases simply because the constants of nature remain the same which means the equations give the same distance. Otherwise, if every distance was increasing, then we would probably not be able to preceive it.
  6. Jun 4, 2004 #5
    A particular photon would redshift because it loses energy as it propogates. It initially started deep in a gravitational potential well (at the time of the big bang). The total energy of the photon is conserved, so as it escapes the well, it loses a portion of the energy corresponding to its wavelength (E=hc/wavelength).
  7. Jun 4, 2004 #6
    so which is supposed to be responsible for redshift again?

    the doppler effect was brought up to say that galaxies were flying away from each other "intact" so to speak. this was an indication that the universe was expanding? or at least the matter in it was moving away from itself in clumps (galaxies).

    now somewhere along the line between Hubble and now we decided that space itself is expanding, not just "objects" flying away from each other, then we say that the expanding space is doing the redshifting (and or not the Doppler effect?)


    Why is this confusing??

    are you talking about "tired light" or the Shapiro effect, where a light loses energy as it passes gravitational fields? Why would it lose energy as it propagated through vacuum?

  8. Jun 4, 2004 #7


    User Avatar
    Science Advisor
    Gold Member

    local effects are much more powerful than gravity. the nuclear strong and weak forces, as well as electromagnetism, overwhelm gravity at short distances by many orders of magnitude. the cosmological constant is even weaker than gravity, thus it is not a player until you go to cosmological distances. an anthropic principle thing.

    universal expansion agrees with observation on many levels [even more than currently predicted]. quantum theory insists upon a non-static universe. thus only two states are possible... it either expands or contracts. the expansion model is attractive and logically consistent with what we observe. an expanding universe must include increasingly complex structures [re: information theory] to preserve causality. this is the history of the universe. it came into being as a highly ordered state and has steadily evolved into balanced states of increasing order and disorder. this, i believe, is absolutely necessary to preserve a net zero state of entropy.
  9. Jun 4, 2004 #8
    It would seem that the present laws of thermodynamics only comes into play once particles are formed. So we have no thermodynamics that applies to the process of first particle creation. This is because we define entropy in terms of the energy of particles assuming they already exist, so there is no definition which applies to the creation of the first particles. There is a conservation of energy as particles collide, interact, and change form. I wonder what the entropy is of particle interaction or decay? How does the entropy increase as an electron and a positron meet to form photons?

    But it seems to me that if energy is conserved even as the first particles came into existence, then the energy gained by the first particles would correspond to a loss of energy somewhere else, probably from the fabric of space-time itself, where else? There is then the possiblity of developing thermodynamic between created particles and the fields/space from which they come. Though I'm not sure which ones entropy is increasing and which is decreasing in the process of first particle creation from whatever. In any event, if something gains energy as another losses it, then wouldn't the entropy of one increase while the other decreases, assuming thermal equilibrium, whatever that means without particles?
    Last edited: Jun 4, 2004
  10. Jun 4, 2004 #9
    The Doppler shift for sound is a change in the distance between crests in sound waves. However, light is not a wave. The Doppler effect for light refers to the energy of the photons. Photons with relatively low energies in the spectrum are towards the red side.
  11. Jun 6, 2004 #10


    User Avatar
    Science Advisor
    Gold Member

    i dont disagree with your argument mike. i believe the rules of quantum physics and and relativity are emergent... they existed prior to the reality we perceive. why? i really have no clue. but, if you accept this premise, it explains everything. this universe exists because it is the most probable state available. and we, somehow, are necessary for it to exist. a universe without sentience is a huge waste of time and space.
  12. Jun 7, 2004 #11
    The forming of the structure of a particle would have to represent a lowering of entropy. That information is probably stored in some sort of topological invariants of the geometry of the particle. For the characteristics of that structure do not depend on its orientation or speed through some coordinate system but on properties independent of such. Of course that would mean that information was lost somewhere else. Now before the first particle was created, the universe must have been a manifold in and of itself. If the original dimensions were all curled up, then the original manifold must have been closed and as such had topologially invariant characteristics of its own. So I wonder if topological invariants are changed or lost if an original manifold opens up so that it acquires a boundary. Does "information" transfer from the original closed manifold to the newly formed boundary in the process of opening? We would then say that the original manifold of spacetime lost information/gained entropy to the newly formed boundary particle which gained (at least a portion of that) information. Thoughts anyone?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook