# Thread: Does time expand with space?

1. Hmmm....This might sound silly but I can't help it.

It is said that the Earth is about 4.5 billion years old, while the Universe is estimated to be 13.5-14 billion years old.
Don't you feel that the earth is comparatively "too old" to be in this Universe.I mean the ratio of both ages is about 1/3(correct me if I am wrong).

All the events like the big bang, matter anti matter collision, formation of galaxies, and gigantic stars, solar nebula and the formation of sun...All these events must have been happened extremely fast compared to the events happened in the period between earth's formation and now(like evolution and plate drifts).
Either of the two estimated ages could be wrong(or both). But if the estimations are true then that leaves us with just one conclusion...Time has also expanded with space....Captura.jpg.

What I mean by this is that 1s at big bang>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>1s now.
Is it logical to think so? If not? Why???

2. Don't you feel that the earth is comparatively "too old" to be in this Universe
No, I don't really feel that way.

But if the estimations are true then that leaves us with just one conclusion...Time has also expanded with space
That isn't a conclusion I would arrive at. Mathematically, in the FLRW metric, the expansion happens only in the space part of the metric, not in the time part.

Is it logical to think so? If not? Why???
It isn't what the maths of the ( currently accepted as scientific consensus ) Lambda-CDM model tell us.

3. Originally Posted by Markus Hanke
Mathematically, in the FLRW metric, the expansion happens only in the space part of the metric, not in the time part.
However, one can coordinate-transform a metric of the form:

to a metric of the form:

If represents a flat three-dimensional space, then the latter metric manifests the conformally flat nature of the FLRW spacetime.

4. Originally Posted by MaxPayne
What I mean by this is that 1s at big bang>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>1s now.
Is it logical to think so? If not? Why???
How can 1s at the big bang not be what 1s is now? 1s is 1s... what else can it be? Note that the notion of redshift does not conflict with this because one still has to have a uniform notion of what 1s is in order to be able to say there has been a redshift.

5. Originally Posted by KJW
How can 1s at the big bang not be what 1s is now? 1s is 1s... what else can it be? Note that the notion of redshift does not conflict with this because one still has to have a uniform notion of what 1s is in order to be able to say there has been a redshift.
It should be remarked that the whole of general relativity relies on the requirement that has the same meaning at every location in spacetime. Only with this requirement in place does the metric have any meaning.

(Actually, one can relax this requirement and deal with conformal covariance, but this takes one outside the realm of standard general relativity.)

6. Thanks for replying KJW and Mark.Why are we separating time from space time, now? ..I mean gravity bends and twist both space and time.How can space expand without time?

Don't you feel that the earth is comparatively "too old" to be in this Universe
No, I don't really feel that way.
Why? I did explain why I feel so,you didn't give any remark on that.

But if the estimations are true then that leaves us with just one conclusion...Time has also expanded with space
That isn't a conclusion I would arrive at. Mathematically, in the FLRW metric, the expansion happens only in the space part of the metric, not in the time part.
Assumes that the spatial component of the metric can be time dependent..This assumption might have stopped time for expanding, along with space.

How can 1s at the big bang not be what 1s is now?
Because of time expansion.
1s is 1s... what else can it be?
I mean 1 sec then may be 1 hour of today.

I think most of our physics is based on the assumption that time is constant.I still feel that earth is pretty old and it just don't seem to fit in a 13.5 bys old Universe.
Time is just a way of comparing the motion of two objects. 1s = duration 1 rot of earth/24*60*60..that's all.If earth's rotational velocity changes, the magnitude of 1s will also change..unless we calibrate it accordingly.

7. Originally Posted by MaxPayne
Assumes that the spatial component of the metric can be time dependent.
It is not an assumption. In the same way that it is not an assumption that 4 is the result of 2+2.

8. Originally Posted by MaxPayne
Originally Posted by KJW
How can 1s at the big bang not be what 1s is now? 1s is 1s... what else can it be?
Because of time expansion.

