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

CMB frame

  1. Jun 2, 2006 #1
    How to understand that the cosmic microwave background (CMB) can define a cosmological frame?

  2. jcsd
  3. Jun 2, 2006 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    roughly on average it should look the same temperature in all directions.

    if you are moving, then it will look hotter ("blue-er") in the direction ahead of you
    and it will look colder ("redder") in the direction behind you.

    the hotspot was already found some time ago----I think in the 1980s----by a microwave observatory carried to high altitude by a U2 airplane. they measured the temperature in all directions and they found that it was roughly A TENTH OF A PERCENT warmer in the direction towards the constellation Leo

    that is called the CMB "dipole"
    so the people with the converted U2 highaltitude "spy" aircraft were able to measure the dipole.
    Later with satellite observatory people could measure it more accurately and also find more complicated splotchy variations in the temperature and make maps of the CMB. The splotchy maps they publish usually have the dipole variation already subtracted out---so you dont see it in the maps.

    now that is the basic picture and you may know all that and be asking something more complicated and sophisticated. What would your follow-up question be?

    the doppler shift in temperature corresponds to our going roughly at the speed 350 km per second in the direction of Leo. So to calculate a REST FRAME we have to imagine something that is going 350 km per second RELATIVE TO US back in the opposite direction from Leo, to COMPENSATE. If we could be on that train then we would not see any dipole. Then the CMB would look roughly same temperature in all directions.

    Actually the comological rest frame idea is very old. It goes back to BEFORE PEOPLE SAW CMB.
    The renowned Edwin Hubble himself saw it. As observed from earth, the Hubble Law has a dipole.
    the hubble law says recessionspeed is proportional to distance
    v = Hd
    but unless you compensate for earth's motion, this is not true. the galaxies in the Leo direction are RECEDING MORE SLOWLY than the law says they should because we are "trying to catch up with them" by our own motion relative to the expansion of the universe
    and the galaxies in the opposite-from-Leo direction are RECEDING MORE RAPIDLY than the law says because our own motion, relative to the universe, adds to the observed recession velocity.
    So to get the hubble law to work out perfectly one has to compensate and subtract off our velocity vector of 350 km per second.
    It is just a small correction compared with most galaxy recession speed so it hardly matters.

    Also I am writing from memory, and it might not be 350. It might be 370 or something. But I think it is in the range 350-370 or somewhere around there.
    and I say "earth's" motion but really mean a complicated combination of the sun's motion plus a small contribution from the earth's orbital motion around the sun (which changes thru the year). the main part of that 350 or 370 is the sun's motion relative to CMB, i.e the same as relative to the expansion of the universe.

    Now Helen Wang, that is a nice question. Do you have a follow-up question? Does your saying HOW TO UNDERSTAND have a deeper curiosity attached to it?
    Last edited: Jun 2, 2006
  4. Jun 2, 2006 #3
    Yes, the velocity explanation is accepted by most of the researchers. What is the best evidence for that?

    Besides, it sounds like, to me, a bit circular argument. You define a CMB rest frame, and then calculate the velocity dipole according to the Doppler effect. What if there is something more in there?
  5. Jun 2, 2006 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Indeed there might be something more in there, but it would be very small. Besides the main part of the dipole due to the sun going somewhere, there might be some temperature fluctuation JUST LIKE all the other temperature fluctuation which we see all over the sky. but that other stuff is very small compared to the dipole.

    Here is your first question again, because I am trying to understand the underlying thing that you are getting at with your questions. What are you challenging and what do you think might be happening that the conventional analysis overlooks?
    Hah! I thought you might have a follow-up.
    Your original question was HOW CAN IT DEFINE and that is a human convention thing: how scientists use it to define the cosmological rest frame.

    I think I answered that enough. Yes?


    Do you know the Friedmann equation that cosmologists ordinarily use to model the universe?

    the solution to this equation is the FRW metric. (Friedmann Robertson Walker metric).

    this model of cosmology, that they use, has an obvious rest frame, and it has expansion------in the sense that the distance between two points which are at rest is always increasing.

    The Friedmann model is just a simplified version of the Einstein equation of General Relativity----simplifed enough to get a solution. so basically (in the simplified case---"homogeneous and isotropic") it is that the Einstein equation of GR tells us to expect a rest frame and to expect this kind of expansion.

    so I would first ask you a question. IS THE FRIEDMANN MODEL ALL RIGHT WITH YOU? Or do you challenge the Friedmann model and do you challenge General Relativity as a good theory of spacetime geometry and gravity?

    I think that is the important issue to settle. the Friedmann equation is the basis of the current consensus model called "Lamda-CDM" which is just one particular version of the Friedmann model and the FRW metric.
    We dont have to get into particulars about it. If you are OK with Gen Rel that means you are probably OK with the Friedmann model in general. And then the CMB rest frame and stuff like that are easy consequences.

    But if you don't like Gen Rel and dont accept the Friedmann model then it all gets a lot harder. Maybe I should think about your question some. Or perhaps you will discuss your questions some more and elaborate.
    Last edited: Jun 2, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook