# If Space Expands,

Where does the “Space” come from to replace it? Or is space just stretching? If so, how far can it stretch? Finally, why the hell is it expanding in the first place?
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The question is not posed right. It’s not “coming from” anywhere. Nor is it “stretching”, which would imply an attenuation. It’s expanding, which is not the same as either of those things.

Expansion means that distances get bigger, but they get bigger in all directions and between all places in the same way.

The basic maths of expansion is simple: if we call the present time t=0, then the relationship between the distance d[sub]0[/sub] between any two galaxies now and the distance d between the two galaxies at some other time later time t will be:

d = d[sub]0[/sub]*a(t)

where a(t) is a strictly increasing function of time with a(0) = 1.

Most of the Universe is Dark Energy. Dark Energy is pushing everything else apart. Or it could be something else entirely.

Am I correct in thinking that time expands in the same way, so it’s a 4-dimensional expansion? What about further dimensions?

Actually, space is static. Everything in side it is shrinking…

Yes, I believe this to be correct since space and time are essentially one thing (spacetime), so as space expands, time dilates with it (relatively though, between two reference frames).

As for more dimensions on top of our familiar 3+1, none have been observed, only hypothesized (or mathematically theorized), so we don’t know the reality on that front.

I could be wrong though.

It’s called cosmic inflation, which unlike the regular kind of inflation isn’t controlled by The Fed.

And Glazer’s comment about dark energy is actually far more precise than what science currently understands – i.e. nobody has any friggin’ clue how D.E. actually works.

This isn’t correct, as Cosmic Inflation refers to a specific period that occurred after the big bang in which the universe very rapidly expanded, which lasted a very tiny fraction of a second.

I wouldn’t say time expands as well, it’s more that the time coordinate parameterizes the expansion of the spatial coordinates. In the big bang model space is taken to be the same everywhere and the same in every direction, but you can see that any given patch of space changes in time and how it changes depends on whether you go forwards or backwards in time.

Take for example a circular cone, this 3D solid could be viewed as a sequence of expanding circles parameterized by an axis which is perpendicular to the circles. This is a crude analogy, but it’s probably a better illustration of the relationship between the spatial coordinates and the temporal coordinate.

It’s worth mentioning that for a large part of the Universe’s history dark energy wasn’t the dominant factor in the expansion, so dark energy alone doesn’t tell you why the Universe expands. Unfortunately big bang theory only really says the Universe must be expanding or contracting as Universes that do neither are either empty or unstable. As well as dark energy, the Universe’s expansion depends on it’s initial conditions and big bang theory doesn’t explain why the Universe had those initial conditions, it merely allows us to calculate what they were.

Well, the initial expansion might have been due to inflation, which appears to have been a phenomenon at least qualitatively similar to dark energy (just much, much stronger). So in that sense, you could say that the expansion is due to dark energy.

But ultimately, first causes aren’t really a topic that’s suited to examination by physics, or indeed any science. It begins to feel like that game that three-year-olds love to play, of asking “Why?” to everything. At some point, you run out of answers before the kid runs out of “why?”.

Is this possible? Could you match observations for a constant-sized universe by allowing the universe’s “constants” to vary appropriately? Which constants would vary, and in what fashion?

Right, c remains constant no matter how fast space is expanding.

Why?

No. Everything inside space isn’t growing in scale, so it doesn’t make sense to say the inverse (i.e. everything inside space is really shrinking).

Cute play on words, but nothing more.

IMHO, dark energy is a fudge factor. If you can’t put it on the table or show its properties (Like Helium in the sun and its Fraunhoffer - absorption - lines ?) how do you show this dark energy ?

The big bang theory has so many fudge factors to get it to work…
Whats the direct evidence ?

Microwave background isn’t consistent, there’s variation in it. Could be that its distorted by the presence of a galaxy-nest nearby when our galaxy-nest had a little bang .

Everything isn’t growing because local forces overwhelm the force of expanding space. It would be exactly the same if things were shrinking.

There’s actually a recent reformulation of expansionary dynamics in terms of a ‘shrinking’ fundamental length scale by Christof Wetterich. Johannes Koelman has provided some discussion of this on his blog.

c remains constant in space. However the expansion of space can move objects apart faster than the speed of light without violating relativity. Relativity only says that nothing can get from point “a” to point “b” faster than light. Not disagreeing with you at all, just throwing out that clarification.

wolfpup: And it’s a marvelous one. It even permits for a “rotating cosmos,” where the most distant objects would be moving much faster than light – except that since space itself would be moving, no object would be moving ftl through space.

(No one outside of a few cranks actually supports a rotating cosmos model – but the GR equations do permit it.)