Why has the cosmic microwave background radiation not already left the universe?

Sooo. As I understand it, the cosmic microwave background radiation is the remnants of the light that was created several hundred thousand years after the big bang. If light is the fastest thing in the universe, surely all that light would have traveled beyond the edge of the universe by now? There wouldn't be any left. Someone pointed out to me that this is because the universe is curved, like the skin of a balloon - is that true? If this is the case then the light that we see, must have gone around that skin several times by now. Not only that, but the light from our galaxy in it's early stages must also have traveled around the universe by now, so is it possible that one (or more) of the galaxies that we see in the sky is in fact our own galaxy? What have I got wrong? Thanks!


Oh hang on. I just saw some more of a documentary (everything and nothing, very good btw.) and the guy explains that because the universe is expanding the light of the CMB hasn't had time to leave the universe yet. WTF? Surely that would mean that the universe must be expanding at close to half the speed of light!?

Update 2:

@Zaghawa - But I thought the speed of light was the fastest anything can go? How could the universe be expanding faster than the speed of light?

Update 3:

@blobrana. So what your saying is that the universe started off expanding incredibly quickly, slowed down and is now speeding up again...? There is definitely something fundamentally wrong here.

Update 4:

@ all very good answers (no Robert B - I don't understand, I have no idea what most of the symbols you are using mean. sorry). So let me get this right. The universe had a period of rapid expansion, whereby the space between objects grew very quickly as if we pumped up a balloon and all the marks on the surface moved away from one another as the surface area grew. Light now travels around the surface of our balloon, but it has yet to reach us from all parts of the surface. Is the observable universe expanding uniformly then, or are there lumps in the surface of the balloon?

7 Answers

  • 9 years ago
    Favourite answer

    <QUOTE>Why has the cosmic microwave background radiation not already left the universe?</QUOTE>

    By definition, the Universe is EVERYTHING. So, there's no "outside". The CMBR you see *now* is the radiation that left from some place some 13 billion years ago and is arriving to you now, 13 billion years later.

    Basically, after the Universe has cooled down enough that atoms were formed (instead of having electrons and nuclei floating around separately and not combining because they had too much energy) the Universe became transparent to radiation. The CMBR is the radiation from when the Universe went through that phase transformation (from opaque to transparent).

    @Paula: Read this http://en.wikipedia.org/wiki/Discovery_of_cosmic_m...

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  • 9 years ago

    Okay, first of all, it IS possible for the universe to expand faster than the speed of light. Nothing can travel faster than light, true, but the cosmic expansion is the increase in distance between galaxies rather than the movement of galaxies themselves. During the inflation era, for instance, which occurred (far) less than a second after the big bang, the expansion rate was such that something the size of an atom would have been blown up to (more than) the scale of the observable universe today.

    Second, the 'light' of the CMB is, more accurately the 'heat' of the universe. Imagine the universe as a room that starts at the same temperature everywhere. This room is so large that the light emitted at one side takes a while to reach you at the centre, so you're effectively seeing into the past. Now imagine that the room starts to get colder. You'll feel the temperature dropping around you straight away, but even though the rest of the room is cooling at the same rate, it looks warmer to you because you're seeing into the past. As time goes on, the area around you will become freezing, but if you look at the room's walls they still seem to be red hot.

    This is exactly why we still see the CMB today: The area around us has cooled to almost nothing, but we can still see the surface of last scattering, apparently still hot, in the distance.

    Incidentally, despite what Paula claims, the CMB is not a myth. There is no reason that radiation would be at the edge of the universe, traveling away from us at the speed of light, and there is no basis for the claim that it can be explained by cold regions of the IGM. The CMB is extremely well measured, including the minute variations in temperature that occur in the exact same pattern as were predicted using inflationary cosmological theory.

