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Microwave background: stupid question

athon

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Ever have on of those questions that you know has a simple answer, yet for some stupid reason you just can't visualise it in your mind...

The Microwave background radiation of the universe has been mapped, and forms a picture of the early universe relatively soon after the big bang when radiation was released. I get that much. And as light travels at the speed of light, the further away the 'source' of light, the further back in time we're looking. No problems there.

I guess I'm visualising this the wrong way (and, oddly, even as I write this I can nearly grasp it, but need comformation). But imagining the universe as 'compact' and then expanding - keeping in mind it's spacetime that is expanding and not the universe getting bigger in a larger medium - I picture seeing that initial burst of radiation moving away at the speed of light. To see it, wouldn't it need to somehow reflect back? I know it seems ridiculous, but I'm imagining there needing to be something that radiation reflects off so we can then see it.

What am I not getting?

Athon
 
It sounds like you are visualising a hollow shell of radiation , surrounding the rest of the universe. I don't do that. I take a simplistic gaslike model. Presumably the radiation not only filled the whole thing then, it still does. It's just that inflation and entropy have dropped the temperature of the whole universe to the 3 degree absolute (or whatever it is) level.

I'm imagining a cylinder of hot gas , allowed to expand . The gas temperature falls everywhere inside the cylinder. In this case, there happens to be no actual "Outside".

Just my simplistic take on this . I'll be totally unoffended if someone more informed says it's wrong. I don't come here to be told when I'm right.
(Which is just as well, really).
 
See, this is what I 'know' it to be like. But I have problems seeing it. For the universe to still be full of this microwave radiation, I picture that it must be 'bouncing around' inside of the universe. Otherwise it just moves away with it at the speed of light. So how do we see the radiation that was released at the beginning?

I'm too much of a visual person. That's my problem with physics.

Athon
 
It's the gas, principally hydrogen, that's emitting microwaves. It's very thin now, but still there, and it emits at a characteristic frequency for its temperature. Also it's not evenly spread out, which stands to reason as if it were, there would be no galaxies, stars or planets for us to live on.
 
Asolepius said:
It's the gas, principally hydrogen, that's emitting microwaves. It's very thin now, but still there, and it emits at a characteristic frequency for its temperature. Also it's not evenly spread out, which stands to reason as if it were, there would be no galaxies, stars or planets for us to live on.
Click to expand...

Ok, that makes more sense. So the hydrogen gas as it was distributed moments after the big bang is now at the edge of spacetime, expanding with it? And it is this hydrogen that has emitted the radiation? I'm satisfied with that. Thanks mate.

Athon
 
I've been troubled a bit about what is actually going on here also.

My take is that some of the universe is receding away from us very fast. EM from that area of the universe is just now reaching us, but the expansion of space has reduced the frequency of that radiation from what it was to what it is that we see it at today. Other parts of the universe are receding away from us even faster such that radiation from those parts hasn't reached us yet.

My biggest problem is constructing any kind of mental model of the universe. I don't really get this idea that it looks the same on average from wherever one is looking although the basic concept seems likely to be true to me.

If one traveled through the universe infinitely fast in one direction would one eventually circle around and come back to where one started? If that isn't true then the idea that the universe looks about the same no matter where one is in the universe would seem to imply an infinite universe. My best cut at this is that the people who understand this issue at all are at least much more knowledgable than me on this and possibly a whole lot smarter and that gives them some ability to understand this at a level that I might not ever have been able to.

ETA: I didn't mean to ignore other posts, especially ones that I think said what I said in a somewhat different way. The posts just came in while I was composing my post.
 
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Disclaimer: I am not a cosmologist (IANAC), but...

Here's how I always thought of it. I could well be wrong.

Picture a balloon, inflating. The balloon represents space. Even if there were waves of radiation moving across the balloon faster than it's inflating, if we were at one spot on the balloon, we would still continuously encounter that radiation. Even long after the balloon started inflating, radiation could still be coming at us the long way around the balloon, and only just arriving.

