What effect does the expanding universe have on spacetime?

[rede96]

If our universe is indeed expanding, would this have an effect on spacetime? For example as space and time are related, if I change one, shouldn’t it have an effect on the other?

Also, wouldn’t it make gravity weaker as it is stretching space?

 

[rede96]

What no answers, lol!

Ok, I guess this is just a really silly question or just not interesting

 

[pervect]

I'd say the problem is that it isn't too clear what you're asking about, it's sort of vague.

 

[Arens]

Btw, if i understood well what you are asking, i would say that the answer is in your question. Universe itself is made of 3 dimensions and a forth which is time.

 

[rede96]

I'd say the problem is that it isn't too clear what you're asking about, it's sort of vague.

Yes it is a bit, sorry. I Wasn't really sure how to ask the question.

As I understand it, if the universe is expanding, it isn't that objects are simply moving apart from each other but rather the space in between is expanding.

Also, as I understand space-time (And my understanding of all this is very elementary!) it is the usual 3 dimensions + time, hence space-time. And as one moves through space, one must sacrifice moving through time.

So my first question is that as time and space are connected, what would be the effect on the time element as the space element expands?

For example, would the amount that time dilates over a set distance and speed, change as the universe expands?


My second question was that as space expands, does the effect of gravity weaken over a set distance as the fabric that causes gravity, i.e. the warping of space time, is changing by expansion. (Sorry if that is badly put!)

EDIT: Another way of putting it is that as space expands, the distance between objects must increase. So either the gravitational force remains constant, (i.e. the warping of space time remains a constant.) Or expansion must also 'straighten out' space-time so the inverse square law remains a constant.

 

[bcrowell]

So my first question is that as time and space are connected, what would be the effect on the time element as the space element expands?

Have you tried to imagine a method of measurement that would measure cosmological time dilation? If you try and you can't, then that probably answers your question.

 

[soothsayer]

rede96, while I do not know the answer to your first question, I can tell you that Paul Dirac himself one hypothesized that the strength of gravity as dictated by the gravitational constant, was decreasing with time. A lot of observations have been done to try to determine whether this is true, we have found no evidence suggesting that G is not a constant. Gravity appears to have had the same strength throughout time.

 

[WannabeNewton]

Time dilation due to relative movement is not affected by the expansion of space as you can tell by the fact that the gamma factor depends on the relative velocity and that alone as a variable. Gravitational time dilation is also unaffected because that depends on the metric tensor for the geometry/mass - energy distribution in question and in general one ignores the large scale structure of the universe when probing the geometry of a local mass - energy distribution.

 

[Fredrik]

If our universe is indeed expanding, would this have an effect on spacetime?

Expanding space is a property of spacetime.

Also, wouldn’t it make gravity weaker as it is stretching space?

It seems that what you have in mind is that Newton's theory of gravity says that the gravitational force between two objects is inversely proportional to the square of the distance. The problem with that argument is that you're applying a theory to a situation where it's known to be really bad. You need to use general relativity to describe what's going on at distances where expansion is relevant, and GR doesn't even describe gravity as a force.

 

[Passionflower]

Have you tried to imagine a method of measurement that would measure cosmological time dilation? If you try and you can't, then that probably answers your question.

So you do not think that expansion of space in the FLRW solution is completely equivalent with a global reduction in the speed of light or alternatively a global decrease of time dilation?

 

[atyy]

So my first question is that as time and space are connected, what would be the effect on the time element as the space element expands?

It so happens that although curved spacetime is more fundamental than either space or time, the curved spacetime that describes our universe is well approximated by a separation into space and time. This is why we say "expanding space".

Usually gravity is said to be weaker than electromagnetism, because the gravitational attraction between two electrons is negligible compared to the electrical repulsion. This weakness is described by the universal gravitational constant G. This is a constant of spacetime, so classically, there isn't a sense in which gravity gets weaker depending on spacetime curvature.

