# The expanding universe now?

1. Sep 15, 2011

### razorheno

I have read a few threads on here and acknowledge i am way out of my depth. Bearing in mind my limited knowledge can someone explain to me how we can say the universe is expanding now. the logic seems reversed to me. ive heard it as the furthest away galaxies are receding from us at the greatest speed, the nearer the galaxies the slower the expansion. a galaxy 10 billion light years is receding faster than one 5 billion light years etc. arent what we are seeing is 10 billion years ago the universe was expanding faster than it was 5 billion years ago, doesnt this mean as time has passed the universes expansion has slowed. the further back in time we go the faster it expanded, and we come to more recent time this expansion slows.

2. Sep 15, 2011

### phinds

You are quite right that what we are seeing was emitted as you state and the logic of how you reached your conclusion is clear, but not correct. If the expansion were entirely IN space, then your logic would be correct, but the expansion is OF space so what is being measured is the red shift and the conclusions drawn from the measurement are what lead to the understanding that the expansion is, as I said, OF space and that the expansion is accelerating.

3. Sep 15, 2011

### razorheno

thank you for your reply. but is it not still the case that any information we garner from something 10 billion light years away is 10 billion years old, old news, can we say anything about what is happening 14 billion light years away now, wouldnt that break speed of light for information to reach us, or am i missing that you can see what is happening to said galaxies as they are now. i assumed anything we could ever know about something 10 billion light years away was governed by c, and so 10 billion years old. we have no way to know what is happening now, beyong our nearest neighbours, and they might be moving towards us.

4. Sep 15, 2011

### phinds

yes, but what we are gathering light from is NOT 10billion light years away. It emitted the light 10 billion years ago. If the expansion were not accelerating, then the space between it at the time of emission and us when the photon reached us would have one characteristic and the light arriving here would have an accompanying characteristic and if the expansion is accelerating, then the space between the point of emission and us would have a different characteristic and the photon arriving here will have a different characteristic. This latter is what has been found to be the case. When the expansion was discovered it came as such a big surprize that those doing the measurements were SURE at first that they had made a mistake in the measurement or that perhaps there was a flaw in the logic that assigned supernovas as the standard candle to use to establish distance. Lots of careful analysis of LOTS of red-shift from different distances has confirmed the result conclusively.

Our nearest neighbors ARE moving towards us (well, one of them is at least) and so are a large number of all the galaxies in existance if you only measure their motion IN space, but that motion is totally swamped by their motion due to the expansion of space so in absolute terms everything outside the local cluster is moving away from us.

5. Sep 16, 2011

### Chalnoth

Well, you have to disentangle a bit what is meant here.

What we do in cosmology is slice up our universe into "equal time slices", at which every location in our universe has the same average expansion rate. It is very true that for each time slicing, far-away objects have greater recession velocities than nearby ones. But the point is that far-away objects are can have greater recession velocities even if the expansion is speeding up, merely because they are further away. It all depends upon the rate of the acceleration vs. how much further away these objects are.

That said, the rate of acceleration that our universe is approaching is such that further-away objects will always have higher recession velocities.

6. Sep 16, 2011

### razorheno

thanks for the replies. i still cant quite grasp how we know what is happening at those distances now. you say it is true that far away objects have greater recession velocities than near ones. is it that they had greater recession velocities when the light we see now left them, i.e. 10 billion years ago. if we know what they are doing right now then dont we have information transmitted over vast distances immediately?

7. Sep 16, 2011

### phinds

Good point, and certainly we do not have FTL information transmission. What we have, if I understand it correctlly, is logical conclusions based on extrapolation of current data.

8. Sep 16, 2011

### razorheno

so its an extrapolation but we cant know what is happening in far regions right now, we can only extrapolate what is happening now from what was happening 10 billion years ago, when the light lef its source?

9. Sep 16, 2011

### razorheno

logical conclusions based on the extrapolation of current data? isnt the most current data in this example 10 billion years old, isnt that the most current data?

10. Sep 16, 2011

### phinds

What we observe NOW, when we see light that was emitted 10billion years ago is something that happened 10 billion years ago. What is happening now in the place where the light was emitted is not knoweable directly without FTL transmission of information. So yes, the only thing we have is extrapolation but it is not flaky in any way if that's your concern.

Stated differently, the information we have now is just that ... it is current information but it is about something that happened in the past.

