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

Cosmological Redshift

  1. May 11, 2003 #1

    (Q)

    User Avatar

    I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?
     
  2. jcsd
  3. May 11, 2003 #2

    drag

    User Avatar
    Science Advisor


    Space-time expands and the EM waves do too.
    I'm not certain about how the energy is
    treated, I suppose you could say nothing
    dissappears and it's just like normal
    space-time curves or there are those theories
    dealing with quentessense or something like
    that and say that it "fuels" the expansion.

    Live long and prosper.
     
  4. May 11, 2003 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    this question is one reason why physicforums is the greatest thing since sliced bread. this is a great question

    i want to hear what some gurus say about this (tom, damgo, etc)


    note that your questions assumes that it goes somewhere
    which means you are assuming a conservation law
    but there is no energy conservation law in GR, I think.
    or any momentum conservation law either.

    you are assuming that if energy is lost (from the light being stretched out and getting a longer wavelength) that it must
    GO somewhere----because in local SR physics energy is conserved.

    but expansion of space is a GR thing----so you must ask if
    there is a conservation law in that context. Maybe there is!!!
    I am no expert. But I do not know of one, for whatever that is worth.

    I heard a neat thing about GR. An amoeba can move in curved space merely by changing shape.

    this seems to contradict conservation of momentum

    he does not need any rockets to push him. he just changes is shape a couple of times and behold! he is in a different place.

    this was at an MIT archive of recent online physics papers or something, they thought it was cool. It is good peer-reviewed physics even tho it sounds wacky.

    so if anybody thinks he knows of a conservation law for momentum in GR he will have to explain this traveling amoeba

    (Q) you are great for asking this question. maybe it will get a good answer from the likes of damgo

    love it! :))
     
  5. May 11, 2003 #4

    Labguy

    User Avatar
    Science Advisor

    I think that Redshift is a change in the wavelength of the light reaching us, not a change in the energy of any particular photon(s) at all.

    Remember, the Alien looking from the "other side" sees the same light source blue-shifted, same light source = same photon energy. The photon energy and the perceived wavelength are two different animals.
     
  6. May 11, 2003 #5
    But doesn't the relation between wavelength and energy for a photon imply that when a photon is Doppler shifted (from our point of view) that the amount of energy we measure for it has changed? So the photons reaching the alien actually have more momentum (if absorbed) than photons absorbed over here.

    Is an "apparent" change in the energy of a photon different than an actual one (what is an "actual" one, anyway)?
     
  7. May 11, 2003 #6

    drag

    User Avatar
    Science Advisor

    Re: Re: Cosmological Redshift

    WHAT ?!?!?!?!?! LINK, PLEASE !
     
  8. May 11, 2003 #7

    FZ+

    User Avatar

    Oh that... well... it's not entirely accurate. It's a study of how amoebas move in water that gave a theoretical way of cheating newton's 3rd law - if space is curved, you can exploit the gravitational gradient by generating motion through moving the craft's centre of gravity. It's sort of akin to tacking a yacht in a headwind....
     
  9. May 11, 2003 #8

    LURCH

    User Avatar
    Science Advisor

    Re: Re: Cosmological Redshift

    Not so. An alien on the other side would see the light red-shifted, just as we do. If he is the same distance away, he'll see the red-shift about the same amount, too.
     
  10. May 11, 2003 #9

    Labguy

    User Avatar
    Science Advisor

    Re: Re: Re: Cosmological Redshift

    Whoa? Let's say we are here and the alien is way over there, and moving about equally relative to us. Place a fast-moving light source smack in the middle between us and the alien. The light source moves quickly away from us and toward the alien. We see a "redshift" (wavelength change) because its moving away from us. The alien sees a blueshift because it is moving toward him.

    Why is this "not so"?

    I am talking redshift from motion, not gravitational redshift.
     
  11. May 11, 2003 #10

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Re: Re: Re: Re: Cosmological Redshift

    Then you are talking SR and there is a conservation law.

    Say a receding spaceship is flashing lights at us. In the spaceship frame the light is not redshifted and retains its energy.
    In the earthbased frame looking at the spaceship flashing lights at us, the light was always of a lower energy lower frequency longer wavelength, so it retains its energy.

    Everything is normal.

    But this guy (Q) was talking about the COSMOLOGICAL redshift which happens from space stretching out while the light is under way towards us. this is a GR thing. It is not relative motion of source and receiver, but an effect of expansion of space itself.

    He is saying 'what happened to all that energy?' I think IMHO that it just went away. What a huge amount of energy to have just ceased to exist! All the light from all the galaxies, for the whole history of the universe, most of which is still flying on its way somewhere, has been deprived of a large percentage of its energy!

