Ok, here goes nothing. If the universe was created in a big bang, and is therefore expanding, and when we look out into space we are looking back in time, how come everything far away is so far apart? Put it another way, say the Universe is 20 billion years old, and we can look back 10 billion years, if we look in opposite directions, things look 10 billion years 'away', in each direction. Yet these two parts of the sky are 20 billion years away from each other, when 20 billion years ago, they should have been in the same place. Am I making any sense? (Ignore the numbers, 20 billion years is probably wrong, but it helps with the maths!)

Geez. A couple of things spring to mind. First off, stars did not form until the Universe had been expanding for quite a long time--maybe .5 to 1 billion years, I think. Even then we couldn't hope to see single stars at the horizon of visibility, so probably the oldest luminous objects we can hope to see are galaxies which, according to current best thinking, formed some time after the first stars. The galaxies seen at the most remote distances do indeed seem to be smaller and less structured than the ones we see nearby. I don't remember, offhand, if they appear to be closer together. Maybe one of the many more knowledgable folks on the BABB can fill in the gaps left by my lazy mind.

The second note of interest is that the most tenable big bang theory includes a period of inflation very early on, when the nascent Universe expanded not by a paltry fraction of a percent per year but at an exponential rate, doubling and redoubling its size in fractions of a second. It's estimated by some that some 100 doubling of size before the special physical state called false vacuum decayed would be sufficient to create a Universe consistent with what we see. But there is no theoretical reason why the period of inflation couldn't have resulted in 200 or 400 doublings instead. If that's the case, the Universe could be vastly larger than the small part we can see...and even at the early stage when the first galaxies were flaring up it could have extended far beyond 10 billion lightyears.

I'll bet you get some other very informed responses on this question. I'm just an interested observer of science, so you must take my comments with a grain of salt. You might delve into <U>The Inflationary Universe</U>, by Alan Guth, and <U>Just Six Numbers</U> by Martin Rees, for layman-level overviews of cosmology.

--Don



<font size=-1>[ This Message was edited by: DStahl on 2001-12-07 00:21 ]</font>

Welcome to the BABB! Just because things are twenty billion lightyears apart doesn't mean that we would see them as they were twenty billion years ago. As you say, we would see them as they were ten billion years ago.

<font size=-1>[Added welcome, sheesh, where are my manners]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2001-12-07 01:45 ]</font>

So, are we in the middle? My point about the 10 billion LY either way is that these things are 20 billion LY appart, and if we are looking back in time to 10 billion LY either way, these things got 20 billion LY away from each other in only 10 billion years. So, as the speed of light is always constant for the observer, and these points have moved apart 20 billion LY in only 10 billion years, then each point has moved away from the other at twice the speed of light. Either something else is going on here, of we shouldn't be able to see past hlfway to the start of the universe.

Surely they are just movng in opposite directions, and hence the distance between them doubles?

Two objects moving away from each other at 1 m/s get 2 m further away from each other every second. Nothing changes if the speed happens to be close to the speed of light.

__________________
Up the Imps!

I know that what I am about to say would drive our old friend JW nuts, but the analogy is not a bad one. Assume you are a bug on an expanding balloon. Now assume that you can only see along lines of sight that run in the surface of the balloon. The balloon has dots all over it, and you can see them as you look along the allowed balloon surface lines of sight. Two things become readily apparent if you let your brain stew on the analogy for a while: (1) Your view of the universe will be somewhat distorted, and (2) You will be unable to locate, or even to define the concept, of the "center" of the distribution of dots on the balloon surface. The reason for this latter observation is that the "center" can only be approached by looking in 3 dimensions, toward the space inside the balloon. But you can't look in any direction thaty does not lie along the surface of the balloon, you can't even directly sense the existence of that 3rd dimsnion.

But you can infer that the 3rd dimension exists by counting dots per unit surface area, making some assumptions about the dots being evenly (or predictably not evenly) spaced on the surface. That count as a function of "distance" from you will reveal that the surface is not a euclidean plane, but has curvature. You can do that with sounts of galaxies and distant galaxy clusters, and come to roughly the same conclusion. The "center" is somewhere else, in a direction that we can infer but not sense.

Tim, amigo, that has to be the best and clearest presentation of the balloon analogy I have ever read. My best compliments.

Notty Imp, the distant galaxies are not moving away from us through space (like a boat through water), the space between is expanding (as if water is welling up between the boats). The galaxies cannot move through space faster than c, but with enough space between us and them the space between can expand at a rate that makes the distance between us and them increase at a rate greater than c.

Ah, I have failed to achieve the TT level of clarity. Two boats can be dead in the water, but if water is welling up between them they may be forced apart anyway. Does that help?

