Reythia
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"Cosmic Sound Waves Rule"
I was reading the April edition of "Physics Today" yesterday and really found the feature article interesting. And since at least a few others on this board are science freaks, I figure I should share.  The full article is "Cosmic Sound Waves Rule" by Eisenstein and Bennett, if you want to borrow the magazine from your local library and read it. It's a tale of the Big Bang Theory, as told by "sound" waves in the Cosmic Microwave Background (CMB).
First, some background for those of you a little rusty on the subject. Remember, after the "big bang" and "inflation" (which I'm still highly skeptical about, I might add!), the early universe was still very hot, very dense, and very small. It was hot enough that baryons (protons, neutrons, and electrons) couldn't clump together -- ie: it was ionized. Which meant that everything was incredible crowded, and the particles -- baryons, yes, but also light-bearing photons -- bounced continually into each other before they could go very far.
The universe kept expanding in this state until about 380,000 years (according to the article) after the big bang, when the average temperature dropped below 3000K. At that temperature, there was no longer enough energy to ionize the baryons -- which means nuclei formed and trapped electrons to from Hydrogen. Suddenly all these particles were bound tightly together -- which meant there was more space between each new atom. As the article puts it, "decoupling was like the lifting of a universal fog." The photons were suddenly able to shoot through all the atoms and escape.
Today we can see this sudden escaping of photons as the Cosmic Microwave Background. The WMAP project recently mapped the current temperature of the CMB across much of the sky and found that it's mostly the same... but there are clumps that are a touch hotter and others that are a touch colder. These depict regions in the old universe (at the time the photons escaped) that were unusually dense or empty.
Okay, so that's the background. Read on if you want to hear about the article!
This article pointed out that when the photons escaped, there were regions that were unusually dense. This anisotrophy ("unevenness") might have been caused by quantum mechanical effects... or just something we don't understand yet. But in any case, some areas were more dense that others.
Now, when ordinary AIR is more dense in one place than another, it creates a pressure wave, pushing the air next to it outwards -- and then that air pushes on the next air -- and the next, etc, etc. This is a sound wave.
The thing I hadn't thought about before was that the UNIVERSE makes "sound" waves too! That is, when the early universe was overly dense in spots, they would have pushed on the spots next to them, which would have started a wave of longitudinally-compressed particles moving outwards from the over-dense spot in a sphere. Which is just how a sound wave works.
Imagine that, when the universe was still ionized, a plasma-filled compression wave began to propagate from an over-dense spot. It moves along, carrying baryons with it, and then suddenly the temperature drops and makes the universe transparent. All the baryons in the wave form atoms -- and STOP MOVING (more or less). Now what you're left with is a clump of over-dense matter (mostly dark but also some baryonic) at the original pressure source. AND you've got a spherical ring of baryonic matter left at a distance:
D = (Speed of plasma "sound") * (Time of plasma epoch)
I won't kill you with any more math (since it rapidly gets messier at this point and even I glazed over for 2-3 paragraphs), but the point is that if you know D and you know (or can guess at) T, you can find S. Translation: new, better estimate for a length scale in the early universe. AND that length scale is true for both "regular" baryonic matter AND dark matter -- which makes it remarkably useful, considering how little we understand about dark matter.
It's a good article. I tried, but I can't do it justice. You should all go read it!
---  -- YAR! |
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5/6/2008, 4:17 am
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QS2
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Registered: 03-2006
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Re: "Cosmic Sound Waves Rule"
You know, that is really really smart. I'm glad some one thought of that. Sounds like you might be able to get some very useful data from it as well.
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5/8/2008, 6:08 pm
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thepinksuicidallemming
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Registered: 12-2005
Location: Australia
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Re: "Cosmic Sound Waves Rule"
Sounds interesting.
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Sentio aliquos togatos contra me conspirare.
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5/9/2008, 1:13 am
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Reythia
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Registered: 11-2005
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Re: "Cosmic Sound Waves Rule"
I know, QS. It was a really well-written article too, which probably impressed me as much as the concept did. I know a lot of scientists and engineers who are really good at understanding things, but really BAD at explaining them in a non-technical way. The authors of this article weren't that way, which impressed me.
--- -- YAR! |
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5/9/2008, 3:51 pm
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