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galacsi
2010-Jan-02, 06:57 PM
Space expansion is central to the Big bang theory and I don' understand how it works.

I mean in the details. I don't have any problem with the general idea of an expanding universe and in this thread I don't want to discuss it's veracity.

So the universe is expanding , but what exactly is expanding ? At the beginning 13 billions years ago the universe was very dense and full or particules then after some cooling protogalaxies and galaxies appeared and seems to spread away from each over.

It is expansion in the macro level. Then we have the cosmological redshift, which suggests an expansion in the smallest level.

Then , is that space is expanding at all scales of distance ? Or is there a barrier or expansion stops. And if so why? I already asked this question but I have never received a relevant response.

korjik
2010-Jan-02, 07:25 PM
It is a bit tricky and I am no expert, but here is my take.

The expansion is in space itself. Take a non-interacting meterstick m1 at time t. Then take a second non-interacting meterstick m2 at time t+dt (some later time). Then compare the two metersticks. The second meterstick is longer.

Now there are a few qualifiers. First, these metersticks arent really physical. The interactions between particles making up a physical meterstick would keep the stick from expanding. Second, you are measuring two different meters at two different times. You cant just measure the same meterstick, cause it will always be one meter long, no matter when you measure. It is that the meter changes between the two times.

The trick is that this only really happens in empty space. As in between galaxy clusters empty. This is due to the fact that gravity is a much stronger force than the expansion. The local group is gravitationally bound, so any recession is cancelled out by gravity, but even fairly nearby clusters are receding. This leads to why I said a non-interacting meterstick before. Any physical meterstick is held together by gravity and electromagnetism, so it wont expand.

So, expansion has a lower bound. Anything smaller than a galaxy cluster dosent expand.

Now for an upper bound, there you get to the nature of the universe. There shouldnt be a barrier to expansion on the universe, because you cant define an outside of the universe for this barrier to be in. Brane theories may say otherwise, but like I said, I am no expert.

galacsi
2010-Jan-02, 08:06 PM
It is a bit tricky and I am no expert, but here is my take.


. . . .
The interactions between particles making up a physical meterstick would keep the stick from expanding . . .
The trick is that this only really happens in empty space. As in between galaxy clusters empty. This is due to the fact that gravity is a much stronger force than the expansion. The local group is gravitationally bound, so any recession is cancelled out by gravity, but even fairly nearby clusters are receding. This leads to why I said a non-interacting meterstick before. Any physical meterstick is held together by gravity and electromagnetism, so it wont expand.

THis is interesting and quite handy in a way because that that mean distance like the radius of the Earth orbit or Moon orbit or diameter of Earth or even the size of one atom has not expanded. However at a very microscopic scale the EM radiation has bee expanded i.E redshifted. SO I see that as a paradox and I don't understand it.


So, expansion has a lower bound. Anything smaller than a galaxy cluster dosent expand.

Now for an upper bound, there you get to the nature of the universe. There shouldnt be a barrier to expansion on the universe, because you cant define an outside of the universe for this barrier to be in. Brane theories may say otherwise, but like I said, I am no expert. This lower bound that what I was calling a barrier. And aS I said it quite handy because it prevent a full load of complications.

there is still the question ; why gravity or electromagnetism prevent expansion at a lower scale than Galaxy . It does not make sense to me. If it is the fabric of space which expands , we should have expansion at constellation or stellar level at the least and not only at galaxy level.

Blue Fire
2010-Jan-02, 08:26 PM
Current theory has it that all of spacetime contains something call Dark Energy. It is this Dark Energy that is driving the expansion of space time. But at galaxy levels, Gravity is much stronger than the dark energy. So gravity tends to pull space time together, and dark energy tends to expand spacetime. At the level of galaxy clusters, however, dark energy wins out and spacetime expands.

Andrew D
2010-Jan-02, 08:27 PM
Just think of it this way: at any given time, any two objects not bound together by gravity are further apart than they were the moment before, and the farther apart they are, the faster they become more further apart.

Scientists can't explain the 'expansion', becuase they don't understand what causes it. You might hear the term "dark energy" thrown around, but that just a smart way to say "we have no idea," without really admitting it.

The math can get pretty complex, but here's a few articles that might help:

http://en.wikipedia.org/wiki/Scale_factor_(Universe) (http://en.wikipedia.org/wiki/Scale_factor_(Universe))
http://en.wikipedia.org/wiki/Friedmann-Lema%C3%AEtre-Robertson-Walker_metric
http://en.wikipedia.org/wiki/Hubble%27s_law
http://en.wikipedia.org/wiki/Redshift

also, Leonard Susskind has a lecture series on iTunes and youtube titled 'Modern Physics:Cosmology' that explains things very well without getting too 'mathy'.

