Flat universe philosophy

[Realitycheck]

So I need to clarify some details about this flat universe ideology.

 

Is the universe supposed to be 2 dimensional?

 

Is it supposed to be, say, one galaxy thick, on average, give or take a galaxy?

 

Or is it supposed to be, say, one nanometer thick?

 

Of course, I just found out that there are other competing philosophies, but I was wondering why this is even considered. Even the hyperbolic philosophy makes no sense. I envision a spiky ball of fuzz.

[5614]

Are you talking about what people mean when they say "the universe is flat"?

 

A simple overview of the shape of the universe, here: Geometry of Curved Space.

 

Wikipedia on the apparent flatness problem.

[Realitycheck]

Again, it is saying that "if the average density of the universe is equal to the critical density--then the universe will expand to a maximum density and remain there for eternity. This universe is flat; it has zero curvature."

 

So tell me where I am wrong. The Big Bang happens. We have this big, gigantic, dense ball of plasma or whatever. It cools down. Does fusion of photons and neutrons release energy? Matter is flung in all directions. Photons travel faster than the matter, right? So the light takes off, with nowhere to go to. Nevertheless, space is just space. It doesn't grow. It is invaded. So the photons keep expanding into space, long before the matter gets there. I can understand how remnants of the CMBR will always roam the universe, becoming cooler and less prominent as the universe continues to grow.

 

However, since the beginning, all of the matter has been flung in every direction, so why call the universe flat? I understand that if the average density and the critical density are not the same than expansion will be irregular, but expansion is expansion, so why call it flat, even if the densities are the same?

[swansont]

It's generally tough to visualize 3-D non-Euclidean geometries, so the nomenclature focuses on two of them. Which is pointed out in Jonathan's first link.

[thedarkshade]

Is the universe supposed to be 2 dimensional?

What:eek: ? Then how could we (3D objects) exist in a 2D space?

[5614]

After the big bang the universe did expand along all 3 of those axes. Flatness / curvature of the universe is to do with how those 3 axes themselves are curved.

 

 

Whilst this image shows distorted 2D planes, the 2D plane is representing the universe. The curvature, or flatness, of the universe is how the 3D (technically +time, so 4D) universe is shaped.

 

You are familiar with the concept of curved space-time, that's what gravity does, it distorts space-time. But space-time is 4D. Including time can be confusing, so working just with 3D; we're quite happy saying gravity distorts 3D space. Just think of a rubber sheet with a mass in the middle that stretches the rubber (as is the usual example). But once again your approximating a 3D universe to a 2D rubber sheet.

 

The reason being:

It's generally tough to visualize 3-D non-Euclidean geometries, so the nomenclature focuses on two of them. Which is pointed out in Jonathan's first link.

[trober]

If the universe is expanding, what is it expanding into?

 

Until now, I always thought it was *space*-time expanding into something. The post above is a new thought for me, space is already there, matter/energy expanding into it.

[iNow]

If the universe is expanding, what is it expanding into?

 

Until now, I always thought it was *space*-time expanding into something. The post above is a new thought for me, space is already there, matter/energy expanding into it.

 

Welcome to SFN, trober. May you encounter endless new thoughts.

[swansont]

If the universe is expanding, what is it expanding into?

 

Until now, I always thought it was *space*-time expanding into something. The post above is a new thought for me, space is already there, matter/energy expanding into it.

 

We aren't expanding into anything.

 

http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

"[T]here 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."

 

(emphasis copied from original)

[5614]

We aren't expanding into anything.

Yeah, the universe includes everything, there is nothing outside. The universe itself is expanding.

 

But at the same time:

 

I was actually having a good talk to an astrophysicist today about inflation. One inflation model (slow roll - it is one of the main ones, I think!) says that near the beginning of the universe the universe underwent a very fast expansion (faster than the speed of light).

 

Now I'd always imagined the "edge" of the universe to be how far light has travelled since the big bang occurred, meaning that the universe would be ct large (c=speed light, t=age of universe. speed*time = distance). However the universe is observed to be larger than ct. There are other observables, including the flatness of the universe and the horizon problem, that all indicate a faster than c inflationary epoc/era occurred during the early universe.

 

Here's a relevant graph from Cosmological Inflation and Large Scale Structure, Liddle & Lyth:

 

 

The "physical Hubble length" line is what would happen if the universe expanded at a constant ct. Note that the scale during inflation is expanded in the graph, but the "physical Hubble length" line has a constant gradient at ct (well, some logarithmic variant of that).

 

The "physical scale" line is what inflation says happens. During inflation there was a massive, >c, inflation. It does not say that light (or any matter) went faster than the speed of light, but that the universe itself expanded.

 

I asked why the "physical scale" line clearly has a gradient less than that of the ct line, wouldn't then light catch up with the "edge", I argued. Apparently the universe expanded so much that light may never catch this "edge" of the universe. The other thing I was told was that when the 2 lines meet for the second time, after inflation, I was told to ignore that! I don't really understand why, after inflation, the inflation model says that the universe expands at less than ct. I also don't understand why, if it is expanding less than ct, that after a finite time light will catch this "edge", the only way to avoid this is to have an infinitely fast inflation, which did not occur.