Originally Posted by KJW
1s is 1s... what else can it be?
I mean 1 sec then may be 1 hour of today.
Hmmm. I thought it was too good to be true... I had a rather tricky explanation to make and I failed. My point is that in order to make a comparison between now and the big bang, it is necessary to have a reference that does not change between these two times. 1 second at the big bang can't be anything other than 1 second now because that is our reference by which we compare now with the big bang. When we observe a redshift, we are observing how long a cycle of radiation from a source takes when it reaches us. But in order for that to be meaningful, we have to know how long the cycle took at the source, and we can only do that if the notion of time itself does not change.

Originally Posted by MaxPayne
If earth's rotational velocity changes, the magnitude of 1s will also change..unless we calibrate it accordingly.
No. The second is not defined by the earth's rotation. The second is defined by the radiation corresponding to a particular transition of the caesium 133 atom.

9. I think it should be noted that reality can't change in a perfectly hidden manner. Any change must manifest itself in some way. What this means is that it is not possible for reality to change in such a way that because all the references also change in the same way, there is no apparent change. Reality can only change in a way that can in principle be observed.

10. Hi,KJW
No. The second is not defined by the earth's rotation. The second is defined by the radiation corresponding to a particular transition of the caesium 133 atom.
yes,true but we didn't know that the transition of cesium 113 takes one second before 1 sec was invented.To me it is just a calibration scale.
Also if time stretches the cesium 113 transition will take say 1.0001 sec. But it will still be taken as one sec, thus one second in the past will feel longer than one sec in the future. But a conscious being time travelling to the past, will be able to tell the difference.It is also worth mentioning that transition can vary with local time.
For constancy of second we should be able to define it in terms of something which is time (stretch) independent.
Isn't it because of time stretching that we are travelling forward in time?Gravity can(?) influence time, so it can alter the way it stretches?

I think it should be noted that reality can't change in a perfectly hidden manner. Any change must manifest itself in some way. What this means is that it is not possible for reality to change in such a way that because all the references also change in the same way, there is no apparent change. Reality can only change in a way that can in principle be observed.
Since we are a part of reality, there is no way to measure the changes in reality(hidden as far as we are concerned). We will need something outside reality to compare it with reality and to measure the relative change. But even if we find a scale to measure the changes in reality, we will assume that it's the scale that's changing and not the reality.

Hmmm. I thought it was too good to be true... I had a rather tricky explanation to make and I failed. My point is that in order to make a comparison between now and the big bang, it is necessary to have a reference that does not change between these two times. 1 second at the big bang can't be anything other than 1 second now because that is our reference by which we compare now with the big bang. When we observe a redshift, we are observing how long a cycle of radiation from a source takes when it reaches us. But in order for that to be meaningful, we have to know how long the cycle took at the source, and we can only do that if the notion of time itself does not change.
Yes,we need time to be made constant to explain the red shift, but at what cost? Dark energy? Accelerated expansion of space? dark matter? etc etc.
There is no doubt that the space is expanding, but if we keep time constant the expansion of space w

space 1 2 3 4
time 1 2 3 4 //constant expansion

space 1 2 3 4
time 1 1 1 1 //accelerated expansion

space 2 3 4 5
time 1 2 3 4 // decelerated expansion

Hi,Strange

It is not an assumption. In the same way that it is not an assumption that 4 is the result of 2+2
2+2=4, under the assumption that 2 = 2 and 2 =/=3; and x+x=2x and not equal to x+x=x/2. In fact without these assumptions you can even make 2 + 2 =;..lol
Just kidding!!

By the time you put the second "2" in your equation it's value might have already changed by an infinitesimal amount.I was talking about something like this.

11. Originally Posted by MaxPayne
Why are we separating time from space time, now?
We are not. Remember that expansion is not the same thing as local curvature ( from which gravity manifests ). The point is just that the expansion coefficient in the metric sits in front of the spatial parts of the line element, rather than the temporal part - however, they still both remain part of the line element.

Why? I did explain why I feel so,you didn't give any remark on that.
I didn't elaborate further because this question is quite subjective. I can understand that you feel the way you do, but to me 5 billion years is an awfully long time, and much has happened during that time in the rest of the universe. I don't really see any issue.

