Expanding Universe or Shrinking Universe?

chris_mb · Aug 31, 2005

Could the universe and all matter in it be shrinking giving the illusion that the universe is expanding? IE: If you put two balloons next two each other then increase the preasure around them, they will shrink and the distance between them will become greater.

Saiph · Aug 31, 2005

well...if the matter is shrinking faster than the universe...then yes. That would work, at first glance. But i don't think you'd get the correct results when you look at galaxies and such, and it would have to contract fast. So all the matter would blink out almost immediately (hits size=0 meters). And if we don't notice it (cause everything is scaled accordingly)...then you end up with the same thing as an expanding universe. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

mrmux · Aug 31, 2005

This is probably a really elementary question, so I apologise in advance, Saiph. The Universe expands. I'm picturing the classic rubber sheet with a grid pattern on it. I draw a light wave across it so one wavelength fits neatly in each grid square. I stretch it and the wavelength gets 'longer', but not from the rubber's perspective. It's still one unit of space to one wavelength. Doppler from moving objects I'm fine with. Where am I going wrong here?

Saiph · Aug 31, 2005

Simple, our meter stick (measuring device, whatever) doesn't get bigger. Do the same thing, but imagine a wooden stick the length of one side of those squares. This is our measuring device. It's small enough that the internal forces and bonds holding it together can easily overcome expansion, and so it doesn't expand. Now, stretch the sheet, and use the stick to measure the wavelength (as we must) and you'll see the wavelength is longer. You notice the length get bigger yourself, because your head/eyes don't expand. But you're right, it isn't any bigger compared to space itself. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

mrmux · Aug 31, 2005

Thanks Saiph. I always assumed the expansion was completely uniform at even atomic levels. ie. Expanding rulers, like you say. How do we tell how much galactic red-shift is due to proper motion (if any), and how much is due to expansion?

Saiph · Aug 31, 2005

good question. Straight off the bat...you can't. There are clues however. The one I can think of is "forest" lines. Hydrogen absorbs specific lines of light. If a nebulae full of hydrogen (i.e. all nebulae) is between you and the distant galaxy, some of the galaxies light is going to be absorbed, and provide a specific pattern. That's fine and all. If nothing's moving, or only "really" moving, you'll just absorb the same wavelength of light over and over again. This is because all the "redshifting" is done at the source. All the nebulae will see the same thing. Result: one set of lines. Wait! maybe all the nebulae are moving relative to the galaxy. If they move in a random fashion (most probably arrangement given enough nebulae), you'll have a random distribution of line patterns overlapping them, likely forming a "gaussian" distribution. Some redder, some bluer, than an average pattern position. It'll look like a garbled mess, but you should be able to identify many different patterns, and measure their offsets. Now...if there is expansion, each nebulae further out is going to see a "redder" galaxy (redshift ~ distance remember). So they'll have their pattern appear in different spots as well. But it won't be in the completely random fashion above. They'll be displaced more, and more, and in one direction. Any random motions they have will merely create a "scatter" if you were to plot them all out...i.e. it wouldn't be a perfect offset forming a nice line, but a sort of scattered trend. We see this last set of forest lines. those with some random scatter, but an overall offset trend. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

mrmux · Aug 31, 2005

Much appreciated and nicely explained. You saved me a right trawl through Astronomy 101. Still, I don't like the idea of accelerating expansion any more than I like the idea of the big crunch. Just seems such an inelegant waste of space (not that it cares, of course). Tell you what though, if a God is actually driving the evolution of the Universe then you could say he is literally the cosmological constant (cue drum roll)... 'I am the One'? <img src="/images/icons/blush.gif" /> (cue tumbleweed)

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