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Big Bang and Doppler

  1. Jul 16, 2008 #1
    Wouldn't the Doppler effect stop us from seeing the beginning of the universe unless it starts approaching us? (or unless the universe stops expanding and begins retracting?)
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  3. Jul 16, 2008 #2


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    the cosmological redshift puts a limit on how far back
    in the standard treatment of cosmology they make a distinction between the redshift and a Doppler effect of motion
    the redshift is not a Doppler but is associated by the expansion factor during the time the light was traveling on its way

    but the bottom line is, whatever you call it, the REDSHIFT does put some kind of limit, depending on the sensitivity of the instruments.

    because light shifted to longer and longer wavelength eventually becomes difficult to detect and extract information from

    but the redshift is not the ONLY obstacle. as far as observing light (instead of say gravity waves and neutrinos) we can only see light that comes from when expansion was about 400 thousand years old
    because before that the universe was opaque to light

    the oldest light we can see is redshift 1100, the CMB light from about year 400,000, which is shifted down to around microwave wavelengths-----the wavelength of the original light has been stretched out by a factor of 1100.

    no older light could get thru because the universe was opaque just like the sun is opaque (the gas is glowing partly ionized, so it is not transparent the way cooler neutral gas is)

    so the main frustration is NOT THE REDSHIFT, it is the fact that earlier space was full of un-transparent gas which blocks our vision.

    but be happy we at least have the CMB. the material that sent us the CMB light was receding from our material, when it sent the light, at a speed of about 57 times the speed of light, and it is currently about 45 billion LY away and receding at about 3 times the speed of light. If I remember correctly.

    it is quite usual for us to be able to see things which were receding several times lightspeed when they emitted the light and which are still receding faster than light.

    superluminal recession speed does not necessarily prevent our seeing objects. if you want to understand this read the Lineweaver SciAm article in my signature. it is a good non-mathematical article with clear diagrams

    A cosmology calculator is here:
    you can plug in parameters like 71 for the hubble and 1100 for the redshift and get out speeds like the thing was initially receding at 57 times the speed of light, and was 40 million lightyears away etc etc. Ask if you need coaching on how to use the calculator.
    Last edited: Jul 16, 2008
  4. Jul 16, 2008 #3
    So what you're saying is that at a certain point, the expansion rate was so great that the wavelengths are shifted beyond the known spectrum?
  5. Jul 17, 2008 #4


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    No, he's saying is that the matter in the universe hasn't always been transparent. According to http://map.gsfc.nasa.gov/news/ the universe became transparent around 3.8*105 years after the big bang. That's why the oldest light that's still around is from that time. That light is definitely detectable. See e.g. the image on that web page.

    Since that's the oldest light around, there's no light that's redshifted more. So light from e.g. the oldest stars must be redshifted by a factor that's less than 1100. (Probably much less, but I don't know how much).
  6. Jul 17, 2008 #5


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    the oldest galaxies have redshift around 7 (rough estimate), apparently
    star-forming only began when expansion was several 100 million years old

    thanks for responding to Dobro's question, Fredrik. I completely agree.

    Dobro, try out Morgan's calculator---the link is in my signature.
    Put in 0.27 for matter
    0.73 for lambda (i.e. dark energy) and 71 for Hubble
    then the calculator will model the standard universe of cosmology for you.

    if you put in any redshift it will tell you the original distance---and recession speed---of the matter which emitted light
    that we now see with that redshift.

    put in z=1100 and it tells you that the CMB is from matter that was 40 million LY away and receding at 57 times the speed of light.
    we are currently receiving that light and it is redshifted by a factor of 1100, which is certainly not off the scale!
    a redshift of 1100 only takes you from reddish sunlight or lightbulb light down the ordinary spectrum to long infrared and microwave

    a factor of a thousand is just micron to millimeter

    redshift is best understood if you do not think of it as Doppler----if you want to teach yourself, forget Doppler for a while and read the Lineweaver SciAm
    Last edited: Jul 17, 2008
  7. Jul 21, 2008 #6
    I see....that is some mind-boggling stuff.

    What makes something opaque? And thanx for everybody's patience with my newbie questions lol.
  8. Jul 21, 2008 #7


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    to be more precise what I was talking about was how when a gas gets hot enough to partially ionize it becomes much more able to SCATTER any light that tries to get thru.

    so if you have a thick enough layer of partially ionized gas, you wont be able to see anything through it.