 

Anyhow, I have this research, conversation & graph because I'm doing a presentation on the early universe this Friday and I just happened to be doing this today! Luckily we're not going into too much detail on inflation!

 

The reason I began this whole thing with a "but" was because whilst it is the universe itself which is expanding, it is possible that the edge of the universe is beyond the furthest bit of light or matter. It's just an interesting model and thought that I only came across today.

[Realitycheck]

...

[fripro]

After the big bang the universe did expand along all 3 of those axes. Flatness / curvature of the universe is to do with how those 3 axes themselves are curved.

 

 

Whilst this image shows distorted 2D planes, the 2D plane is representing the universe. The curvature, or flatness, of the universe is how the 3D (technically +time, so 4D) universe is shaped.

 

You are familiar with the concept of curved space-time, that's what gravity does, it distorts space-time. But space-time is 4D. Including time can be confusing, so working just with 3D; we're quite happy saying gravity distorts 3D space. Just think of a rubber sheet with a mass in the middle that stretches the rubber (as is the usual example). But once again your approximating a 3D universe to a 2D rubber sheet.

 

The reason being:

 

I could not help but comment on your spheroid drawing. In recent photos in 3d steroscopic placed on the

fourm (I refer you to--Fripro's Gyroscopic Universe Vision) on this SFN forum string. It referes one to a spheroid version of the Universe with photographic

 

http://www.scienceforums.net/forum/showthread.php?t=30901

[Realitycheck]

I still don't understand this. Are they trying to depict hyperbolic space has having accelerated growth, like in the following picture? Whereas, spherical space would refer to slowing growth?

[Martin]

I still don't understand this.

 

that picture fits nicely with the spherical space one.

 

think of each of the circular rings as a 3-sphere.

 

Present day LCDM model and presentday data tend to favor two cases. Omega=1 infinite and Omega>1 finite spherical space (3-sphere).

 

in either case the scalefactor increases as they have drawn the radius of the rings increasing, in the picutre. At first very fast (early universe inflation) and then slowing down (while matter dominates) and then accelerating (as dark energy begins to dominate). In the picture the acceleration is shown by the horn flaring out.

 

The picture fits the spherical space case much more neatly than the alternative---the infinite case. Because you can't draw infinite volume so well.

 

Are they trying to depict hyperbolic space has having accelerated growth, like in the following picture? Whereas, spherical space would refer to slowing growth?

 

No, you seem to have it backwards. People USED to think that spherical space involved slowing growth. That was before 1998. No dark energy or cosmological constant was assumed. Now positive cosmo constant has been measured, so spherical space case involves continued, and now accelerating, expansion.

 

That picture is especially nice as I see it because it is a perfect illustration of the history of a universe with finite spatial volume----where space is a 3-sphere----the 3D analog of the balloon surface that everybody refers to.

What the picture shows is the history of the balloon expanding, except in order to be able to draw it the balloon is reduced to a ring.

 

To be exact, nothing should be drawn inside the cone, or horn. The pictures of galaxies should be dots on the expanding rings. But then the artist could not use them to illustrate the evolution of galaxies that was also going on. The artist risks a bit of confusion by trying to illustrate two processes in the same graphic. Putting stuff inside the spacetime diagram horn-picture of the expanding spatial rings. But I guess most people can avoid getting confused by that mild inconsistency. Pictorial license similar to poetic license.

[Blue Fire]

If the universe is expanding, what is it expanding into?

 

Until now, I always thought it was *space*-time expanding into something. The post above is a new thought for me, space is already there, matter/energy expanding into it.

The universe is expanding because space itself is expanding. In general, galaxies are moving away from one another because the space between them is expanding - they are not moving Through space, they are simply moving with the expansion of space. This is similar to the way pennies that are pasted onto a balloon will move away from each other as the balloon gets bigger and bigger. The pennies aren't moving across the surface of the balloon, they are just moving as a result of the expansion of the balloon's surface. If you can imagine that the surface of the balloon Is the universe, then you can see that the surface isn't expanding Into anything - the surface is all there is. Now try to translate this into thinking about a hypersphere (4D, excluding time) expanding. That might help to understand the theory. At the time of the Big Bang (meant to be an inclusive term of inflation theory), there was no space, no time - they were created and continued to expand, and this was all there was to our universe. Of course it is possible that our universe expanded and continues to expand into something "else" but that something else would not be our universe as we understand it now.

[Dark matter]

Isn't there 2d, 3d, and time? without either one of those the universe would not make any sense, and all if our physics would change.

[swansont]

Isn't there 2d, 3d, and time? without either one of those the universe would not make any sense, and all if our physics would change.

 

Yes, we have these dimensions. I don't think there's anything in these explanations that contradict this.

[antimatter]

The Universe is made up of an infinite number of four-dimensional spaces.