For a real explanation, Google on "Charlie Lineweaver" and go from his personal page to the papers he has written (with Tamara Davis) on "superluminal" redshifts and on the "unchained galaxy" problem. When I get time I'll post links, but right now real life calls... [img]/phpBB/images/smiles/icon_frown.gif[/img]

Gotcha on the balloon analogy. But surely, if the balloon (of space) is expandidng, this applies to all space. Stars and galaxies move apart because the fabric of space is expanding, but then, so should the space within atoms expand also, making these bigger. Net result, no observable phenomonen. To go back to the balloon analogy, if we observe the dots moving away from each other, we would also observe the dots themselves getting bigger. And our bug, if he was 'enbedded' within the fabric of his universe (the rubber of the balloon), he too would expand at the same rate, and therefore wouldn't notice the expansion of the rest of the universe. All the tools he would use to measure the expansion would also be expanding along with the universe. I've never understood this notion that somehow the whole universe can expand without effecting the spaces between and within atoms, only the spaces between stars and galaxies.
Red shifts? Are these observed from stars within our own galaxy? Or just stars from other galaxies? If our own, does this mean our galaxy is expanding?

It seems logical that everything would expand, but it doesn't work that way. The expansion of space is very "weak" and is overcome even by the gravitational attraction between galaxies in clusters. The gravitational force binding stars into galaxies and planets into the Solar System is much stronger than that, and overcomes any expansion of space at local scales. And of course the electrostatic forces between molecules in matter is many times stronger even than the gravitational forces in the Solar System, so we and our yardsticks are not expanding.

As I understand it, we only see expansion between objects that are so far apart that they are not gravitationally attracted to each other by any appreciable amount. I guess that means for most purposes only the space seperating superclusters of galaxies is expanding.

<font size=-1>[ This Message was edited by: DStahl on 2001-12-07 20:45 ]</font>

So, the red shift should not be observed from stars within our own galaxy? Is this what we actually see?

The stars within our galaxy are far too close to us to show much effect of cosmological redshift, even if they weren't tightly bound by gravity as explained above.

We do see redshift in some stars within the Milky Way galaxy -- and blueshift in others. There are significant "proper motions" of stars besides their common rotation around the galactic center of mass. So we see them moving toward or away from us (or rather, that component of their proper motion that is directed toward or away from us). Of course, there are some that have little or no motion of that sort, and so we see them with little or no red- or blueshift.

In short, any cosmological redshift is overwhelmed by the effect of proper motion when you're looking at nearby objects.

Using the balloon analogy, Is the universe thought to be finite in size, yet has no edge.
If you could travel for long enough, fast enough, would you return to your starting position from the opposite direction?

Tom K

I thought that red shift was used to measure the distance to stars where the paralax shift due to the size of the Earths orbit was too small to measure, beyond a few hundred light years. Does that mean we can't measure the distances to stars within our own galaxy?

Not using redshift. There are various ways to measure stellar distances within the Milky Way, parallax being the one used for the nearest stars. Another method can be used for certain special stars called Cepheid variables -- that technique can be used as a tape measure for nearby galaxies too.

Redshift is useful as a distance measure only for relatively distant galaxies.

So, how do we know that red shift is caused by the galaxies receding, and not some other hither to unknown reason?

Excellent question. IMHO, we take our best guess at the cause of the redshift. Whatever the explanation, it has to satisfy the observations:

1. Galaxies that are farther away have higher redshift. (Problem: it's hard to judge how far away galaxies are, you have to find a standard candle like a supernova which is both bright enough to see at great distance and also has a consistent maximum luminosity. Quite difficult.)

2. The characteristic spectral signatures of the elements has to be preserved despite redshifting (or blueshifting). In other words, the emission lines characteristic of ionized hydrogen must be shifted down- or up-spectrum but not scrambled.

3. Redshifted or blueshifted light must not be scattered by whatever mechanism changes its frequency, or else we would not be able to resolve images at any great distance.

4. The explanation should account for redshifting and blueshifting by some plausible mechanism. I guess we would like to say "This would definitely cause light to be redshifted" rather than simply say "We dunno what could account for this."

There are probably some more conditions that I am forgetting. But the expanding Universe hypothesis meets these criteria, while some other possible explanations ("tired light," Compton scattering) don't. Again, IMHO as an amateur, there could be another explanation for the redshift phenomenon. It's just that the general consensus, and it's a pretty firm consensus, is that an expanding Universe is the best hypothesis for the facts as we know them.

--Don

Another thing to keep in mind is that prior to the BB, space and time as we know it did not exist. You always hear about how all (known) matter was contained in an area the size of a pin head just before the BB, but this comparison is really kinda misleading, since space/time (again, as we know it) did not exist until after the BB. Not sure what point I'm trying to make here, or even how accurate it is, but I guess what I'm trying to say is when considering our current view of the universe, it may help to consider that everything really didn't start out at "point A" necessarily, since our perception of what "point A" is is limited to our 4 dimensional view of the universe, which didn't exist until AFTER the BB occurred. Help me out here guys. Am I on the right track???

