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Cosmic background radiation - redshift, blueshift, anisotropies

  1. Mar 7, 2010 #1
    I've been trying to find out more about cosmic background radiation, but I am unable to find any information on the variations in the measured cosmic background radiation.

    I'm talking about these WMAP images. Most of the images you'll find on Internet are the versions after being corrected, e.g.

    But it appears that the actual measurements are closer to this?:

    1) First, there appears to be a "dipole" in the measurements. The radiation is a number of microKelvins higher in one part of the sky than in the other. There is a redshift in one part nd a blueshift in the other.

    What causes this dipole?

    2) Then there is this band in the middle which also is quite some microKelvins warmer than the rest.
    Again, what is the cause of this?

    3) Finally, what does the fact that the anisotropies we observe after correcting for the two major shifts imply? That the universe is uniform in every direction?

    I have little background knowledge of physics or cosmology (I'm a high school student), so I hope I have been able to explain my question clearly enough.
    Last edited: Mar 7, 2010
  2. jcsd
  3. Mar 7, 2010 #2


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    Yes, it's fine, apart from question 3.

    (1) The cause of the band in the middle is microwave radiation from our own galaxy.
    (2) The cause of the dipole is that our own galaxy and solar system are moving through space relative to the radiation. Hence there is an additional blue shift from the direction towards which we are headed, and an additional red shift in the rear.
    (3) I think there's a word missing in question 3; I don't know what you are asking.

    Welcome to physicsforums! I find these are amazing pictures; looking back at what is almost the start of the universe.

    Best wishes -- sylas
  4. Mar 7, 2010 #3
    Sorry, forgot to finish the question. What I meant was:

    3) Finally, what does the fact that the anisotropies we observe after correcting for the two major shifts are very minute imply? I mean, what does it mean that the fact that the differences in light temperature are very small mean for the shape of our universe? Does it imply that the universe is nearly uniform in whatever direction we look?
  5. Mar 7, 2010 #4


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    Yes, you're correct. The fact that the CMBR is very uniform in every direction (once the dipole is corrected for) is a very important piece of evidence for what we know about cosmology, including the isotropy of the Universe (a fancy way of saying 'the same in all directions').

    The fluctuations themselves, even though they are small, are also very important as they tell us about the structure present in the early universe which 'seeded' the structures we see today i.e. the galaxies and galaxy clusters. It also tells us about the physics that drove the initial rapid expansion of the Universe, known as 'inflation'. Different models for inflation predict different types of fluctuations, so measuring the CMB carefully tells us about this very very early era in the history of the Universe.
  6. Mar 8, 2010 #5
    What I still do not get is how the unifomity of the universe supports theory of the rapid expansion and/or the Big Bang theory. I mean, intuitively I would believe that if there was a Big Bang, then the radiation coming from the location where the initial Big Bang happened should be stronger than from "the other side".
  7. Mar 8, 2010 #6


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    Ah! You've hit upon one of the classic speed bumps in this topic, and the CMBR is one of the best ways towards understanding it.

    There is no such thing as the "location" where the Big Bang happened. The Big Bang was not an explosion in space at some location. It was a state of affairs in which the entire universe was in a condition of extreme density.

    What we are looking at in the CMBR is a hot plasma of mostly hydrogen, glowing hot, at about 3000 K. The thing about a plasma like this is it is opaque. The gas is ionized; the atoms are too hot to hold on to their electrons, and the gas is mostly a mix of protons and electrons, plus a few other atoms (helium, heavy hydrogen, lithium, mainly) Light keeps colliding with the protons and electrons in this plasma.

    However, if you have a plasma that is cooling and expanding (that is, the density is dropping as everything disperses away from everything else) then the atoms get to a point when they can hang onto electrons, and now it is just a regular gas; not plasma... and it is transparent. Light can pass freely through the gas.

    About 380,000 years after the initial conditions where physics falls apart and cannot describe what is going on, the whole universe was filled with this plasma, and it was just at the point of forming hot transparent hydrogen gas. As this point, light becomes free to pass through the universe without bumping into all that ionized plasma.

    We are looking at that light now; this is the light from the time of "last scattering" of light.

    That light comes to us from every direction in the sky, because every part of the universe was equally part of the big bang. You are looking at the Big Bang, in a way; at the hot gas that filled the early universe 13.7 billion years ago, and which subsequently formed into stars and galaxies and people; people who look out and can still see that radiation, which fills every corner of the entire universe.

    This radiation filling the universe was one of the early predictions of the Big Bang model, and discovery of it was a powerful confirmation of the theory which predicted it.

    Cheers -- sylas
  8. Mar 9, 2010 #7
    Then what exactly is the source of this light/radiation we measure? I now understand that the hot plasma blocks the light from passing through and that as it cools down and becomes "matter" in the form of atoms etc. it can let light through. But where do the photons come from? From the plasma itself or from some other source? Or have they always been trapped in the universe since the Big Bang?

    And since the radiation we measure is becoming increasingly weaker (right?), does it mean that this hot plasma is slowly vanishining, or simply that the universe is expanding so fast that everything, including the plasma, gets farther away from everything, including the earth?
    What will eventually happen to the plasma? Will it at some time completely vanish? If so, would we then no longer be able to measure background radiation? Or would there always be some background radiation stemming from the Big Bang kept in the universe?
  9. Mar 9, 2010 #8


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    They come from the plasma itself.

    The plasma we are looking at is now long gone. It filled the entire universe 13.7 billion years ago. Since then it has cooled and clumped together into galaxies and stars and so on. We also are formed from that same plasma, and the universe, "it goes on forever -- and -- oh my God -- it's full of stars!"*

    One of the basic ideas in cosmology is that the universe is roughly the same, everywhere. It is (now) filled with galaxies, and they go on without bound in every direction.

    But the universe is not the same at all times. It changes. It used to be filled with hot plasma. Now it is filled** with galaxies and with the background radiation.

    What we are looking at is light that has been traveling through the universe, unhindered, for 13.7 billion years. The stuff we are looking at in that light is hot plasma, which is (presumably) now formed into galaxies as well; galaxies that are now far far away. Defining "distance" is a bit of a problem in cosmology, because of a few subtleties with how expansion works. Another commonly used definition is called "proper distance", and by that definition, the plasma we see as background radiation is now formed into galaxies that are some 46.5 billion light years away; you'll often see that number quoted as the radius of the observable universe.

    The plasma forms into galaxies. We know this, because by looking shorter distances we can see what the universe looks like as time goes by.

    The background radiation will continue to travel through the universe, indefinitely. For any observers of the radiation, it will appear more and more redshifted as time goes by, until, billions upon billions of years into the future, it will be redshifted so far as to be virtually undetectable.

    To get an idea of what the universe is like now, I recommend the amazing website An Atlas of the Universe. It also gives a bit of an introduction to the Big Bang and the Cosmic Background Radiation in the pages associated with looking at the universe at the very largest scales.

    Cheers -- sylas

    * A quote from "2001, a Space Odyssey", in case you don't recognize it!
    ** I use the word "filled" loosely. Galaxies are arranged into clusters and great streams or filaments of clusters, and between them are vast empty voids.
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