Good luck

Strange
2010-Jan-02, 09:30 PM
THis is interesting and quite handy in a way because that that mean distance like the radius of the Earth orbit or Moon orbit or diameter of Earth or even the size of one atom has not expanded. However at a very microscopic scale the EM radiation has bee expanded i.E redshifted. SO I see that as a paradox and I don't understand it.

It is not directly related to scale. It is simply the fact that the gravity within denser areas (inside galaxies, for example) "holds things together" and stops/slows the expansion. When the EM radiations is between galaxies (or clusters) it is affected by the expansion - hence the red shift. So, no paradox.

korjik
2010-Jan-02, 09:49 PM
THis is interesting and quite handy in a way because that that mean distance like the radius of the Earth orbit or Moon orbit or diameter of Earth or even the size of one atom has not expanded. However at a very microscopic scale the EM radiation has bee expanded i.E redshifted. SO I see that as a paradox and I don't understand it.

Doppler shift in EM isnt a microscopic effect. That solves that apparent paradox quite nicely.


This lower bound that what I was calling a barrier. And aS I said it quite handy because it prevent a full load of complications.

there is still the question ; why gravity or electromagnetism prevent expansion at a lower scale than Galaxy . It does not make sense to me. If it is the fabric of space which expands , we should have expansion at constellation or stellar level at the least and not only at galaxy level.

The 'force' of the expansion is alot of orders of magnitude less than the force of gravity. So much so that until you get to millions of lightyears of separation, you can completely ignore the expansion.

Cougar
2010-Jan-02, 11:24 PM
It is this Dark Energy that is driving the expansion of space time.

Well, since about 1929 it was known some sort of expansion was going on. It appeared to be coasting, so there wasn't any thought of what was "driving" it, other than the initial condition that produced it. There was no need for any idea of dark energy until 1998 when observations appeared to indicate that the expansion was not slowly slowing, but slowly accelerating, which would logically call for some sort of input. That input has taken the name of dark energy, but nobody knows much of anything about it, other than being able to model it pretty precisely using the cosmological constant term in the Einstein field equations.


At the level of galaxy clusters, however, dark energy wins out and spacetime expands.

Actually, gravity is still dominant within clusters, maybe even superclusters. It's not until you start looking WAY out there that the cosmological redshift becomes unmistakable.

aastrotech
2010-Jan-03, 12:54 AM
Nobody knows "how it works" because "big bang", "expansion" is something of a cheat to explain an observed phenomenon, red shift. In short, at close range, we know that light coming from objects moving away redshifts. It is a reasonable assumption that the cause of the red shift at cosmological distances could be caused by the objects moving away. If true this can also be used as a measure of object's distance from us. It is assumed to be true and is used as a measure of distance. A philosophical problem arises with assuming "moving away" is the sole cause of red shift. Everything is red shifted uniformly in direct relationship to its "distance" from us. That would require us to be at the center of everything. Astronomy, since getting burned so bad for insisting geocentrism was a fact, is justifyably uncertain of conclusions that hold that we are at the center of the universe. So enter stage right the expansion speculation. Usually this is put as like a balloon expanding. Any point on the surface of an expanding balloon has all other points moving away from it directly proportional to its distance from it. So, although we see all other objects redshifted in direct proportion to their distance from us, all other observers anywhere would see the same thing. This neatly sweeps aside objections to "big bang" conceptions that (it would be pointed out, in spite of enormous improbabilitiy) we are at the center of the universe because we are not at the center of the universe. It just appears that way due to the uniform red shift from us because the universe is expanding and every point in the universe sees the same thing.

This is all to explain one observation, red shift. It should be pointed out that red shift as a distance measurement matches intrisic brightness measurments of distance based on assumptions of type 1a supernovae, cephid variables and ultimately at very short range geometric measures of distance by parallax. But all of the measurements exept for the very short range parallax measurements are tempo based measurements. Anything that may affect the wavelength (or tempo) of light over time or distance will also affect all tempo based measurements. If light is affected in some way due to distance or time by an amount sufficient to account for the red shift it would not be detectable in the short distances and time we here on earth can observe it.

korjik
2010-Jan-03, 09:03 AM
Nobody knows "how it works" because "big bang", "expansion" is something of a cheat to explain an observed phenomenon, red shift. In short, at close range, we know that light coming from objects moving away redshifts. It is a reasonable assumption that the cause of the red shift at cosmological distances could be caused by the objects moving away. If true this can also be used as a measure of object's distance from us. It is assumed to be true and is used as a measure of distance. A philosophical problem arises with assuming "moving away" is the sole cause of red shift. Everything is red shifted uniformly in direct relationship to its "distance" from us. That would require us to be at the center of everything. Astronomy, since getting burned so bad for insisting geocentrism was a fact, is justifyably uncertain of conclusions that hold that we are at the center of the universe. So enter stage right the expansion speculation. Usually this is put as like a balloon expanding. Any point on the surface of an expanding balloon has all other points moving away from it directly proportional to its distance from it. So, although we see all other objects redshifted in direct proportion to their distance from us, all other observers anywhere would see the same thing. This neatly sweeps aside objections to "big bang" conceptions that (it would be pointed out, in spite of enormous improbabilitiy) we are at the center of the universe because we are not at the center of the universe. It just appears that way due to the uniform red shift from us because the universe is expanding and every point in the universe sees the same thing.