Assumes that the spatial component of the metric can be time dependent..This assumption might have stopped time for expanding, along with space.
Sorry, I don't really get what you are trying to say.

12. Originally Posted by MaxPayne
Isn't it because of time stretching that we are travelling forward in time?
No, I don't think so. I would rather put that down to the laws of thermodynamics, but then I believe there is a large philosophical element to this question also.

Gravity can(?) influence time, so it can alter the way it stretches?
Gravity and metric expansion are not the same things - be careful not to mix them up.

We will need something outside reality to compare it with reality
I'm afraid this is a rather meaningless statement, at least so far as physics is concerned.

13. Originally Posted by MaxPayne
Also if time stretches the cesium 113 transition will take say 1.0001 sec. But it will still be taken as one sec, thus one second in the past will feel longer than one sec in the future. But a conscious being time travelling to the past, will be able to tell the difference.
If time changes, then it changes for everything. If the cesium atom is producing a lower frequency, then this "consciousness" will be slower as well. 1 second is 1 second.

14. Originally Posted by MaxPayne
yes,true but we didn't know that the transition of cesium 113 takes one second before 1 sec was invented.To me it is just a calibration scale.
Yes, it's true that the second was originally defined in terms of the rotation of the earth, and it's also true that when we redefined the second in terms of the caesium 133 atom, we made this definition numerically match the original definition. But by defining the second in terms of an atomic property, we have ensured that we now have a stable definition of a unit of time because our definition is ultimately based on the laws of physics.

Originally Posted by MaxPayne
Also if time stretches the cesium 113 transition will take say 1.0001 sec. But it will still be taken as one sec, thus one second in the past will feel longer than one sec in the future. But a conscious being time travelling to the past, will be able to tell the difference.
No, an observer would not be able to tell the difference because the changes to the caesium 113 atom would also produce changes to the rates of all chemical reactions, including the biochemical reactions that govern thought processes and the sense of time. This is because the time-related properties of everything are governed by the same laws of physics.

Originally Posted by MaxPayne
For constancy of second we should be able to define it in terms of something which is time (stretch) independent.
We do. It is defined in terms of the caesium 133 atom.

Originally Posted by MaxPayne
Since we are a part of reality, there is no way to measure the changes in reality(hidden as far as we are concerned). We will need something outside reality to compare it with reality and to measure the relative change. But even if we find a scale to measure the changes in reality, we will assume that it's the scale that's changing and not the reality.
But that's the whole point. We are inside of reality and have no access to anything outside of it. Everything we measure in reality is always in terms of reality itself. Therefore, everything we measure is relative to the physical objects that we define as standards. This "intrinsic" view of reality is ultimately the basis of relativity.

15. Originally Posted by KJW
But that's the whole point. We are inside of reality and have no access to anything outside of it. Everything we measure in reality is always in terms of reality itself. Therefore, everything we measure is relative to the physical objects that we define as standards. This "intrinsic" view of reality is ultimately the basis of relativity.
Because we measure everything in terms of physical standard objects, any change that occurs that equally affects the thing we are measuring and the standard objects will not be recognised as a change in the thing we are measuring. And because this is true for everything we measure, we will not recognise any change at all. If we can't recognise any change in reality, then in what way can we say the change happened? Of what significance would the change be to us? In other words, why would we want to consider such a change? The only reason to consider such a change would be to explicitly disregard it, thereby creating a symmetry from which we can derive a conserved quantity using Noether's theorem.

16. But that's the whole point. We are inside of reality and have no access to anything outside of it. Everything we measure in reality is always in terms of reality itself. Therefore, everything we measure is relative to the physical objects that we define as standards. This "intrinsic" view of reality is ultimately the basis of relativity.

Yes, I agree with you.

I don't know if you people realize this, relativity is just one of the many ways to explain the obvious. I am not sure whether or not it is the shortest way. I can't criticize it without studying it in depth.

17. Originally Posted by MaxPayne
I can't criticize it without studying it in depth.
Absolutely right - I really wish everyone on these forum had this same level of insight as you do. It would save us a lot of unnecessary discussions

18. ..lol, the joke is on me

I thought that it's better to ask dumb questions than to sit quiet and cry about it.Slowly but surely I am plugging in the gaps in my knowledge.So I am gaining a lot from this contrary to your belief.

