# Is the universe slowing down?

1. Oct 7, 2012

### NWH

So, from a laymans point of view we're taught that we live in an accelerating universe. But I was thinking, the further we look into the distance the faster objects appear to be moving away from us; but the further away we look the further back in time we are actually looking. Doesn't that mean that the expansion of the universe is actually slowing down, since objects closer to us are moving slower than those far away from us? Perhaps that assumes we were the centre of the universe or some other fallacy, I'm not rally sure. If someone could shed some light on this thought that would be appreciated.

2. Oct 7, 2012

### Staff: Mentor

Those distant objects now have a speed well above c (in appropriate coordinates!), while they had a lower speed in the past.

Speed is time- and distance-dependent. "Accelerating" refers to objects which follow the overall expansion of space, and therefore have a larger distance today than they had in the past.

3. Oct 7, 2012

### Chronos

The Hubble flow has been slowing down since the end of the inflationary epoch. As of October 2012, the Hubble constant was estimated to be about 74 km/s/mpc [based on Spitzer data] - up from about 70 km/s based on WMAP 7 data. In the past it was much higher and in the distant future it will decline to around 60 km/s/mpc. The cmb, for example, was originally receeding at about 65c, but, is currently receeding at 'only' about 3.4c.

4. Oct 7, 2012

### Staff: Mentor

The velocity of receding objects is based on their distance from us. Currently expansion causes objects to recede from us at an increasing velocity of about 74 km/s per megaparsec in distance. So for every megaparsec, or about 3 million lightyears, objects are from us, they will on average be moving away at another 74 km/s. Very distant objects are many thousands of megaparsecs away from us, leading to very high recession velocities. For example, an object that is 10 billion lightyears from us is receding at about 250,000 km/s, almost the speed of light! At 40 billion lightyears from us, almost as far out as we can see, recession velocity is almost 1 million km/s, over 3 times the speed of light! (Which is not a violation of any laws. General Relativity allows for velocities to be greater than c between distant objects in an expanding universe)

5. Oct 8, 2012

### Chalnoth

Come on now, km/s/Mpc. Megaparsecs, not milliparsecs ;)

6. Oct 12, 2012

### NWH

Thanks for the responses. Though I'm still a little confused. Aren't the most distant objects we observe just a billion year old image of what once was? On average the further away an object is from us the greater the red shift, so wouldn't that indicate that the universe was expanding at a faster rate in the past, more than it is now in the present?

I just don't understand why we have to look back in time to see the greatest red shift. I ask my self, where did these objects get their velocity from? Why do objects closer to our position not have the same or greater velocity when they have been around just as long?

Sometimes I get the feeling red shift is just an optical illusion and not an actual description of the state of our universe, because it never makes sense to me. Of course I'm just an amateur so my logic is probably flawed in many ways, but I can never wrap my head around it.

7. Oct 12, 2012

### Chalnoth

Not at all. The redshift itself actually has nothing at all to do with how fast the universe has expanded. In fact, the redshift is a measure of the total amount of expansion since the light left that galaxy, not how quickly it got to that amount of expansion.

Perhaps the best way of thinking of this is to think in coordinates where things are stationary with respect to the expansion. In that situation, the far away galaxy and our own galaxy aren't moving (much) with respect to one another. In these coordinates, the redshift doesn't stem from the velocity of the far-away galaxy at all, because the far-away galaxy isn't moving due to the expansion.

Instead of the galaxies moving, then, we can say in this picture that the space between galaxies is being stretched. As it turns out, light rays are stretched by the exact same amount that space is stretched. So if we see a galaxy at $z=1$, then this means the expansion of the universe has carried objects to be twice as far apart as they were then (twice because $z$ is defined as the redshifted wavelength being $z+1$ times the rest-frame wavelength).

In fact, this picture, where we think of the galaxies as stationary and space as stretching to produce the expansion creates a better picture of the behavior of the expansion than does the picture of the redshift stemming from recession velocity.

