Hello,
Very informative forum. I've learned much just browsing here. I get the feeling people here are friendly and willing to answer even simple and basic questions. So here goes:
After reading the "Big Bang Interpretation" thread, the question that comes to mind is "Into what is spacetime expanding?" I've been searching the web, but I can't find the answer. I've seen the balloon model of the expansion, but no answer to my question.
Klaus

That's a hard question. I won't even try to answer it myself. Here are some answers from Q&A websites I like:

"Perhaps the simplest way to look at these questions is the following: if the universe includes, by definition, everything -- all of space, time, matter, energy -- than there can be nothing outside of it (and hence no edge), nothing for it to expand into. Its true that this is contrary to our everyday experience, as is much else in physics and astronomy; but of course our everyday experience does not extend to the entire universe....Another way to look at it: if there were a higher-dimensional space in which the universe were embedded and into which it expands (like a two-dimensional balloon expanding into three-dimensional space), we could have no way of ever measuring the existence or characteristics of such a space. Whether such an unobservable space can truly be said to exist at all is a question best addressed by philosophers...!"

Reference: Ask a high-energy astronomer at NASA's Imagine the Universe website.

Here's Ned Wright's answer:

"This question is based on the ever popular misconception that the Universe is some curved object embedded in a higher dimensional space, and that the Universe is expanding into this space. This misconception is probably fostered by the balloon analogy which shows a 2-D spherical model of the Universe expanding in a 3-D space. While it is possible to think of the Universe this way, it is not necessary, and there is nothing whatsoever that we have measured or can measure that will show us anything about the larger space. Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about. Just as Dali's Corpus Hypercubicus is just a 2-D picture of a 3-D object that represents the surface of a 4-D cube, remember that the balloon analogy is just a 2-D picture of a 3-D situation that is supposed to help you think about a curved 3-D space, but it does not mean that there is really a 4-D space that the Universe is expanding into."

Reference: Cosmology FAQ from UCLA astronomer's Ned Wright's excellent set of pages.

Hmmmmmm, I agree with DShatl, that is a VERY hard question. One thing I've always wondered is that our universe is just one out of many thriving inside one gigantic "superuniverse". Of course, if there was such a superuniverse, than what lies outside that? An even bigger universe? OUCH my brain hurts.

__________________
-Zap
If you didn't like the opinions expressed in this post, get over it!

Klausnh,
Welcome on board.

visit this link with nice slides:

http://www.astro.utoronto.ca/~barkho...rt5/sld139.htm

Zap: "One thing I've always wondered is that our universe is just one out of many thriving inside one gigantic 'superuniverse'."

There have been speculations about this. One example: suppose black hole singularities can start new big bang universes in sets of dimensions separate from ours--some of these universes would be governed by physics which also allow black hole singularities to form, and these would seed new universes ad infinitum. If we could tally this array of universes from some impossible extra-dimensional viewpoint, we might find no sign of an ultimate beginning, no prospect of an end.

Others have speculated that the inflationary epoch has only collapsed in our particular region of the big bang universe--a region which, however, might still be larger than 10¹⁰ times our current visible horizon--and in other regions the collapse of inflation may have resulted in certain different physical constants. The physical universe we know may be one bubble in a larger, more diverse multiverse; in fact, depending on the details of the collapse of the false vacuum, some have speculated that inflation will, in a larger sense, never end; and we find ourselves in one of a near-infinite number of bubbles of "plain vanilla" vacuum in which the false vacuum has collapsed but there are more such bubble-universes coming into existence all the time in the exponentially-expanding super-universe.

This is speculation, though, and possibly something that beings operating inside a coherent region of spacetime with set physical laws can never possibly test.

If we follow the "superuniverse" line of speculation, and call our spacetime "spacetime(8)" and the spacetime ours is expanding into, spacetime(9), could spacetime(9) be affecting our rate of expansion? This would do away with the need for dark energy/matter.
Second question, any speculation by anyone what spacetime(7) would look to us as it expands into our spacetime(8)?
Klaus

Yikes! The questions just get harder and harder! [img]/phpBB/images/smiles/icon_wink.gif[/img]

There's an April 2002 paper by one of the big guns of theoretical cosmology, Andre Vilenkin, which goes into the idea of eternal inflation.

