# Is big bang 13.7 billion years ago for all observers?

1. Jun 18, 2011

### Singham

Will the big bang be seen as 13.7 billion years ago by all observers in the universe regardless of their velocities ?

I mean I read that in SR one has to abandon notions of simultaniety. And an event which has already occured for one observer may be in future for another observer.

I guess for cosmological purposes GR is used so things must be drastically different from SR.

So my basic doubt is that since we have abandoned notions of simultaneity , how can big bang be 13.7 billion years ago for all observers.

Note - newbie here and don't have physics background,so please bear if this question is outright silly. Please point out where I am goofing up. also if this has been already discussed please point out the thread. cheers.

2. Jun 18, 2011

### Trinitiet

Well, the problem is, the big bang is a singularity in the time-spacedimensions. Special relativity indeed says we should stop the notion of simultaneity, but SR acts as if our spacetime is an Euclidic space (R^4).

As that is not the case in macro cosmology (the universe expands, so the premisse that we live in an Euclidic space is gone), I think it IS a simultane event for all observers.

The simultaneity problem from SR lies in the fact an event happens at a specific spot and the light goes at the speed of c to the observer. But the big bang didn't happen at "13.7 billion lightyears" from us. It's not the same physical situation as the normal simultane situation problems.

But not sure about my explanation though :p

3. Jun 18, 2011

### WannabeNewton

No you are correct in saying that observers don't necessarily have to agree on the age because of simultaneity issues. A single observer chooses a time coordinate so that if he takes a "slice" of the universe at any given time then everything looks roughly the same in all directions(isotropy) and there is roughly the same amount of matter everywhere (homogeneity). Any observer who is at rest relative to this "slice" of the universe will also agree on the time no matter where they are on this "slice". There can be other time coordinates for which this isn't true, you are correct, but the cosmological expansion is slow enough so that on the large scale (the scale cosmology applies at) the random velocities of galaxies and observers are non - relativistic. Also, since there is only ONE universe as far as GR is concerned we might as well choose a reference frame (such as the one this observer has taken with his special time coordinate) so that the observed homogeneity and isotropy is satisfied in this frame. This way we are only worried about cosmological time which is time as measured by clocks in co - moving galaxies (they are at rest relative to each other so their coordinate times are their proper times).

4. Jun 18, 2011

### bcrowell

Staff Emeritus
5. Jun 19, 2011

### Chalnoth

In principle, no. In practice, yes.

Basically, in order to see an age of the universe much different than everybody else, you would not only have to be moving fast compared to the local universe right now, but would also have to have been moving fast for a significant fraction of the history of the universe.

But the problem is, objects that are moving quickly with respect to the background expansion tend to catch up to things moving away from them. You see, in a universe where everything is moving away from nearly everything else, when I move in one direction, any direction, I am moving towards things that have greater and greater velocities in the opposite direction. Thus, without changing my speed at all, I rapidly catch up with matter moving just as fast, and so I reach the overall expansion.

So, in practice, all observers see a universe that is the same age, because all observers end up being drawn to the same average expansion rate.

Basically, any observer which is stationary with respect to the average expansion will see a universe that is the same age relative to its own clock.

6. Jun 19, 2011

### Singham

Nice explanation by all of you. cheers.

7. Jun 22, 2011

### phinds

I would appreciate it if you could expand on this a bit. I am completely puzzled by it. I suppose it sounds perfectly logical to you, but I'm getting nothing but nonsense. I'm not saying it IS nonsense, I'm just saying that I don't understand it at all.

Thanks

8. Jun 22, 2011

### bcrowell

Staff Emeritus
I'm also not having much luck understanding the paragraph from Chalnoth's #5 quoted in phinds' #7.

9. Jun 22, 2011

### Chalnoth

It is, perhaps, easiest to understand in one dimension (and with a chalkboard, but whatever, we can work with this...).

Consider that we have the following setup. There are a set of points listed as:

-3d -2d -1d 0 1d 2d 3d

0 is where we are, with 1d being one unit of distance away, 2d being two units of distance, etc. Now, this is an expanding universe, so we can write the velocities of stuff at these positions as follows:

-3v -2v -1v 0 1v 2v 3v

For simplicity's sake, let's imagine a coasting universe, where "v" is a constant, but "d" increases as d = vt.

Now, with this set up, what happens to the bullet if I fire a gun off to the right at a speed of 2v?

Well, this bullet obviously has no trouble passing point 1, because it is moving quite a bit faster than the stuff at point 1. The bullet can never reach point 3, because it is moving slower. The bullet eventually passes every point < 2d, because every point < 2d is moving at less than 2v. And every point it passes along the way, it is moving more and more slowly with respect to the stuff around it.

Does that clear things up any?

10. Jun 22, 2011

### phinds

Yes, that makes perfectly good sense. How does that tie into the statement "I am moving towards things that have greater and greater velocities in the opposite direction" ?

11. Jun 22, 2011

### Chalnoth

Yeah, I think I misspoke there. Greater and greater velocities in the direction I'm moving would be the accurate statement.

12. Jun 22, 2011

### Chronos

With instantaneous communication, all observers would agree the current age of the universe is 13.7 billion yeats. Of course instantaneous communication is impossble by physics as we know it, so, distant alien cosmologists would report seeing a younger universe commensurate with the light travel time from their place to ours.

13. Jun 22, 2011

### Chalnoth

That all depends upon when they report.

14. Jun 24, 2011

### Chronos

'When' is squishy. They can only report what they observe in their reference frame, by the time the message reaches us, the universe will have aged by the light travel time of the message.

15. Jun 24, 2011

### Chalnoth

Well, it all depends upon whether you want to depend upon when we receive the message, or when they sent it. For instance, if you take a hypothetical situation where they emit their message when the CMB temperatures reaches 2.725K, then they will all report the same age we do, but we will receive the messages much later (or never, if they are far enough away).

16. Jun 26, 2011

### Singham

Yes. Thanks for these further illustrations. :) And the FAQ article is also good.