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jaBleV
2003-May-29, 11:40 PM
I have been wondering, about some questions concerning the cosmic microwave background raditation (CMB):

1. Why do we see the CMB uniformly in all directions?
And maybe related:
2. Where has it "been" since the Big Bang?

more questions to be posted..... :wink:

Grey
2003-May-30, 12:32 AM
These are in fact closely related questions. The CMBR is the echo of the Big Bang. In the early universe, the temperature was very high, and hence there was a large amount of intense radiation. As the universe cooled and expanded, this radiation remained, but shifted to lower energy.* Today, the CMBR is this radiation cooled to a temperature of about 3K (that's three degrees above absolute zero). We see it from everywhere in the sky because the radiation filled the universe, and continues to do so. You can take a look at the WMAP (http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html) results to find out more.

* You can either view this as a change in the blackbody spectrum of a photon gas as the temperature drops, or as a redshifting to higher wavelengths due to the cosmological expansion, depending on whether you want to look at it thermodynamically or relativistically.

jaBleV
2003-May-30, 08:19 AM
Well, I believe my question rather should be:
What property of our universe keeps radiation inside it?
Should the universe be seen as a sort of expanding black hole?
and also related:
Why do we see the universe as uniform from earth? Its hardly because we are in the middle of the universe, but why??????

:o

Grey
2003-May-30, 01:34 PM
Well, I believe my question rather should be:
What property of our universe keeps radiation inside it?
It's mostly a question of geometry. If the universe is open or flat (and the latter seems more and more likely), then space is infinite. Radiation keeps moving in all directions, but it never reaches a boundary, since there isn't one. If the universe is closed, then it's easiest to think of it as analogous to the surface of a sphere. Whatever direction you travel, you eventually come back to where you started and never reach a boundary. It doesn't look like the universe is nearly old enough for light to have cirumnavigated it, even if it is closed, so we wouldn't notice this, but might many billions of years in the future.


Should the universe be seen as a sort of expanding black hole?
If the universe is closed, this is in some sense a fair characterization, but it's using the term "black hole" pretty loosely.


Why do we see the universe as uniform from earth? Its hardly because we are in the middle of the universe, but why?????
The classic image is of the surface of a balloon (with glaxies painted on it) expanding. Put yourself at any one of them, and you'll see the same pattern of galaxies, more or less, all rushing away from you.

As for the uniformity of the CMBR specifically, it fills all space, so we're just seeing the bit that happens to be colliding with us at the moment. If we went somewhere else, we'd see the bit that happens to be colliding with that point, and it would look the same. Although some of it gets absorbed (like when we detect it, or it runs into a star), the universe is mostly empty space, so there are plenty more background microwave photons travelling.

If you're specifically wondering why they aren't coming from a specific point ("the spot where the Big Bang happened"), and are imagining them bouncing back from the edge of the universe, that's not quite the right way of looking at it. I'll eleaborate on the previous paragraph. Let's go with a closed universe for a moment; that's the easiest to visualize for me, since it's finite in size. The universe can be viewed as three-dimensional space, curved into a four-dimensional sphere. Going down a dimension to visualize, the universe is the surface of a sphere, not the sphere itself. If you leave the surface, you've just gone at right angles to reality and left the universe. It may be possible that black holes do that very thing by warping the fabric of space through gravity, but it's important to note that there's no way to just launch a rocket in that direction.

At the Big Bang, the sphere was much smaller (theoretically a point at the very beginning). This sphere was not only the matter in the universe, but space itself, so the distance to circumnavigate the universe was much shorter. Space itself is expanding, carrying the matter with it; it's not that matter is expanding into pre-existing space.

Very early on, the temperature is high enough that matter and radiation could easily switch back and forth, so you'd have photons turning into particle-antiparticle pairs when scattering, and matter and anti-matter colliding with each other to produce more photons. This is happening throughout the universe, so there are photons everywhere, going in every direction. Eventually the universe cooled enough that the photons were no longer energetic enough to create particles, the matter and anti-matter annihilated to produce a final batch of photons (and apparently leaving a small surplus of matter; we're not quite sure why, but it's worked out well for us!) This is the point at which photons "decoupled" from matter.

At this point, there are still photons everywhere, moving in all directions. As the expansion continues, they'll continue moving, redshifting to lower and lower energies. Occasionally, they'll be absorbed by something, but since the universe is pretty empty, most of them just keep going. Since the photons are starting from every point in the universe, and there are a lot of them, we'll see some coming from every direction. Since there were always some that started out travelling in the same direction, but started just a little further away than what we're seeing now, we'll see the same thing tomorrow. The reason that many are travelling in our direction isn't that we're someplace special; there are just many travelling in every other direction, too. Since they're evenly distributed throughout the universe, if we moved somewhere else, we'd see the same distribution of photons, more or less.