In this paper Vilenkin first discusses the theoretical framework for an initial singularity nucleating via quantum processes from 'nothing'--a condition without the attributes of space or time. On page 10 of the paper he notes that while theory allows inflation to be eternal into the future, it appears that Arvind Borde, Alan Guth, and others have shown that the DeSitter equations do not allow a universe which is eternal into the past (I don't understand the theory, I have to take Borde's and Guth's word for it).

My statement that there is no sign of a beginning is, if their work is correct, wrong.

On the testablity of the eternal-into-the-future hypothesis, Vilenkin writes:

[Pedantic self-correction: Vilenkin is speaking of the testablity of the quantum-nucleation origin of a singularity, not the eternal-into-the-future part of inflation theory. Sorry.]

"There are also some bad news. In the course of eternal [into-the-future] inflation the universe quickly forgets its initial conditions. Since the number of thermalized regions [ie regions where the false vacuum has collapsed--DS] to be formed in an eternally inflating universe is unbounded, a typical observer is removed arbitrarily far from from the beginning and all memory of the initial state is completely erased. [Here of course Vilenkin is not talking about human memory, but about the 'memory' preserved in the universe through evidence like the cosmic background radiation.--DS] This implies that any predictions that quantum cosmology could make about the initial state of the universe cannot be tested observationally."

Now, as to the question above--what would be different in these these different 'thermalized' regions:

"The only case that requires special consideration is when there are some constants of nature, [alpha_j], which are constant within individual universes, but can take different values in different universes within the ensemble. (One example is the cosmological constant in models where it is determined by a four-form field.) In this case, the memory of the initial state is never completely erased, since the values for [alpha_j] are always equal to their initial values."

abstract in html, full paper in PDF.

So: perhaps one of the constants which might vary between 'thermalized' regions in Vilenkin's proposal for an eternally-inflating universe would be the 'dark energy' of the vacuum.

As far as the eternal-into-the-future inflation hypothesis goes, I don't see how separate regions, separate bubble-universes, could affect each other's expansion rate. It would appear that they are cut off from each other by an exponentially expanding gulf of false vacuum, and no conceivable signal--no force, no energy--could travel between them.

I'm not sure about the bubbles coalescing or colliding. One of the key concepts of the eternal-inflation set of hypotheses is that because the false vacuum expands at an exponential rate--doubling and redoubling its size in fantastically short time periods--and the bubbles only expand linearly (I think), then the bubbles of thermalized, 'collapsed' vaccum cannot all merge and fill the super-universe. What the likelihood of two bubbles merging would be, I just don't know.

<font size=-1>[ This Message was edited by: DStahl on 2003-02-10 01:21 ]</font>

<font size=-1>[ This Message was edited by: DStahl on 2003-02-11 17:13 ]</font>

An easier question?
If I use the analogy of spacetime as a rubber sheet, and I pull that sheet tighter (expand spacetime), any object placed on the sheet will deflect the sheet less as the sheet gets stretched (weaker gravity?).
Does gravity get weaker as spacetime expands, or is the analogy, not valid?

Gravity definitely gets weaker as spacetime expands, but not in the way you're illustrating it. There is no "tension" in spacetime as there is in a rubber sheet (no spring constant). Okay, that's not exactly true, but it's not a straightforward analogy in any case. Every object warps flat spacetime in EXACTLY the same way no matter where it is. The reason expansion weakend gravity is because things get farther away from each other.

Yes, i believe that universe is expanding, but i don't know if it started in a "singularity" or started "in all parts at the same time"

well, iron4, you're not alone in that doubt.

cyreks reply:
The comments above are an example of the unrealistic nature of the BB. To add further comment and to answer the above question simply, is that the BB is expanding into NOTHING. There is no space outside of the BB. We cannot step outside of the BB to see what our Universe is like. We are prisoners of this BB.
Our universe is finite in dimension but infinite in light travel time. The light keeps bouncing (or reflecting) off the edge of this BB and just keeps on circulating.

Actually, michael, this is exactly wrong. There is no "reflection" because we're fairly certain that the horizons are an observational limit and not a physical limit. In other words, somebody looking at the universe 10 billion light years away will not see the same parts of the universe as we will.

If those parts are not observable here, do we consider them in the same universe? Or do they become part of our universe when time passes long enough for them to become observable?

Or do you mean they are 10 billion lights years away and 10 billion years ago?