8. Oct 12, 2012

### Staff: Mentor

Think of what expansion means. If the universe is expanding, then as you increase in distance away from us objects are receding faster and faster. If you could divide space into blocks that are expanding, you would see that the recession of objects in one block far away from us is greater than a block near us because there are more blocks in between to push it away. The RATE of expansion may or may not have been different in the past, but the recession velocity is more due to distance than anything else. (I'm actually not certain if the rate was different back then. I think it was, but I'm not sure)

Remember that the rate of expansion and the recession velocity of objects due to that expansion are not the same thing. The rate of expansion is how much larger a volume of space gets over time. The recession velocity is dependent upon both the rate of expansion and its distance from us.

We have to look far back into time because light has a finite speed and takes many many years to arrive from distances that are far away. Had we been alive 10 billion years ago we wouldn't even be able to see past about a few billion light years since light would have only had 3.7 billion years of time to get to us.

It is difficult to wrap your head around a challenging concept when it's something that you've never really had to try to understand before and don't have the benefit of a college education in cosmology to teach you the nitty gritty details of WHY and HOW this is happening. Luckily you and I have people like Chalnoth to spend all those years learning and can explain it to us!

Last edited: Oct 12, 2012
9. Oct 13, 2012

### NWH

So really it's got nothing to do with the velocity of a distant galaxy but simply to do with how light is warped and stretched over time? Since light from the farthest galaxy traveled a greater distance it has a greater red shift, that was the basic principle I always understood about how light travelled over distance, but I think I've got ideas from other places that really add more confusion than is necessary.

Is it true though, that the galaxies farthest away have a greater velocity that those near to us? Is that velocity relative only to our position? I might be getting a little off topic here, I just want to get some closure on this question as this is what is really adding the confusion.

10. Oct 13, 2012

### Staff: Mentor

The further away a galaxy is, the faster it is receding from us. The velocity is independent of where the observer is at. If I were 500 million light years away, a galaxy 5 billion light years away from me would have an equal recession velocity to a galaxy that is 5 billion light years away from you. IE I measure galaxy A and get a redshift measurement, then you measure galaxy B and get a redshift measurement, both are the same if galaxy A is 5 billion ly away from me, and galaxy B is 5 billion ly away from you.

11. Oct 13, 2012

### Chronos

Correction noted, my failure to properly capitalize compromised my credibilty.

12. Oct 13, 2012

### Staff: Mentor

Hey, I panicked when I saw it. My whole world felt like it was expanding out from under me.

13. Oct 13, 2012

### Chalnoth

One of the things that makes this discussion a bit complicated is that there is no one, unique way to talk about velocities of far-away objects. I can correctly say that far-away objects are not moving with respect to us at all (aside from local motions, such as the fact that we orbit our Sun, and our Sun orbits the Milky Way). But I can also correctly say that the further away objects are, the faster they are moving away from us.

This weird fact of General Relativity can be admittedly very confusing. The problem is that the way General Relativity works, no absolute definition of relative velocity between far-away objects is possible. So we come up with sensible definitions that make the universe easy to understand.

It just turns out that to understand redshift, thinking of the expansion not as galaxies moving through space but instead as space being stretched makes that understanding easier.

If instead we were to discuss what the rate of expansion does to redshift when we describe the expansion as galaxies moving away from us, we would find the exact same effect: that the redshift describes the total amount of expansion instead of its speed. It would just take longer to get to that same conclusion.

Neither description is more correct than the other. It's just that the description of stationary galaxies and expanding space happens to be the easy way to get to this particular answer.

14. Oct 13, 2012

### NWH

I get it now. I guess I understand why the balloon analogy is so relevant, because it's the material of the balloon that is stretching, not a galaxy moving at velocity across the surface of the balloon.

I still don't like the idea of an expanding universe as much as the evidence suggests it, I always find my self on a fine line between illusion and reality and I don't like that. On the contrary relativity is one of the most fascinating areas of physics, such a double edged sword. I hate the notion that I'm the center of the universe and everything accelerates away from me, red shift really gives that illusion.

15. Oct 13, 2012

### Chalnoth

It does, but it's just an illusion. Any observer sitting in a galaxy anywhere else in the universe sees the same overall picture.

16. Oct 14, 2012

### Staff: Mentor

Knowing the magicians trick allows you to be impressed by the illusion, and awed by the truth.