Of course, there's also the fact that in the expanding balloon model, there's no point on the balloon that you could point to and say, this is the point where the big bang occurred. All of space is expanding from the initial explosion, so there is no point from where the initial radiation must be arriving from.

All that, plus what others have pointed out -- that space may be expanding faster than the speed of light.
 
No you could never travel around the expanding universe and come back at the same point. The time required is increasing too fast. For example it may take you two years longer to do so because you started travelling this year rather than last year.

The microwaves were emitted when energy and mass became two different things. So it was not emitted by anything other than the universe.

But then I am not an expert in this field.
 
Let me try to explain in terms of the "reflection" language:

Before a certain time t0, the matter in the universe was dense enough that the light was "reflecting" off matter often. (Here, "reflection" stands for absorption and emission of photons). So at t0, the light was in equilibrium with the surrounding matter and the energy distribution of photons had information about the temperature of the surrounding matter.

After time t0, the matter became rarer, so that most of the light did not have anything to reflect from (get absorbed or emitted from), and it continued travelling. That's the light we see in the microwave background radiation now: it tells us what the energy distribution of the photons was at time t0, and consequently, what the conditions (temperature etc.) were at time t0.

-Dorman
 
athon said:
Ok, that makes more sense. So the hydrogen gas as it was distributed moments after the big bang is now at the edge of spacetime, expanding with it? And it is this hydrogen that has emitted the radiation? I'm satisfied with that. Thanks mate.

Athon
Click to expand...

I think your use of the word 'edge' shows the problem here.

I think maybe, you're still picturing the universe as a 3D expanding globe like structure? Like, spherical?


There is no 'edge' of spacetime. Impossible (at least for me) to picture a 4D expanding space, but there you go.
 
rjh01 said:
No you could never travel around the expanding universe and come back at the same point. The time required is increasing too fast. For example it may take you two years longer to do so because you started travelling this year rather than last year.

The microwaves were emitted when energy and mass became two different things. So it was not emitted by anything other than the universe.

But then I am not an expert in this field.
Click to expand...

I'm a big believer in the idea of the constancy of the speed of light and the impossibility of just zipping around the universe at some hyper light speed.

My question was a thought experiment to try to understand a little bit more about the nature of this idea of a universe that looks about the same no matter where you are in it.
 
It's a good question, davefoc, and we don't know the answer yet. The issue is known as 'The Horizon Problem' (horizon_problemWP) and to cut a long story short, it is one of the reasons that inflation is popular since the fast expansion of inflation allows regions that are not causally connected to look the same.

I'll be back this way to answer some more later. Today is...a busy day for me!
 
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Back to Athlon’s original question… I think your question has been answered in parts by several posts (though I’m no cosmologist!):

The microwave background was created when matter and light split, soon after the big bang. The split occurred throughout the universe, so the microwave background is present everywhere (why the 'temperature' of space is 3 K). I see it a bit like a stretched out piece of pastry: the pastry started as a small blob, but now it’s large, thin and still present everywhere.

Incidentally, have you heard the story of how the microwave background was discovered?
 
Ooh, can I have a go?

Imagine you're sitting on a rubber sheet (this may be easier for some members than others) with a can of silly string. Next to you is someone else with a can of silly string.

When someone shouts, "GO!" the rubber sheet between you starts stretching very fast. At the same time you start spraying the silly string at each other.

So you see him receding from you really fast, but the silly string he is shooting sort of keeps pace with his recession from you so the leading edge of his spray of string stays with you as he shoots off into the distance. The end of his string that is next to you is, of course, the string he launched at the beginning. The string he is launching now is still near him and is on its way to you. Also remember that from his perspective he thinks the same about you.

Or it's not so much that he has shot silly string at you, more like he attached a rubber band to you and strummed it as he started to recede so that the really short wave length with which the rubber band was vibrating has now stretched to a much longer wave length.