    Hey drag! I do not have the URL. There is an MIT archive of online physics goodies and I found it there. Maybe I will come across it again. Of course it was not a real amoeba, I just called it that. The journal article was about an object that can change its shape and, by using locally curved spacetime, can move around by changing shape thru a cycle that leaves it the same original shape but in a different place. If I see it again I'll post it.
     
    Last edited: May 11, 2003
  12. May 11, 2003 #11

    drag

    User Avatar
    Science Advisor

    Re: Re: Re: Re: Re: Cosmological Redshift

    Well it seems like an EXTREMELY "slow and
    painfull" proccess, unless you can do
    some near c movements of course but that's
    a bit too much for an amoeba I guess.
     
  13. May 11, 2003 #12

    FZ+

    User Avatar

    Oh it is extremely slow... but there is speculation on using nanotechnology to do this on a large scale....
     
  14. May 11, 2003 #13

    Labguy

    User Avatar
    Science Advisor

    Marcus said:
    And also some things about all of the "missing energy".

    Ok, I'll buy that in the context of "cosmological" redshift. But, even GR, Hawking, Wheeler, et al won't throw out thermodynamics; they require it. So, the energy takes an equivalent transformation to "something". Your (Marcus') description of the huge amount of energy available from this effect since expansion began might be "the" source of the "repulsive force" ("dark energy"). Just thinking out loud. This might seem to fit in with what Linde recently proposed, which was posted a day or two ago.(Expansion then contraction)
     
  15. May 11, 2003 #14

    LURCH

    User Avatar
    Science Advisor

    Re: Re: Re: Re: Cosmological Redshift

    But that's just it; he won't be (moving equally, that is). If things at great distances from one another were stationary relative to one another, there would be no cosmological redshift.* If we look at a light source one billion lightyears away, and an alien is also one billion lightyears away from that same light source (but on the "far side), he will see about the same redshift as we. If the alien were to look at us, and we at him, we would each measure the other's redshift to be about twice what it is for the intermediate object (the light source).


    *At least, not in theory.
     
  16. May 11, 2003 #15

    russ_watters

    User Avatar

    Staff: Mentor

    Therein lies the problem - the photon doesn't change its wavelength. It is created at that wavelength because the source is moving. There is no missing energy.

    Doppler shift works the same way for sound as well. With a moving source, the sound is created with a doppler shift.
     
  17. May 12, 2003 #16

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Take a galaxy that we see receding, and its light red-shifted. People living in that galaxy do not see their light redshifte, it seems normal to them and it doesn't occur to them that any energy has been lost. What they do see red-shifted is our light. And maybe they ask where the energy went in our case.

    Neither we nor the aliens are speeding, the distance beteen us is growing and it produces the effect of speed including red-shift. The stars in galaxy X are glowing with their usual spectra.
     
  18. May 12, 2003 #17

    drag

    User Avatar
    Science Advisor

    selfAdjoint and russ, if you maintain a certain
    permanent rest frame in the Universe you will
    see the expansion of space-time decrease the
    energy of at least photons and possibly
    also the kinetic energy of all rest-mass particles
    (though the rest masses appear to stay the same).

    Live long and prosper.
     
  19. May 12, 2003 #18

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    the original post by (Q) called attention to the energy that has been lost from the universe by wavelength stretching caused by expansion of space.

    this energy can be calculated---most simply in c=G=hbar=k=1 units.

    Almost all the redshift energy loss since year 300,000 after BB has been from CMBR photons which were released around year 300,000 and have experienced redshift 1100.

    This loss is overwhelmingly greater than that in light emitted by galaxies.

    The temp of CMB is 2E-32
    The energy density (energy per unit vol) of CMB is equal
    to the fourth power of the temp, multiplied by π2/15.
    that factor is about 2/3
    So the density comes out close to E-127
    and multiplying by 1100 one gets about E-124.
    It is a small energy density compared with dark energy
    have to go will proofread later
     
    Last edited: May 23, 2003
  20. May 12, 2003 #19

    russ_watters

    User Avatar

    Staff: Mentor

    Aether? Unnecessary. Just like sound - but now both the observer and the source are moving. And it doesn't matter what frame of reference you use (Relativity) you still get the exact same doppler shift. The frequency of the light does not change while it is traveling unless acted upon by a gravitational field.
     
  21. May 14, 2003 #20

    (Q)

    User Avatar

    Labguy

    I think that Redshift is a change in the wavelength of the light reaching us, not a change in the energy of any particular photon(s) at all.

    A change in the wavelength IS a change in the energy.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?