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Please explain the Big Bang and universal expansion

  1. Jun 27, 2010 #1
    I think I understand the basics of the Big Bang but after trying to explain it to someone else, I've discovered that there are many details lacking in my knowledge. So I have come here in the hopes of alleviating my ignorance...

    I understand that in the very early universe, all the energy in the universe was concentrated in comparatively little space, so much so that it formed an opaque plasma which is the source of the cosmic microwave background radiation. What I don't understand is why this is the very last thing we can see. Why can't we see anything past the surface of last scattering after the universe has cooled from its plasma state?

    Is there anything past the surface of last scattering? There must be since you'd expect photons to be emitted from the surface...

    ...which leads to the question of how fast the universe is expanding. How fast is the edge of the universe expanding? In a sense, I'd expect that space is expanding rather slowly since we can still see the surface of last scattering (for now!). However, if the Big Bang is the source of space-time and photons on the edge of the universe are traveling towards this edge, does that mean that space is expanding at the speed of light?

    This leads to yet another question: does the universe have an edge? If the geometry of the universe were elliptical then I'd understand how space can expand from a singularity. However, if it's Euclidean, then I don't understand the nature of this expansion. Does it have an edge? If it doesn't then what does it mean for the Big Bang to be the origin of space?

    As you can see, I'm actually quite confused by the nature of the Big Bang but I'm eager to fill out my ignorance on the subject. I have a couple more questions but I think this is a good start...

    Thank you!
  2. jcsd
  3. Jun 28, 2010 #2


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    Because before then, our universe was opaque. In principle we may find ways to see earlier parts of the universe using non-optical methods.

    I think the thing you might be misunderstanding here is that the surface of last scattering is a surface in time, not a surface in space. Obviously there has to be stuff before the surface of last scattering: an opaque universe!

    We see no edge.
  4. Jun 28, 2010 #3
    One way to look at it…One of the challenges to understanding the big bang expansion is that we need to think in terms of more spatial dimensions than we usually deal with. A way around this is to considered a lower dimensional analog, and extrapolate from there…for instance.

    Consider a universe, U2, with only 2 spatial dimensions instead of our real universe, U3, with three spatial dimensions. All objects and beings in U2 would be flat 2-D objects and they would occupy an area – part of a surface - as opposed to occupying part of a volume as do the 3-D objects in U3. U2 beings would only be able to move or even look in some combination of the two directions (call them D2-1 and D2-2) spanned by the surface – they would not be able to point or look “up” – to do so would be to point out of the universe. To make things easy, let’s say the U2 surface is that of a sphere. All objects and beings in U2 could move anywhere throughout U2 and they would never reach any edge just as a bug crawling ot the surface of a sphere would never reach a boundary– See Abbott’s “Flatland” for more on this. U2 would be finite but unbounded. Imagine how U2 expands, the spherical surface grows and the direction it expands in is perpendicular to both D2-1 and D2-2. That is, it expands in a direction the U2 beings cannot point to. Similarly, we in U3 cannot move or look or point in the direction our 3-D universe is expanding. And, it’s interesting to consider where the Big Bang originated…Imagine a U2 being trying to point to where the BB occurred.
  5. Jun 28, 2010 #4
    After reading Chalnoth's response, I'm beginning to think I don't understand the Big Bang theory at all! Incidentally, are you the same Chalnoth that was on Christian Forums?

    Can this be right?

    There is only one temporal dimension in time so there can't be a "surface." Also, the wikipedia page describes the surface of last scattering as a "spherical surface," implying that it's a surface in space...

    I've also heard somewhere, from Lawrence Krauss' lecture I think, that plasma reflects electromagnetic radiation and that the surface of last scattering is the radiation reflected off the plasma state universe from long ago. I must admit that I don't know how this is supposed to work...

