# What effect does the expanding universe have on spacetime?

• rede96
In summary, if our universe is indeed expanding, it would have a minor effect on spacetime. However, because space and time are related, this would also have an effect on gravity. Additionally, due to the expansion of space, the distance between objects would have to increase, which would keep the inverse square law of gravity constant.
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?

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

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

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.

pervect said:
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.

Last edited:
rede96 said:
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.

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.

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.

rede96 said:
If our universe is indeed expanding, would this have an effect on spacetime?
Expanding space is a property of spacetime.

rede96 said:
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.

bcrowell said:
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?

rede96 said:
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.

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.

Last edited:
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"

pervect said:
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.

pervect said:
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

bcrowell said:
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.

Thanks for the replies guys.

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

## 1. What is the expanding universe?

The expanding universe refers to the observation that the distance between galaxies is increasing over time. This phenomenon was first discovered by astronomer Edwin Hubble in the 1920s and has since been confirmed by numerous experiments and observations.

## 2. How does the expanding universe affect spacetime?

The expanding universe has a profound effect on the fabric of spacetime. As the universe expands, it causes the space between objects to stretch, resulting in an overall increase in the size of the universe. This stretching of spacetime is known as cosmic expansion.

## 3. What is the impact of cosmic expansion on the speed of light?

According to Einstein's theory of relativity, the speed of light is a constant in a vacuum. However, as the universe expands, the wavelength of light also stretches, causing the light to travel a longer distance to reach us. This can result in the observed redshift of light from distant objects.

## 4. Does the expanding universe have an impact on the curvature of spacetime?

Yes, the expanding universe does affect the curvature of spacetime. As the universe expands, it causes the gravitational pull between objects to weaken, resulting in a decrease in the curvature of spacetime. This is known as the cosmological constant and is a fundamental part of Einstein's theory of general relativity.

## 5. How does the expanding universe impact our understanding of the universe's age and the Big Bang theory?

The expanding universe plays a crucial role in our understanding of the universe's age and the Big Bang theory. By studying the rate of cosmic expansion, scientists can estimate the age of the universe and trace its origins back to the Big Bang. Additionally, the expansion of the universe provides evidence for the Big Bang theory, as it suggests that the universe was once in a highly compact and dense state.

Replies
7
Views
1K
Replies
28
Views
925
Replies
5
Views
1K
Replies
10
Views
2K
Replies
29
Views
2K
Replies
6
Views
1K
Replies
21
Views
2K
Replies
6
Views
2K
Replies
60
Views
2K
Replies
8
Views
1K