I think the most important feature is that all of those 'distant' realms of the Universe have not really 'gone' anywhere. They're just as adjacent to us now as they were 300,000 years after the Big Bang when the Universe became transparent to radiation and the CMB radiation began to pass freely through space. I'm not a physicist amd my amateur understanding peters out, but I suppose there must be some measure of distance between points that has remained constant despite the expansion of spacetime. Is it something like the light-travel time between two points? Intuitively I think that something must have stayed constant if the whole thing is a zero-sum game, but I think I need a grown-up physicist to tell me what it is.
 
Thanks everyone. In some ways I think I got it, although some answers I've read seem (to me) to conflict a little. I think it's the way I'm reading it rather than poor responses.

BSM, your analogy is the one I came close to picturing when I thought about it; that the expansion of spacetime pulled the radiation away quickly (hence stretching it into microwave frequencies). However, I guess I always thought the expansion was either at or faster than c, so the radiation would take an infinite amount of time to reach us.

The 'balloon' model and my misapplication of space as a sphere is a mistake I keep making. I don't know why my head can't cope with the geometry of it. But Angus, I'm getting the sense you're implying that the radiation is taking a longer distance because it's coming 'around' the 'balloon's' surface. Am I misreading this, or am I taking the analogy too far?

Another related question; will this radiation eventually disappear?

Athon
 
Another question: Is it inspired by the will it go away question:

Is it plausible that a change in frequency with time could be detected for the background radiation?

The frequency of maximum energy has changed by a factor of about 1000 in about 10 billion years.

This is a factor of about 1 in 10 million per year.

Or (160 ghz/10 million) about 16 khz per year.

A change of 16 khz seems like it might be detectable. Of course the CMB has a blackbody radiator distribution so that would make it really tricky I suspect since there isn't just one frequency mixed in to the CMB. And then there's this whole inflation thing that suggests that the universe expanded very rapidly at the very beginning so maybe the change is much less than I am estimating on a per year basis now.

And since I have never hear of anybody even considering this I suspect it is a non-starter, but still is it even remotely plausible is the question.

Note that calculations were done mostly be rounding alot to make calculations easy.
 
athon said:
BSM, your analogy is the one I came close to picturing when I thought about it; that the expansion of spacetime pulled the radiation away quickly (hence stretching it into microwave frequencies). However, I guess I always thought the expansion was either at or faster than c, so the radiation would take an infinite amount of time to reach us.

The 'balloon' model and my misapplication of space as a sphere is a mistake I keep making. I don't know why my head can't cope with the geometry of it. But Angus, I'm getting the sense you're implying that the radiation is taking a longer distance because it's coming 'around' the 'balloon's' surface. Am I misreading this, or am I taking the analogy too far?

Another related question; will this radiation eventually disappear?

Athon
Click to expand...
When matter and radiation decoupled every point in the Universe had the same temperature, so that the spectrum of the photons that were emitted was a blackbody and was the same everywhere.

Now, when we look anywhere in the Universe we are looking back in time (due to the time it takes light to reach us), and the further away we look the farther back in time we look. When we look far enough away what we are looking at are the photons that were emitted at that instant of decoupling. They have such a high redshift that the blackbody spectrum, instead of being that of a 3000K emitter, is typical of a temperature of about 3K.

We will always be able to see the CMB, as photons from parts of the Universe that were further away at the time of decoupling arrive at the Earth. As time goes by the spectrum will be further redshifted, as we are looking further away, and, because we'll have been around longer, further back in time.
 
The way I think of it is... the radiation sent our way from billions of light years away--isn't really that far away, in a sense... it's just a long time ago. You're not seeing something 15 billion light years away... you're seeing something that was right here next to us 15 billion years ago. The stuff that really is billions of light years away, you're not seeing at all.

I don't know if that makes sense to anyone but me.
 
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