    Finally, if the surface of last scattering is a sphere surrounding the universe, which it is often depicted to be, then that explains why it appears uniform in all directions. If it were from every point in a spherical universe, then we should see different intensities depending on which direction we look...

    As you can see, I really have no understanding of the source of the cosmic microwave background radiation...

    I can understand not "seeing" it but there must be one in an expanding Euclidean space, mustn't there?

    If there were no space and just a point singularity and it expanded into Euclidean space, how would that space look, geometrically? The only image I can think of is a bounded Euclidean space expanding in all directions but then it would have an edge: the bound. Can this be correct?

    Thank you...
  6. Jun 28, 2010 #5
    I can imagine elliptic space well enough, even in three dimensions. However, from what I've heard, we're pretty sure that the universe we live in is macroscopically Euclidean. How do we reconcile this with space expanding from a singularity?

    Thank you...
  7. Jun 28, 2010 #6


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    What is meant by a 'surface in time' is simply that the last scattering surface corresponds to photons originating from a specific event in the history of the universe, namely the decoupling of radiation and matter. The photons originating from this event appear to us on earth as coming from a spherical surface in space (with earth at the center) -- literally the last scattering surface. The physical distance to the last scattering surface is growing in time, since the CMB photons that reach Earth tomorrow will have originated further out in the universe than those that reached the Earth today.
  8. Jun 28, 2010 #7


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    The universe might be Euclidean if it has a flat geometry. The latest constraints on the geometry of the universe come from WMAP, with the result that the universe is flat to within about a percent. However, the universe might still be closed or open (just obviously not too closed or open). The geometries of these universes are spherical and hyperbolic, respectively. So, at present, all geometries are still in agreement with observations. To make sense of this, if you could only carry out local experiments around your neighborhood, you would conclude that the earth was flat to high accuracy, when, in reality of course, it is actually a sphere. All of our cosmological data comes from our observable universe, which may well be only a small neighborhood of the universe in its entirety.

    With regards to the singularity, it needn't be reconciled since current cosmological models don't address the initial time. The appearance of singularities in physical theories are largely understood as indicating a breakdown of the theory.
  9. Jun 28, 2010 #8


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    Indeed, I hanged around there for a little while. Also a few other forums online.

    My meaning was that it is a surface spread across space separating what happened before from what happened after. Everywhere in our universe, at nearly the exact same time, the universe transitioned from opaque to transparent. That is what I meant by a "surface in time".

    Now, we only can see a part of this surface. The surface exists at about 13.7 billion light years in the past, so we only see the portion of it that has taken 13.7 billion light years for the light to reach us. That part, as bapowell mentions, is a sphere. But it is only a cross-section of the whole surface.

    Well, I think you're misunderstanding slightly. The reason why the universe was opaque before the surface of last scattering is that before that time, the electrons had enough energy that they didn't hang around the protons: they ran around freely. This is the definition of a plasma. Light tends to bounce off charged particles, so it can't get very far in a plasma. A photon will be emitted from some interaction, travel a short distance before knocking into an electron or proton, and shoot off in some other direction with a different energy. So you see, you can't get a clear image of anything for much distance within a plasma, because the light is bouncing all over the place.

    Once our universe cooled sufficiently, however, it transitioned from a plasma to a gas: the electrons became bound to the protons, and instead of electrons and protons moving around, we had neutral atoms. Neutral atoms only react with light of very specific wavelengths, so once this happened most of the light just went on a straight line forever after.

    Not really. It's a much more subtle point than that. The fact that it's the same in all directions is a statement that the entire visible universe cooled from a plasma to a gas at almost the exact same time. From classic big bang theory, this makes little sense, because much of the surface of last scattering that we can see isn't now and never has been in causal contact. That is to say, light could never have communicated the information about the temperature of one region to the temperature of another.

    This is one of the reasons why inflation was proposed: by proposing an accelerated expansion early on, the universe can come to equilibrium quite easily, and there is no problem with it not being in causal contact.
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