Please explain the statement "the big bang happened everywhere at once"

[Drakkith]

I thought he was including the relativistic length contraction... the farther we look, the faster things are receding, so their longitudinal diameters are seen as contracted, as is the space between them, as would be the meter sticks they hold out there to measure things... wouldn't the measure of distance used locally at those distant objects appear to have changed (our view would be that their meters are (were?) contracted compared to ours), right?

I'm not sure we'd see any length contraction since they are moving directly away from us and length contraction acts in the direction of motion. I also don't think the space between moving objects is contracted either. If we take two spacecraft at rest and separated by 10 km, relative to another stationary observer, and then accelerate those two ships in the direction perpendicular to the line of sight between them and the observer until they are close to the speed of light, they will be noticeably length contracted but I don't think the distance between them will have changed.

 

[phinds]

bahamagreen, your first paragraph seems right, but the rest of your post made no sense to me

 

[bahamagreen]

bahamagreen, your first paragraph seems right, but the rest of your post made no sense to me

That may be because I'm incorrect..? :)

But here is my thinking:

- we accept relativistic length contraction of observed moving objects, the faster, the more contraction
- we accept that length contraction includes space, objects, meter sticks
- we observe red shift inferring universal recession increasing with distance

those three things together suggest:

- distant receding objects (galactic super-clusters, etc.) appear contracted progressively with distance
- the space between these objects similarly appears contracted
- approaching the limit, these objects appear thinner and thinner, and likewise the spaces between them
- approaching the limit, there is room for an infinite amount of these objects and the spaces between them

the geometry of this suggests:

- this appearance will be the same from anywhere
- there is no center (everywhere looks like the center)
- any observer will view their location as a density minimum with observed density increasing approaching the limit
- the "edge" or limit is not an observable edge of the universe, it is the surface distance before which everything we can observe exists, and within that surface, because of the progressive contraction, there is room for an infinite number of objects (all locally "space normal")
- this is a finite sphere with infinite volume within because as you observe distant objects they appear thinner, as does the space between them
- the curvature is observed contraction and density increasing with distance approaching the limit

I think this line of thought is direct from and consistent with what SR means for an expanding cosmology... but let me know if not. Maybe GR has more to say on this.

 

[phinds]

A "finite sphere with infinite volume" ? That's a good trick.

 

[rootone]

Here's some bit of the NASA site that I discovered only recently.
I think it gives a concise explanation of where things are in the practical sense.
http://map.gsfc.nasa.gov/universe/bb_concepts.html

 

[Drakkith]

- we accept that length contraction includes space, objects, meter sticks

What do you mean by 'space'? Certainly the distance between objects is shortened, but I don't know about space itself.

those three things together suggest:

- distant receding objects (galactic super-clusters, etc.) appear contracted progressively with distance
- the space between these objects similarly appears contracted
- approaching the limit, these objects appear thinner and thinner, and likewise the spaces between them
- approaching the limit, there is room for an infinite amount of these objects and the spaces between them

As I've said already, I'm not sure you can observe length contraction of an object moving directly away from or towards you. Can anyone with more experience with relativity chime in here?

 

[Ken G]

The expansion is already a general relativity effect, so all relativistic factors are already included-- no need to insert any kind of length contraction. You would only need to do that for objects moving relative to the expansion, i.e., objects that have a motion in the comoving frame of the expanding universe.

 

[dragoneyes001]

i think he's referring to the observable contraction of distance as objects are further away from an observer. two objects ten miles apart seen at one hundred miles look pretty far apart but look almost on top of each other at a distance of a million miles. items at the limit of visibility would need to be incredibly far apart to be seen as separate from each other. but also because there is a limit of distance we can see objects at. that becomes the edge of space and objects on that edge would have a contracted measure of distance because of the distance from the observer regardless of where an observer happens to be that observable limit would be the same.

I'd expect some objects at our limit of observation would have a stretched measure since they would be traveling at some seriously high speeds as groups and individually many of which should be traveling across our field of observation creating some elongation.

 

[bahamagreen]

dragoneyes001, no, I'm not talking about parallax, convergence, reduced subtended angle with distance or any ordinary geometry of vision.

Ken G, not clear what you mean... you are saying there is no contraction for comoving? Or that there is no need to insert contraction because it is already included? I'm assuming it is already inherent in distant observation - recession of distant objects showing contraction...?

 

[Ken G]

Ken G, not clear what you mean... you are saying there is no contraction for comoving?

Correct.

Or that there is no need to insert contraction because it is already included?

That's the same thing, there's no contraction because after you include everything, you don't get any contraction, you get expansion-- if you take the comoving frame coordinate system.

I'm assuming it is already inherent in distant observation - recession of distant objects showing contraction...?

It is inherent, and it is also absent. The expansion of the universe is a solution of the equations of general relativity, so all relativistic effects are included. But it requires two additional assumptions-- you need to apply the "cosmological principle" (that the universe is essentially the same everywhere, at a given age), and then you need to choose the comoving frame for your frame of reference to talk about distances and times. If you do both those things, GR just gives you the time evolution of the "scale parameter" (which all distances between comoving objects evolve in a way that is proportional to), with all relativistic effects included. In our universe, that evolution is expansion with time, that was first slowing due to the gravity of dark matter, but now appears to be accelerating due to the antigravity of dark energy. Yes it is disturbing that both key players in that story are of unknown physical origin, but the observations tell a consistent tale.

 

[bahamagreen]

"the comoving frame coordinate system"... an expanding system of coordinates, very interesting, I'm looking into this... I think many questions will be answered. Thanks, Ken G!

 

[Buzz Bloom]

If the Universe in now flat, it must also be infinite. If it is infinite now, it must have always been infinite, even at that singularity at time zero.

Please see the thread: Big Bang Singularity.

 

[PeterDonis]

If it is infinite now, it must have always been infinite, even at that singularity at time zero.

No, this is not correct. The singularity at "time zero" is not part of spacetime, so it is meaningless to ask whether or not the universe was spatially infinite there. (Also, the model in which this singularity appears is not the actual Big Bang model used in cosmology; that model leaves it open what came before the inflationary era, and does not make any claims about an initial singularity.)

 

[Buzz Bloom]

No, this is not correct. The singularity at "time zero" is not part of spacetime, so it is meaningless to ask whether or not the universe was spatially infinite there.

Hi PeterDoris::

Thank you for you comment.

I am OK with the concept that the singularity is not part of spacetime, but that would also imply it is not a point either, wouldn't it?

I also understand that the current thinking about the singularity is that in first Planck time (5.4 x 10^-44 secs) of the universe, a General Relativity (GR) model would have to be replaced by some currently unspecified quantum model. However, my comment is about extraolating the geometry of the spacetime of an infinite GR model to time zero. This extrapolation would not not result in a point; the spacetime would remain infinite.

I started a thread on this point and my impression of the consensus of the responses there seem to agree with the above point of view. Please see
Big Bang Singularity.

 

[phinds]

...that would also imply it is not a point either, wouldn't it?

Correct, and this is exactly what was already explained to you in the other thread. "Singulaity" does not mean "point" except in the phrase "this is the point where the math model breaks down" in which case it mean "place" not a dimensionless point.