Note by the way, that the "center" of this expansion is not part of the universe. It's off somewhere that you need to travel in the fourth dimension to get to. We don't know if there's really any physical sense in which you could travel there. And where is the point in the present universe where the Big Bang happened? Well, at that point in time, all of the space we see was compressed down into a single point, and was all in the same place. So the answer is "everywhere"! So in some sense, we do see the background radiation coming from the location of the Big Bang, but that "location" is really the entire universe, including where we're standing now. :)

If I'm still not answering your question, feel free to ask further, or perhaps someone else can jump in and provide a clearer explanation. :D

aporetic_r
2003-May-30, 02:39 PM
Grey: Thanks for a great explanation!

Any suggestions on how I might visualize a 4-dimensional sphere?

Aporetic
www.polisci.wisc.edu/~rdparrish

Spaceman Spiff
2003-May-30, 02:54 PM
Very early on, the temperature is high enough that matter and radiation could easily switch back and forth, so you'd have photons turning into particle-antiparticle pairs when scattering, and matter and anti-matter colliding with each other to produce more photons. This is happening throughout the universe, so there are photons everywhere, going in every direction. Eventually the universe cooled enough that the photons were no longer energetic enough to create particles, the matter and anti-matter annihilated to produce a final batch of photons (and apparently leaving a small surplus of matter; we're not quite sure why, but it's worked out well for us!) This is the point at which photons "decoupled" from matter.

In many discussions of the early universe, the term "decoupling" refers to when matter and light stopped interacting due to the drop in opacity and corresponding dramatic increase in the photon mean free path - i.e., when the universe became transparent (such is the case in the photosphere of a star). It is this (blackbody) radiation that we now view as the microwave background radiation. I assume you know what you mean, but to the non-specialist out there who comes across the term, they should be aware of the two different uses of "decoupling".

Nice description of the origins of the CBR, by the way!

Grey
2003-May-30, 05:29 PM
Any suggestions on how I might visualize a 4-dimensional sphere?
Hey, if you figure it out, tell me. :) But I still think I have an easier time with that than imagining an infinite four-dimensional hyperbola. I have an especially hard time dealing with a Big Bang that starts out infinite in size (but still compressed to infinite density) which then expands, which is what we get for either a flat or open universe. The math all works out and I have no real problems with it, but I have a terrible time trying to visualize it.


I assume you know what you mean, but to the non-specialist out there who comes across the term, they should be aware of the two different uses of "decoupling".
A valid point. Thanks for catching it! :)

zwi
2003-May-30, 08:14 PM
I look at it this way

1 The CMB is part of the Big Bang

2. Everything in the Universe is inside the BB

3. We, you and me, brother, are inside the BB

4. As space keeeps expanding it just gets bigger

5.. Therefore the microwaves have to be our background, everywhere we look. We cannot get the other side of the curtain, just like a child finds out that there is nowhere behind the mirror


Note this may not be scientifically correct, or relativistically pure, but it kinda makes sense to me.

A lot of your questions get an answer that way.

Zwi

John Kierein
2003-May-31, 12:40 PM
It is not at all certain that the CMB comes from a big bang. If there wasn't a big bang, but if the universe extends indefinitely or infinitely in all directions, then the red shifted light from radiating sources in the universe produces a smooth background radiation. This was predicted by Noble laureate Max Born to be the same as the observed CMB. The big bang predictions were for a much brighter background.
For references see:
http://www.dfi.uem.br/~macedane/history_of_2.7k.html

Manchurian Taikonaut
2005-Sep-22, 08:37 PM
Planck Qualification
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=37956
Planck's primary purpose is to use its telescope to focus radiation from the sky onto two arrays of highly sensitive detectors. They will measure the temperature of the cosmic microwave background radiation over the whole sky, searching for regions slightly warmer or colder than the average.
http://www.esa.int/esaSC/SEMQBE1A6BD_index_0.html

Only with very sensitive instruments, such as COBE and WMAP, can cosmologists detect fluctuations in the cosmic microwave background temperature.
http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html

Blob
2005-Sep-23, 01:13 AM
Hum,
Think of it as an archaeologist digging down through layers.
The deeper the layers the further back in time he/she looks at.
The CMB layer is just above the bedrock.

The CMB looks the same all over the sky, because of a property of the very early universe called super inflation.