This is all to explain one observation, red shift. It should be pointed out that red shift as a distance measurement matches intrisic brightness measurments of distance based on assumptions of type 1a supernovae, cephid variables and ultimately at very short range geometric measures of distance by parallax. But all of the measurements exept for the very short range parallax measurements are tempo based measurements. Anything that may affect the wavelength (or tempo) of light over time or distance will also affect all tempo based measurements. If light is affected in some way due to distance or time by an amount sufficient to account for the red shift it would not be detectable in the short distances and time we here on earth can observe it.

Too bad none of this is correct.

Tempo is not a physics term.

Wavelength shifts are far more complex that you understand. How to pick out a redshift is pretty well known.

Oddly enough, you arent the first person to realize that knowing what frequency shifts do to spectra is important.

galacsi
2010-Jan-03, 09:28 AM
Thanks everybody for your answers . I am currently digesting them ,except the second link , given by Roobydo , I cannot stomach ! :D

Edit : The link !!!, not Roobydo LOL

mmaayeh
2010-Jan-03, 11:39 AM
Having read all the above then, can I ask are the voids within the universe getting larger as the expansion of space-time is accelerating?

antoniseb
2010-Jan-03, 12:13 PM
Having read all the above then, can I ask are the voids within the universe getting larger as the expansion of space-time is accelerating?
It appears so.

Andrew D
2010-Jan-03, 12:45 PM
Thanks everybody for your answers . I am currently digesting them ,except the second link , given by Roobydo , I cannot stomach ! :D

Edit : The link !!!, not Roobydo LOL

yar!

The FLRW metric is merely a description of a coordinate system that can be used to plot objects within the observable universe. Theres just a few things you need to understand in to grasp what it, regardless of the math.

The FLRW metric starts with the assumption of homogeneity and isotropy of space. In other words, every point in the universe is the same as every other point. This is not only for simplification, but also because as far as we can tell, (with the exception of matter, which accounts for an extremely small amount of the universe's total energy density) the universe is isotropic and homogenious, appearing to every observer as though they are in the center, with everything (distant) 'moving' away from them.

It also assumes that the spatial component of the metric can be time dependent. In other words, the distance between x=1 and x=2 can change with time. This is the 'scale factor' from the first link.

The FLRW describes a curved geometry. If you draw a right triangle on the surface of the earth, at a small scale the triangle will have 180 total degrees (one 90 degree angle and two 45 degree angles, for example). However, because the earth is curved, on a large enough scale a triangle will have more than 180 degrees. For example, if you draw two lines perpendicular to the equator, even though they are both at 90 degree angles with respect to the equator, they will meet at the north pole to form a triangle with three "curved" faces.

Others here can probably go into more depth if you ask, thats pretty much all i'm confident enough to explain.

Keep learning, the secrets of the Universe await!

aastrotech
2010-Jan-04, 12:14 AM
Too bad none of this is correct.

Tempo is not a physics term.

Wavelength shifts are far more complex that you understand. How to pick out a redshift is pretty well known.

Oddly enough, you arent the first person to realize that knowing what frequency shifts do to spectra is important.

Your statement seems to be based entirely on your ignorance of the meaning of the word tempo. "A characteristic rate or rhythm of activity; a pace" http://www.answers.com/topic/tempo .Tempo is a broad enough term to include both frequency and pulsation period.

DrRocket
2010-Jan-04, 12:23 AM
Now for an upper bound, there you get to the nature of the universe. There shouldnt be a barrier to expansion on the universe, because you cant define an outside of the universe for this barrier to be in. Brane theories may say otherwise, but like I said, I am no expert.

You need not worry about an upper bound. It is not necessarily the "diameter" of the universe that is expanding. It is the distance between points that is increasing, as measured by the metric of spacetime.

Think of an infinite rubber sheet with some dots on it. Put the sheet into uniform tension. Then the space between the dots gets larger, but the sheet is still an infinite sheet. It is only in the case of a closed universe that the volume increases, since it is only in that case that the volume is finite.

BTW you can pull off this trick in two different, equivalent ways. The universe can expand. Or all of your meter sticks can contract. You can't tell the difference. Such is the nature of an intrinsic manifold -- it is not embedded in anything.

antoniseb
2010-Jan-04, 12:28 AM
This particular thread is getting a lot of answers that are outside the mainstream. Because there really isn't much concrete to add to the answer... i.e. the mechanism for space's accelerated expansion depends on models that haven't been confirmed yet. So the short answer to the OP is we might know more later.

In any case, I'm closing the thread.