# Why would gravity slow down the Universe's expansion?

by rfoshaug
Tags: dimensions, expansion, gravity, space
 P: 2 I've read a bit about "dark energy" to explain how the expansion of space can accelerate and overcome gravity's tendency to pull space together again. I'm just a curious layperson, so please have me excused if my questions don't make sense or are very naive. I've tried searching for answers in the forum search, but didn't find specifically what I was looking for. My questions are in short: 1: Is space actually the 4D "surface" of a 3D sphere? 2: If so, why is it assumed that gravity works in 4D and not 3D? 3: And why is it assumed that the expansion of space is governed by forces and rules in 3D space and not 4D? And here a better explained version of what I'm trying to understand: To explain why every point in the universe sees it as being homogenous in every direction and it seems that we are in the center of the expansion, it is often assumed that space is the 3D volume that forms a surface of a 4D sphere, and to illustrate this, it is often reduced by 1 dimension and shown as analogous to a 2D space (with ants!) on the surface of a 3D sphere (ie. balloon that is being inflated so that it expands). So my first question is: is this a correct representation of the Universe? Is it actually (at least) 4 spatial dimensions? Secondly, in this analogy, the true expansion of the balloon is from the 3D center of it. The true center of the balloon is outside 2D space and at a 90° angle to every point in 2D space. Why would gravity, which is only (or is it?) observed to attract massive objects toward each other along the 2D surface be expected to slow the 3D expansion that is happening in relation to the 3D center of the balloon? Shouldn't gravity work only along the 2D surface of the balloon? Thirdly, if this 3D balloon's expansion is accelerating, shouldn't that force (or dark energy or whatever is causing the acceleration) be the one that actually works from the center of the balloon to the surface (in 3D space) due to processes happening inside the balloon, and not necessarily within the 2D universe that we mere ants can observe? To a layperson it might seem that the ants are trying to create complex mathematics for forces and dark energy working along the surface of the balloon in 2D to explain why the 3D balloon is expanding faster and faster while the real answer lies deep within the 3D balloon, in other words outside our entire 2D universe - perhaps not detectable in 2D at all as the true expansion and its associated forces and mechanisms are exactly at a 90° angle to every point in the 2D universe (assuming they work from the center of the balloon outward). Just increase all I've written by one dimension and you have our universe. But as a layperson I also know that this may be an over-simplification of things, and I'm not trying to yell "hey, all those scientists have got it all wrong!", I'm just here to learn from those who know a lot more than me about this stuff. :)
 Sci Advisor PF Gold P: 9,435 The 'force' of dark energy is everywhere. In the early universe dark energy was very feeble and expansion actually slowed down due to gravity until about 5 billion years ago. At that time dark energy had increased to the point it overwhelmed the braking effect of gravity and expansion began to accelerate. The modern view is dark energy is an intrinsic component of empty space and once enough empty space was created it began to flex its muscle.
P: 570
 Quote by rfoshaug 1: Is space actually the 4D "surface" of a 3D sphere? 2: If so, why is it assumed that gravity works in 4D and not 3D? 3: And why is it assumed that the expansion of space is governed by forces and rules in 3D space and not 4D?
You mean 3D surface of a 4D sphere, right?

I'm strictly an amateur, but here's my impression. When I was a kid it was thought that General Relativity was the whole story. The dominant force was gravity, and what mattered was the density of the Universe. If it was above a certain density it would collapse back, and if not then it would expand forever. Later things like "inflation" came up that weren't explained by General Relativity. The idea was that the Universe expanded with great rapidity for a while: why? As far as I know no mechanism has been accepted. It's consistent with the data, that's all.

Then came the observation that the expansion is accelerating. Some other force besides in the General Relativity model must be at work, and that is called dark energy.

As a reader all this stuff tends to get mushed together in a confusing way. Old textbooks and stuff are still around, and contemporary sources may not be up-to-date.

You're other questions confuse me. General Relativity is a 4D theory, or what is nowadays called a 3+1 theory with three spatial and one time dimension. So as far as I'm concerned our theory of gravity is 4D. You are postulating a 4th spatial dimension, but until you show it to me I don't have to accept that. It is true that if we live in a sphere then that suggests a 4th spatial dimension, but we don't know that space is a sphere so that's a guess at this time.

We've gotten to the end of what I know. With that balloon model one can hypothesize a 3D dimension and plot a sort of center to the sphere. I don't know how realistic that is. Like, with the current measurements of curvature of the Universe, is that curvature such that an extradimensional center can be inferred, or does the direction of curvature depend on how one looks at it so that no center can be defined? I don't know. I never thought about it until now.

As to whether gravity slows down the expansion, as far as I know this is a conclusion of General Relativity. I don't know how one my test whether or not this might be true, since the expansion is still a mystery.

P: 2
Why would gravity slow down the Universe's expansion?

 Quote by ImaLooser You mean 3D surface of a 4D sphere, right?
Indeed. Sorry for getting things mixed up. :)

Thanks for your answers. Is the problem that the balloon analogy is only valid to show how everything can move away from everything else with no single place being the center of the expansion, and NOT further valid to conclude that space is the 3D surface of a 4D sphere?
PF Gold
P: 6,283
 Quote by rfoshaug Indeed. Sorry for getting things mixed up. :) Thanks for your answers. Is the problem that the balloon analogy is only valid to show how everything can move away from everything else with no single place being the center of the expansion, and NOT further valid to conclude that space is the 3D surface of a 4D sphere?
Yes, that is correct. Try this for a detailed analysis of the balloon analogy:

www.phinds.com/balloonanalogy

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