# Is space really expanding?

1. Apr 1, 2005

### Starship

This brought me to the question: How can space expand? Does it mean that i can push off the vacuum? Is space some kind of aether of energetic particles? Do we really know what gravity is (i mean except the curvature of spacetime by mass-energy)? If space has a finite shape but is unbound then what lies outside it?

Last edited: Apr 1, 2005
2. Apr 2, 2005

### Nereid

Staff Emeritus
Welcome to Physics Forums, Starship!

Perhaps the easiest way to answer your excellent (primary) question is to ask you another one - what is 'space'? It looks simple, doesn't it?

Let's take a closer look at what might be behind the question.

First, why do we say 'the universe is expanding'? Because there are lots of good observations showing that objects such as galaxies and quasars are moving away from us, and a quick bit of simple math shows that they are all very likely moving away from each other.

Next, in terms of how we understand physical reality - in the large, not at the microscopic, quantum level - what does 'moving away' mean? No matter how you gnaw on this, you quickly end up acknowledging General Relativity - whether you run with it, or rebel against it, the ideas you work with (in your head, or in equations) have GR in their bones. So, 'how can space expand?' becomes 'within GR, what does the concept of the expansion of space mean?' or 'apart from GR, what theoretical framework is there within which the expansion of space can be understood?'

As to whether 'we really know what gravity is', you quickly get into philosophy, or you get something like 'gravity is whatever GR says it is, because GR is a highly successful theory' or "'gravity' is a theoretical concept; it means whatever the theory you are looking at says is means; for example, in LQG, 'gravity' is [insert], in String Theory, 'gravity' is [insert]".

For the 'if space has a finite shapre but is unbound' question, please take the time to read some of the many threads here in PF on this topic.

3. Apr 6, 2005

### cosmoboy

Like you and me and I think many others are bothered by expansion of space. There are many questions related to space some of them are as follows:

a). What is means by space ?
b). In case the universe is finite what lies beyond it ?
c). Apart form the fact that matter or energy needs space for their existance is there any other meaning of space ?

I think there is no independent existance of space. It exists only with matter/energy, I mean whenever there is matter/energy there is space. I think there is no effect of non local space on any matter/energy.

Last edited: Apr 6, 2005
4. Apr 6, 2005

### lawtonfogle

im guessing he means space as the stars and their moons (planets), and their moons

all space is expanding

scientist thought at first it will expand, slow down due to gravity pull form all matter, and then stop, then it would again begin to move, but it would move backwords untill all matter colided.

what would happen next? It does not matter, because galaxies are actually acellerating apart. After so long we will hit the speed of light. I heard that matter, when it goes the speed of light, converts to pure energy. So their is a place, X number of miles away from where all matter began to move, where a event-horizon lies. It might not be a perfect sphere for some 'areas' of the universe have more matter, so the pull of gravity will afect the accelleration.

Now i have a question pertaining to this. Does a event horizon have a with?

5. Apr 6, 2005

### Garth

"Is space really expanding? " It depends how you measure distances. Edit: That is: "What standard do you define to be the measure of distance?"

The evidence for space expansion is the Hubble red shift.

Also the next question is: "What has it expanded from?" The expanding solution to Einstein's GR cosmological case teaches us that it has expanded from a singularity, or near singularity, called the Big Bang. Nucleo-synthesis in the BB then predicts that the universe should be 3/4 hydrogen and 1/4 helium with very little of anything else. This ties in with the observed cosmic makeup once the elements created in the nuclear furnaces of stellar cores have been taken into account. Also the Cosmic Microwave Background is good evidence of the radiation given off by that BB, now red shifted into the microwave region of the electromagnetic spectrum.

We can say that if our rulers, made of atomic material, are defined to be of fixed size, and this means defining the rest mass of their atoms to be constant, then the Hubble red shift is Doppler in nature and the galaxies are rushing away from us; the universe itself is expanding everywhere because the space in which all stars, galaxies etc. are embedded is itself expanding.

However if we instead choose a different ruler, then the universe may indeed be static in size and it is atoms that are shrinking within it. An expanding universe with fixed rulers is replaced, or interpreted equivalently, as a static universe with shrinking rulers.

A ruler, or rather the atoms it is made of, may shrink over time because they are gaining mass over cosmological time. Edit: (Because other things being equal, the radius of an atom is inversely proportional to its mass) Hubble red shift is then understood to be an effect caused by the increasing mass of the apparatus used to measure it rather than a Doppler effect.

A second effect of atomic rest masses increasing is that atomic clocks would speed up. Edit: (The frequency of an 'atomic' vibration is proportional to its mass) This would have the effect of projecting the BB back, as measured by this new time, into the infinite past.

These ideas were first mooted by Fred Hoyle in his 'Mass field theory', a conformal gravity theory, in which he tried to explain the microwave background in a steady state model. ( Hoyle, F. "On the origin of the microwave background" Ap. J. 196:661-670, 1975 March 15). They also appear again in Self Creation Cosmology

Garth

Last edited: Apr 7, 2005
6. Apr 6, 2005

### Phobos

Staff Emeritus
Well, that wasn't "the first" scientific cosmology, but yes, that "closed universe" model is losing ground vs. an eternally expanding universe (although the closed model is not out of the running yet!)