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  • 9 years ago

    There is no edge to the universe simply because an edge would be silly. Space was created by the big bang and so was all matter, so there will be nothing "outside" the universe. Either the universe is infinite in every direction or it is finite and unbounded. The latter option simply means if you fly off in one direction you will eventually reach where you started coming from the opposite side of the sky (for example you would land at the South Pole if you left in your space ship going directly up from the North Pole).

    And light ALWAYS travels at the speed of light. The only exceptions are when photons at the front end of a light beam get absorbed by the material they are passing through. So, it would be more accurate to say that light beams may be slowed through materials, but individual photons never slow down.

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  • 9 years ago

    "...Someone pointed out to me that this is because the universe is curved, like the skin of a balloon - is that true?..."

    This may be what's tripping you up because it's generally accepted that the universe is finite but unbounded. Sounds like double-talk I know, but look at it this way. Consider an ant crawling around a huge beach ball and never coming to the end. It would consider the beach ball as infinite. If you now consider the ant as only a two dimensional creature and crawling round a three dimensional beach ball, you could understand why the ant would consider the beach ball to be infinite, the three dimensional picture, that shows how restricted its movement really is, is simply not available to it. Thus with the universe, from our perspective, restricted to our view from within the universe, it appears to be infinite, but this is just an illusion, we are confined to the limits of our universe and cannot escape from it. We are bounded within a finite universe.

    Another way to say all this is you can start from anywhere you like, travel away from that point and eventually you’ll simply come back to where you were. You’ll never come against and edge or boundary, you’ll just end up back where you started.

    "...How could the universe be expanding faster than the speed of light?..."

    I know what you're thinking...Einstein's relativity showed us that nothing can move faster than light. The catch is that word "nothing" (..no thing..) No thing with mass can move *through* space faster than light, but there's nothing wrong with space itself expanding faster than the speed of light. In fact an event called "inflation" happened almost at the moment of the Big Bang -- space expanded faster than the speed of light for a brief period. That means that there are immense volumes of the universe that we'll never be able to examine because light, nor any other information, can reach us from there. A good reason to specify which universe we're talking about, the *observable* universe we live in, or the *entire* universe to which we only have limited access.

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  • ?
    Lv 6
    9 years ago

    First of all nothing can simply leave our universe. Then, the universe is expanding, and in the beginning had a phase where it was expanding much quicker than the speed of light.

    As for a curved universe, the most recent data suggest that this is probably not the case but that it is more or less flat.

    Even disregarding this, the theoretical "perimeter" of the universe is more than the speed of light times its age, and so nothing can have done a complete orbit.

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  • 9 years ago

    Indeed you are correct.

    But.....what if the universe were to have experienced a period of rapid expansion that was trillions of times faster than the speed of light, or perhaps even an infinitely fast expansion rate?

    What would that do?

    I assume that because its space-time i'm expanding, i can cheat, (as the light speed limit only applies to 'stuff' IN space-time). ie Space-TIME has no speed limits, it is only 'information' or 'things' that reside in space-time are limited by the velocity of light.

    This would mean that the 'real' universe were much, much larger than what we can see.

    (we can only see about 13 billion light years in that -------> direction, and that is just a small portion of the total distance). Light hasn't had enough time to travel any further. If we wait around for another 10 billion years we will see light from 20 billion light years away.

    The new bits of of the universe come into view at the speed of light; that would mean the observable universe is expanding at light speed.

    Source(s): Jedi skill
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  • Paula
    Lv 7
    9 years ago

    As I understand it, the Cosmic Microwave Background Radiation (CMBR) is a myth.

    It is one of those things invented to explain something, but it does not explain it.

    If there were radiation it would be in the outer region of the universe and going away from us at the speed of light.

    For some strange reason, CMBR is widely held to exist by cosmologists.

    But in reality, CMBR is merely what you would expect to receive from cold (4 K) regions of inter-galactic space.

    So you are correct in your thinking.

    Now we have to wait for cosmologists to realize their error.

    Get ready for a long wait.

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