__________________
. . . My moustache is touching my brain!!!!

Let me see if I get the premise of the original question.

You have 3 points A, B, & C spaced at 10BLYs.

(A)-9-8-7-6-5-4-3-2-1-(B)-1-2-3-4-5-6-7-8-9-(C)

We are at B looking one direction and seeing A 10BLYs away and C at 10BLYs away in the oposite direction. So, would someone at C look across and see us at 10BLYs away and see A beyond us at 20BLYs away? Or because of the curvature of space see a different perspective? Would the view from C be something like this?

(B)-9-8-7-6-5-4-3-2-1-(C)-1-2-3-4-5-6-7-8-9-(A)

Or some variation such as A and B appearing to be at some acute angle with respect to C.
(B)
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9
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8
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7
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6
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5
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4
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3
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2
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1
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(C)-1-2-3-4-5-6-7-8-9-(A)

Okay, can we borrow somebody's warp-drive space ship so we can check this out?

I think the question is why don't we see "A" and "C" close together, since we're looking back to when the universe was close together - the farther back in time we look, the closer together things should be.

I'm sorry, the Dilithium backend has gone on mine, its in the garage for repairs [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
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Are you a Bright?

Look here for stuff on why the big bang is wrong.
http://www.angelfire.com/az/BIGBANGisWRONG/index.html
If folks think that light must blur sources if there is "scattering" then why can we see light going through water or glass? Surely the light interacts with such a transparent medium; it's significantly slowed! But there is not necessarily blurring. This is because the light wave is reconstructed via Huygens' secondary wavelets.

I hate to argue with Tim, but even on a balloon, if it's expanding, the stars should look closer together as you look back in time.

Let's not confuse what happens in the actual physical mechanism with our model for explaining it.

Are you trying to say the model's wrong? Because we don't see a big bang, we can still create a mosel for one? How many angels will fit on the head of a pin?

What is this supposed to mean? That something is trivial? If you want trivial try the Compton Effect to explain redshift.

I'm just saying that light waves are not reconstructed because of Huygens' construction. And you ought to know that.

John, I don't understand this:

"If folks think that light must blur sources if there is 'scattering' then why can we see light going through water or glass? Surely the light interacts with such a transparent medium; it's significantly slowed! But there is not necessarily blurring. This is because the light wave is reconstructed via Huygens' secondary wavelets."

I'm confused because it seems to me that transmission of light through a transparent substance is quite different from scattering off reflective surfaces, refractive scattering, and Compton scattering. Aren't all these photon-electron interactions quite well described by quantum physics, and tractable to mathematical analysis? My admittedly limited knowledge leads me to think that of the four--transmission, reflection, refraction, and Compton scattering--only transmission always results in photon propagation preserving both the wavelength and directionality of the incident photon. Am I incorrect?

--Don

http://www.people.cornell.edu/pages/jag8/spacetxt.html


Check out the page with the Map, as it does a very good job in explaining the geometry one must be OK with to accept an expanding universe theory. I was able to understand this map, and no other explanation to date has layed it out in such laypeople terms.

I'm currently reading "The Nature of Space and Time" by Penrose and Hawking. One of the more interesting lectures is by Hawking where he talks about the inflationary models and some of their problems. Though this book is a few years old, I have not seen recent information which addresses his concerns.

We keep talking about surfaces, curvatures, etc. but the balloon uses the best analogy. Except, we keep talking about things on the surface of the balloon. But nothing is a surface unto itself, so what about the middle of the balloon? Are we living in 3 dimensions on top of time, which in this case would be the ever expanding surface of the balloon? If so, there ought to be some direction I can look in where I see the future... (If I was sitting on top of a balloon, for example, I would just look "up")

Philosophically, I have an easier time with this. Since things are not really "moving away from each other" on the grand scale, but instead everything in between is actually expanding like the example of upwelling between boats, then in essence, I am in the same place I always have been... only time has gone by.

So it's ok to still say that I'm in the center of the universe... it's just that the center of the universe is much larger than it used to be.

12 billion years? Doesn't seem like enough time for someone as complex as me to develop.

DJ

In the "balloon" analogy, objects (and observers) are not "on" the surface. They are "in" the surface. Two dimensional objects (and hypothetical beings) have length and width but no depth. In a two dimensional world, there is no "up". By analogy, we live "in" the three (or four) dimensional surface which is curved through higher dimensions. We cannot "look" in the direction of these higher dimensions.

_________________
TANSTAAFL!

<font size=-1>[ This Message was edited by: Kaptain K on 2001-12-20 12:01 ]</font>

Funny how a lower can never see a higher.

Now THAT works on all levels.

Now, DJ, are you sure it works on levels higher than your own, levels which you can not see? [img]/phpBB/images/smiles/icon_wink.gif[/img]

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

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