You're thinking of matter moving through space. The separation of galaxies due to the expansion of space itself is different. The galaxies are still moving through space at sub-light speeds, but their relative velocities compared to us will keep increasing until they're out of sight.

7. Apr 6, 2005

### lawtonfogle

Um, i was talking about galaxies and all matter in them. are not they increasing in speed as they move from us.

Or maybe i just misread something.

8. Apr 7, 2005

### cosmoboy

expansion

We would not be knowing about the expansion of the universe if everything were expanding, however, we know that bound objects do not expand so we can feel the expansion of the universe. My question is that since gravity is scale free then how this scale comes, below
which objects do not expand.

9. Apr 7, 2005

### SpaceTiger

Staff Emeritus
I'm not sure what you mean by gravity being scale-free. In the Newtonian limit it goes as 1/r2, meaning it's stronger at smaller distances. As for the scale on which gravity is dominant over expansion, that depends on the age of the universe. In the CDM paradigm, as the universe gets older, more matter falls into gravitationally bound structures, making them larger, both in comoving units and physical units.

However, this depends on the parameters of the universe as well. If it continues to accelerate under the influence of a cosmological constant, then structure basically freezes into place and no further growth occurs, setting a sort of special scale for structure in the universe. I don't know what the currently predicted number is for this, but I suspect it's in the ballpark of 100 Mpc.

10. Apr 7, 2005

### cosmoboy

Plese look somewhere else this is an important propery
of grvaitational intercation unlike to other fundmental
intercations gravity shows this property.

11. Apr 7, 2005

### SpaceTiger

Staff Emeritus
I can't tell if you're a crackpot or just very poor at expressing yourself. Would you mind pointing me to some references that can show me what you mean?

12. Apr 7, 2005

### Starship

I think i am wrong here. It is not "space" (by space i meant vacuum and not distance) which is expanding, it's space-time which is expanding. Since we can't visualize a four dimensional universe, Riemannian geometry and tensors are the only way to demonstrate it.

Space in theoretical physics has various definitions:

1. The structure defined by the set of "spatial relationships" between objects.

2. A manifold defined by a coordinate system where an object can be located.

3. The entity that stops all objects in the universe from touching one another.

4. The condition within the conceptual field of Existence that provides the 'ground' for any manifested form and as such it enables the movement and all physical dynamics.

We don't know what space-time is. Space-time has been treated as a question of geometry, in this case it's a Minkowski space.

Last edited: Apr 8, 2005
13. Apr 7, 2005

### Garth

No actually, space as a hypersurface within space-time is said to be expanding, space-time may suffer curvature, or not, depending on the average density of the universe, but it cannot itself expand as that is movement which requires the passage of time, and time is part of the space-time that you have said is expanding!

It depends on the perspective from which you look at it. We experience a (3+1)D universe which is expanding; a fully relativistic perspective is to 'see' 4D space-time, which has to be a static view.

Garth

14. Apr 8, 2005

### Starship

Maybe something like http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/pics/LightCone.gif [Broken].

Last edited by a moderator: May 2, 2017
15. Apr 8, 2005

### Staff: Mentor

No one replied to this:
No, what we percieve as a recession velocity isn't a real speed. Its a manifestation of the expansion of space between the galaxies. Locally, galaxies are very nearly at rest.

16. Apr 8, 2005

### Starship

More accurately, the metric space is the one which is expanding. Or when viewed as a Riemannian metric the distance in the metric is the one which is growing.

Last edited: Apr 8, 2005
17. Apr 8, 2005

### Phobos

Staff Emeritus
Just to help clarify this...
We're talking about two kinds of motion here. One, galaxies move through space based on gravitational interactions. Two, galaxies move apart from each other (except in local groups) due to the expansion of space.

lawtonfogle - When you were talking about the relativistic effects of moving at the speed of light, that applies to an object moving through space, not to the expansion of space itself. As an example, note that during the first instants of the universe, it appears that space expanded faster than the speed of light...this does not violate relativity, which applies to things moving through space.

18. Apr 8, 2005

### Mike2

What about the Unruh affect? Isn't this where accelerating frames feel a temerature? Then what about space very far away from us, aren't we accelerating with respect to that that frame? Shouldn't we see a temperature difference somewhere due to acceleration? Thanks.

19. Apr 11, 2005

### hellfire

As far as I know, general relativity is scale invariant (conformally invariant, guv -> k guv) if no matter is included in the theory. If matter is considered and one wants to preserve conformal symmetry then there are some conditions to be imposed on the energy-momentum tensor, or one has to leave the framework of general relativity. Correct?

20. Apr 11, 2005

### Staff: Mentor

You can still detect a kind of doppler shift (that's where the idea of expanding space first came from), but that still doesn't mean its the same type of motion.