19. Originally Posted by MaxPayne
But that's the whole point. We are inside of reality and have no access to anything outside of it. Everything we measure in reality is always in terms of reality itself. Therefore, everything we measure is relative to the physical objects that we define as standards. This "intrinsic" view of reality is ultimately the basis of relativity.

Yes, I agree with you.

I don't know if you people realize this, relativity is just one of the many ways to explain the obvious. I am not sure whether or not it is the shortest way. I can't criticize it without studying it in depth.
The "theory of relativity", as it is commonly understood, is actually a subset of a more general notion that is applied to "gauge theory". When I use the term "relativity" without a qualifier ("special" or "general"), I'm actually referring to the more general notion.

20. Originally Posted by MaxPayne
I thought that it's better to ask dumb questions than to sit quiet and cry about it.Slowly but surely I am plugging in the gaps in my knowledge.So I am gaining a lot from this contrary to your belief.
Actually, I was quite serious - a lot of people I have discussions with refuse to acknowledge their own lack of understanding of that which they so vigorously oppose ( like relativity, for example ). Your statement that you can't criticise something without in-depth study is a refreshing change from the usual drivel of many others out there.

21. Actually, I was quite serious - a lot of people I have discussions with refuse to acknowledge their own lack of understanding of that which they so vigorously oppose ( like relativity, for example ). Your statement that you can't criticize something without in-depth study is a refreshing change from the usual drivel of many others out there.
Yeah, that is because science nowadays isn't giving the answers that they are seeking.People want things to be made as simple as possible. They believe that the Universe always does all the things in the simplest possible way.

22. Originally Posted by MaxPayne
Yeah, that is because science nowadays isn't giving the answers that they are seeking.People want things to be made as simple as possible. They believe that the Universe always does all the things in the simplest possible way.
Yes, and that is a dangerous logical fallacy. There is no requirement for the universe to be "simple", and the term itself is highly relative.

23. Yes, and that is a dangerous logical fallacy. There is no requirement for the universe to be "simple", and the term itself is highly relative.
Straight line motion of objects through space is a good example.Since the shortest path between two points in space is a straight line objects prefer straight line motion.
Is this a good example?

The Expanding Spacetime Theory

25. 1's are not always 1's, time differs depending on gravity. General Relativity...

26. Hi, Valtiz
But the question here is, Is the time expanding with space?
But since we don't consider time expansion, the space seems to be expanding at an accelerated pace?
Do you agree with my views?

27. Originally Posted by MaxPayne
Since the shortest path between two points in space is a straight line objects prefer straight line motion.
This is true, but only in flat Euclidean space. In all other cases the situation is a little more complicated.

28. When the Universe was young, right after the big bang, it was more dense and the perception of time would have been different. So yes, I think time is expanding with space.

29. Originally Posted by Vatzis
When the Universe was young, right after the big bang, it was more dense and the perception of time would have been different. So yes, I think time is expanding with space.
That isn't what the mathematics of the Lambda-CDM model tell us, though.

30. "That isn't what the mathematics of the Lambda-CDM model tell us, though. "

The model assumes that general relativity is the correct theory of gravity on cosmological scales. So how can it contradict one of the rules on General Relativity. I'm still getting up to speed on mathematics of it all so cannot comment on that.

Lambda-CDM model - Wikipedia, the free encyclopedia

31. Originally Posted by Vatzis
The model assumes that general relativity is the correct theory of gravity on cosmological scales.
Yes, you are right. Currently though there does not seem to be any indication that GR might not be valid on cosmological scales, although I must admit that there is always a remote possibility that that may be the case.

So how can it contradict one of the rules on General Relativity.
What rule are you referring to ?

I'm still getting up to speed on mathematics of it all so cannot comment on that.
You may be interested in my GR primer over on our sister forum : General Relativity Primer

32. You commit an error. I suggest it to discuss. Write to me in PM, we will communicate. с