The object that emitted the light 10billion years ago might have gone dark exactly one year later and we won't know about it for just a bit more than one year from now. Same thing if it had gone nova.

11. Sep 16, 2011

### razorheno

so can we know that its expanding now? not that it was expanding then

12. Sep 16, 2011

### razorheno

im sorry if im not clear. you state the information we have now is about the past, so can we know whats happening there now, as in its accelerating expansion now or it was. does the information we get from the light tell us anything about whats happening now or like u said about it going darkly, we cant know what has happened since 10 billion years ago

13. Sep 16, 2011

### phinds

It was expanding then, when the light was emitted. There is absolutely every reason to believe that it is STILL expanding now even though we cannot directly observe it now.

We have observed pretty much all time from now back to about 12billion years in the past and it has been expanding all that time (and the expansion has been getting faster for at least the last 6 or 8 billion years).

This is not direct evidence that it is still expanding way out there NOW, but it would be quite a revolution in physics if we were to discover that the expansion has stopped or turned around. This is not unthinkable and I believe there are some hypothetical models that posit such behavior but it is not the currently accpeted model.

14. Sep 16, 2011

### razorheno

so the expansion got faster about 6 to 8 billion years ago. does this then mean that light from galaxies 5 billion years ago( 5 billion light years away ) is more red shifted than light from 10 billion years ago (1o billion light years away)?

15. Sep 16, 2011

### Chalnoth

What? No. All it means is that light that has been traveling for 6-8 billion years comes from objects that look further away than we would otherwise think if there had been no acceleration of the expansion (e.g. if we are measuring distance by brightness, they appear dimmer than we would otherwise think). Similarly, objects from earlier times, when the expansion was decelerating, appear closer (i.e. brighter) than we would otherwise think if our universe hadn't decelerated in the distant past.

The magnitude of the acceleration/deceleration simply isn't enough to make further away objects to ever have smaller redshifts.

16. Sep 16, 2011

### razorheno

ok so the further away objects(older light) were receding at slower speed than nearer objects (more recent light) but they dont have a smaller redshift. i thought red shift was used as a measure of the acceleration but u seem to imply that the magnitude of the acceleration isnt enough to affect the red shift?

17. Sep 16, 2011

### Naty1

Here is a detailed explanation:

http://arxiv.org/abs/astro-ph/0310808

Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe

Tamara M. Davis, Charles H. Lineweaver

(Submitted on 28 Oct 2003 (v1), last revised 13 Nov 2003 (this version, v2))

We use standard general relativity to illustrate and clarify several common misconceptions about the expansion of the Universe. To show the abundance of these misconceptions we cite numerous misleading, or easily misinterpreted, statements in the literature. In the context of the new standard Lambda-CDM cosmology we point out confusions regarding the particle horizon, the event horizon, the observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests. Attempts to restrict recession velocities to less than the speed of light require a special relativistic interpretation of cosmological redshifts. We analyze apparent magnitudes of supernovae and observationally rule out the special relativistic Doppler interpretation of cosmological redshifts at a confidence level of 23 sigma.

18. Sep 16, 2011

### phinds

HUH? No, your logic is exactly backwards. What would make you think that?

19. Sep 16, 2011

### Chalnoth

It's the relationship between redshift and distance that allows us to infer the acceleration of our universe. So yes, it absolutely does affect the redshift, though because the redshift is so easy to measure compared to distance, we usually take the redshift to be fixed and the distance as being the thing that changes depending upon our cosmological model. This is arbitrary, of course, because the cosmological model doesn't say that either is fixed, and only sets the relationship between the two.

But we have a number of different ways of measuring the relationship between redshift and distance, and they all come up with basically the same picture of how our universe has expanded over time.

20. Sep 19, 2011

### LaurieAG

Surely you would have to agree that a doppler x-ray observation that captured one complete circuit of a source(s) rotating around a galactic centre would produce the images that we actually do see? i.e. half of the rotation is going away from the observer and the other half is going towards the observer. This has nothing to do with universal expansion. If two sources are rotating around each other then force and gravity are only relevant to these two masses and their continuing rotation. The real time curved light paths from these sources are created by no other external force or gravity apart from the source(s).

The real impact of the force and gravity comes into play when you realise that in a minimalist model we only require 2 balanced solar masses spinning around a central point not millions or billions as we hypothethise now. Once again we must agree on what we are actually seeing in our observations from first principles before we attempt to integrate force and gravity into any model.

Last edited: Sep 19, 2011