    The sun is mostly just hydrogen gas. Hydrogen is normally transparent. The reason we cannot see into the inner bowells of the sun is because the outer layer is hot, like 6000 kelvin, and partly ionized. So it scatters visible wavelength light like crazy and any light that did get thru would have been scattered so many times on the way that it would be totally randomized.

    Or like a neon sign in front of a bar or strip joint. If it is turned off, the neon is cool and not ionized---thus transparent. But if you turn it on the neon in the tube ionizes and you cant see clearly thru it.

    To be very careful we might not want to call that opaque. But whatever you call it, you can't see thru.

    So that is like the hot hydrogen gas layer called the surface of last scattering, from which the CMB comes. It was a bit over 3000 kelvin, back then. that is not as hot as the surface of the sun but already partly ionized with potential to scatter light.

    but that was like the threshold. a little more expansion allowed the gas to cool just enough so that it lost enough ionization that it was no longer scattery---then whoops any light that was already in motion could continue on without being scattered!

    you should be asking yourself why ionizing a gas (like hydrogen or neon) makes it more able to scatter light-----essentially why are loose electrons floating around loose in the gas more able to wiggle in sympathy with a wave and steal or deflect its energy

    why are conductive things often more reflective. they too have loose electrons able to repond to waves and redirect them. one can think of a partially ionized gas as more conductive and hence more reflective.

    it is something you could take to the general physics forum and start a thread about.
    Like ask them "why does ionized gas scatter light more than neutral gas?" or something like that.


    good questions, I sincerely hope the above helps some.
    Last edited: Jul 21, 2008
  9. Jul 21, 2008 #8


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    In this case, the answer is "charged particles", and in particular "electrons". The universe was so hot and dense back then that atoms couldn't exist for very long. If an electron and a proton combined to form a hydrogen atom, it was very likely that a collision with something else would smash the atom to pieces very soon. Because of that, there were plenty of free electrons flying around, and photons (light) interacts with electrons. The free electrons absorbed most of the photons, but the universe kept expanding and cooling, and after a while it was cold enough for atoms to stay intact much longer. That made the universe transparent because atoms are electrically neutral and therefore much less likely to absorb low-energy photons. High-energy photons would still interact with the component particles of the atom, but the photons with lower energy would just see the atoms as neutral particles with irrelevant internal structure and therefore not interact too much with them.

    Edit: Man I'm slow...I started writing this before Marcus posted his answer (but I also left the computer for a while during that time).
  10. Jul 21, 2008 #9
    G'day from the land of ozzzz

    Marcus said

    Part of email by Prof Oliver Manuel

    Marcus you can check this out.

    Compaction of Neutrons holds the key to the formation and long life of stars and the cyclic process that involves active galactic nuclei that reforms galaxy formation.
  11. Jul 21, 2008 #10


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    You are also writing clear concise style. You said it in a lot fewer words. If I had known you were responding I would have kept quiet. In any case thanks. The answer is definitely charged particles.
  12. Jul 22, 2008 #11
    Alright it's official; I'm in way over my head. :redface:

    I think I'll focus on the basics before trying to understand the news from deep space. I should mention I've only done 2 university level physics courses, but it can't hurt to explore the leading edge of physics. Thanks again for the answers & links, they've been very useful. I'll try not to jump to conclusions in the future.
  13. Jul 22, 2008 #12


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    don't necessarily give up on this way of learning! you are doing fine. you have a strong background comparatively with lots of PF cosmologyites. I'm having fun.
    You asked two good questions, one about doppler and redshift limits to what we can see,
    and the other about what makes stuff untransparent.

    Your doppler question was very good guess BTW. Compliments.

    Did you read that SciAm article by Lineweaver and Davis, or the other one by them? I'm too lazy to look back and see if you said you did.
    There's a link to Lineweaver SciAm in my sig. If you want the basics, that's definitely the basics!

    Anyway you ask good questions and make good guesses. Stick around and try some more out on us if you want.

    Another thing that helps is getting familiar with Ned Wright's cosmology site and with some of the stuff on his cosmology calculator (which implements the standard universe model).
    You can spend hours going over things at that site and playing with the calculator. Good pictures. Ask for links if you are interested. And help interpreting what you find there if you need it.
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