Also the expanding space description is only approximate. It is good if we average over large distances, such that the average density of matter is uniform. This is, of course, not true on small scales like the solar system. On that scale, the universe is not expanding.

Sean Carroll wrote a funny and informative piece about this:
http://blogs.discovermagazine.com/cosmicvariance/2007/09/10/brooklyn-is-not-expanding/

A good, but perhaps technical explanation, as to how gravity has apparently contradictory effects like making things clump together on small scales and making them spread apart on large scales:
http://arxiv.org/abs/0707.0380

The scale over which one has to average to make the universe homogeneous is discussed about half-way down Ned Wright's http://www.astro.ucla.edu/~wright/cosmoall.htm.

 

[pervect]

Certain popularizations do talk about "expanding space". I gather that the concept can be made self-consistent, but personally I find that it misleads more than it helps.

Somewhere or other, though, there was a paper by the "expanding space" proponents that showed how one could get the correct answers to physical questions out of the paradigm. I think that it would be more helpful to the OP to try to explain how the idea can be made to work properly rather than to criticize it ...but unfortunately...I can't find the paper.

It was sort of a semi-rebuttal to the Davis-Lineweaver paper, "Expanding Confusion"

 

[atyy]

Certain popularizations do talk about "expanding space". I gather that the concept can be made self-consistent, but personally I find that it misleads more than it helps.

Somewhere or other, though, there was a paper by the "expanding space" proponents that showed how one could get the correct answers to physical questions out of the paradigm. I think that it would be more helpful to the OP to try to explain how the idea can be made to work properly rather than to criticize it ...but unfortunately...I can't find the paper.

It was sort of a semi-rebuttal to the Davis-Lineweaver paper, "Expanding Confusion"

Not http://arxiv.org/abs/0707.0380? They do mention Davis-Lineweaver on their p5.

 

[bcrowell]

Certain popularizations do talk about "expanding space". I gather that the concept can be made self-consistent, but personally I find that it misleads more than it helps.

Somewhere or other, though, there was a paper by the "expanding space" proponents that showed how one could get the correct answers to physical questions out of the paradigm. I think that it would be more helpful to the OP to try to explain how the idea can be made to work properly rather than to criticize it ...but unfortunately...I can't find the paper.

It was sort of a semi-rebuttal to the Davis-Lineweaver paper, "Expanding Confusion"

The paper that atyy posted was the one that also came to my mind.

IMO there is no empirically testable difference between the expanding space interpretation and the kinematic interpretation. It's like the Copenhagen interpretation versus many-worlds: two different verbal ways of describing a single theory. What it all comes down to is that GR doesn't have a uniquely defined way of talking about the velocity of one object relative to another, if the objects are distant from one another.

-Ben

 

[pervect]

Not http://arxiv.org/abs/0707.0380? They do mention Davis-Lineweaver on their p5.

That was the one I had in mind - thanks.

 

[pervect]

The paper that atyy posted was the one that also came to my mind.

IMO there is no empirically testable difference between the expanding space interpretation and the kinematic interpretation. It's like the Copenhagen interpretation versus many-worlds: two different verbal ways of describing a single theory. What it all comes down to is that GR doesn't have a uniquely defined way of talking about the velocity of one object relative to another, if the objects are distant from one another.

-Ben

I tend to agree, a well-formulated concept of expanding space is going to make the same physical predictions as a formulation that doesn't use expanding space.

But as I personally don't tend to really think in "expanding space" terms at all, it's a bit hard for me to put someone who appears to be "going off the rails" with some malformed expanding-space idea get back on the right track.

I also agree that GR doesn't have a uniquely defined way of talking bout relative velocities. I have one other axe to grind, but I don't want to digress, so I'll refrain. Meanwhile, I hope some part of http://arxiv.org/PS_cache/arxiv/pdf/0707/0707.0380v1.pdf is useful to the OP, even if they don't grasp all the fine details.

 

[rede96]

Thanks for the replies guys.

Hopefully I'll get chance to read & absorb later this week. Just wanted to say thanks for the info.