They are the same thing, as far as we know - unless matter at some point in the past travelled faster than light for a significant period of time, or unless, as some fringe arguments state, the speed of light has changed significantly over the history of the universe.

That's a philosophical question more than anything else. Sort of the rich man's version of "if a tree falls in the forest and nobody is around to hear it..."

Consider, those parts of the universe that are observed 10 billion light years away but not from here will not be causally linked to you because their future light cones have not intersected yours yet. If you map these possibilities in conformal diagrams, you can see some interesting things, but for the most part this is all really theoretical stuff. FOR ALL PRACTICAL PURPOSES are universe is what we can see and only what we can see. Until we really get a coherent theory proving otherwise, it's probably best to stick with that.

michael cyrek:
"Our universe is finite in dimension but infinite in light travel time. The light keeps bouncing (or reflecting) off the edge of this BB and just keeps on circulating."

JS Princeton:
"Actually, michael, this is exactly wrong. There is no "reflection" because we're fairly certain that the horizons are an observational limit and not a physical limit. In other words, somebody looking at the universe 10 billion light years away will not see the same parts of the universe as we will."

Chip comments:
Yes. If we could magically jump instantaneously to a planet circling a star in a galaxy 10 billion light years away from us, and brought along a telescope, we'd see stars and galaxies in different arrangements, but we'd also see on the largest scale, a universe expanding away from us at the new location, (just as we do from the old location - earth.) Our distant "deep field" and cosmic background views from the new location would be just as remote in time, as they are from earth, and would show distant earlier stages, and high red shifted galaxies. We might even find the Milky Way, as it looked 10 billion years ago.

(By hypothetically "jumping" instantaneously in the mind's eye, I'm also muddying the waters of comprehension a bit by actually adapting a single observer coordinate frame set up just between the distant location and us, and thereby setting up an arbitrary, hypothetical universal "now" separate from the distances and the light cones between us. But I was just trying to clarify that based on current observations and physics, the universe has no "center" yet does have a "beginning" in the past, and is expanding as well as finite-yet-unbounded.)


<font size=-1>[ This Message was edited by: Chip on 2003-02-14 15:24 ]</font>

A question for Chip or even JS for that matter from cyrek:
What if you increased that hypothetical distance to 13 billion years, would you see any evolution? If you do then you are close to the center of the BB while instantaneously, the BB is still there with all its galaxies and etc.
Therefore, the BB has (one)_a center.

No... switching to 13 billion years would not show any evolution for precisely the reason that there is no center of the Big Bang.

JS makes the point succinctly. Even if someone could jump away 13 billion light years at once, they'd still observe a universe expanding everywhere away from them, with the most distant red-shifted objects, as well as the CMB appearing as they do from earth. Remember that even if you're on earth or a distant world, you're seeing remote objects as they looked 13 billion years ago. (In the past.)

The BB is not like a conventional explosion, with matter flying out in all directions from a center, filling a pre-existing space. The BB is the beginning of space, time, and matter.

One could think of the universe as not having a "center." If you want a center, you could perhaps think of the "center" of the universe as being everywhere. You could also think of it as being outside the observable universe - in the past. Notice that the CMB is closer to the beginning than we are, and is everywhere in the sky, whereas more concentrated objects (like galaxies) are located in specific locations in the sky.

Dark energy seems to make up most of the universe and is apparently fundamental to the expansion and acceleration. Understanding dark matter and dark energy are fields that will be keeping astronomers and physicists busy for a long time to come.

Having confirmed my foolishness on this question many times, completely understand the feeling of exasperation it causes. Any defined volume of the usual three dimentions does have a center, this has been taught and proved many times to my complete satisfaction. Why this should break down at a cosmological scale, confess it still escapes my limited, and diminished understanding. If we could locate the micropoint of the orgin of the BB, it would be 'close enough' for me to call it the 'center' of our universe, then that's just me. Since all time and all the matter of our universe would have been contained within that point as well, it would be the same as saying the 'center' is everywhere, now.

Charles Bennett, the WMAP principal investigator at NASA's Goddard Space Flight Center, was asked to discuss what the WMAP data says about the ultimate fate of the universe. As a joke, the writer added "in five words or less."

Bennett replied: "The universe will expand forever."

Just wish me were half that sharp and quick, am not, sorry.