# What's our time?

1. Jun 7, 2010

### lufbrajames

Hi

Two part question. I'm just curious is there a place in the universe, possibly at the same location as the big bang occurred, that time runs at the fastest we know of? And if there is, what is our passage of time relative to theirs?

I hope that makes sense.

Thanks

2. Jun 7, 2010

### bcrowell

Staff Emeritus
The big bang wasn't an explosion at a particular point in space. It was an explosion in which space began enlarging itself at the same rate everywhere.

The rate at which time passes depends on the gravitational potential. The places in the universe with the greatest potential would be regions very far from any big concentrations of matter, like the voids between superclusters of galaxies. I don't think the difference in the rate of flow of time between there and the surface of the earth would be very big. Certainly less than 1%.

3. Jun 7, 2010

### lufbrajames

Ok perhaps i worded it wrongly, what i meant was that, surely if you trace all of the galaxies motions back in time, you will get a single point. At this point, there is surely the least gravitational effect, because everything is moving away from it?

4. Jun 7, 2010

### Staff: Mentor

All of the galaxies are moving away from every point.

5. Jun 7, 2010

### lufbrajames

Yes but if you track all the galaxies paths, you'll get to a single point, that point must still exist surely? The universe would have expanded around it but surely it should still be there?

Jim

6. Jun 7, 2010

### Staff: Mentor

No. You misunderstand my post completely. There is no such single point. Every point is the "center" of the expansion as you describe it.

You may want to ask this question in the cosmology forum.

7. Jun 9, 2010

### stevmg

Get acquainted with the mathematics of topology (I tried and it is damn near impossible for me) or "rubber sheet" geometry. In essence when you map a "space" (R1n to another space R2n each point in the second space forms a homemorphic image of the first space even though if one looks at both "spaces" (R1 and R2 at the same time they may look nothiing like each other. Each point is one-to-one and onto each point in the other.

So, looks like the original big bang invovled one space and it is being converted to another as a homeomorphic image.

Here's some tricky concepts unrelated to the original question:

1) On the globe (the Earth) if one follows a geodesic great circle around trhe globe there are at least two points on this great circle where the temperature, barometric pair are identical to a second temperature, barometric pair. Has something to do with continuity and no "jump" discontinuities as one travels along the Great Cirlce.

2) There is such a "thing" as a completely undifferentiable yet continuous curve or line. For each point in this line, the incoming slope is different that the outgoing slope and they are never arbritrarily close. This concept could be helpful with the Heisenberg uncertainty principle of quantum mechanics - something I know less than nothing about.

I guess we don't have to talk of singularities here except for black holes, where everything disappears into a zero point (singulairty.)

And by the way, whoever said there ever had to be a beginning to it all? Pope John Paul II did but who was he? He was a Cardinal and not a scientist. But JPII was a good guy and he beleved in science and not the Creationist garbage that Fundamentalist religions (that applies to Christians, Muslims and Jews) push.

Einstein and Bohr got into it about the Heisenberg Uncertainty Principle when Dr. Einstein stated "God does not play dice with the Universe." (Einstein thought that if you kept at it you would find an answer to everthing and nothing was trully spontaneous.)

Bohr replied, "Don't tell God what to do."

8. Jun 9, 2010

### Mike_Fontenot

That's a common misunderstanding. At an arbitrarily small time after the big bang, the spatial coordinates were already bilaterally infinite. I.e., space was ALREADY infinite. So the big bang essentially happened EVERYWHERE at once. And as time progressed, the real distance between any two coordinate points grew larger, but that was happening equally fast everywhere. That's why the microwave background radiation (the remains of the big bang) comes to us uniformly from all directions. (This is for the case where the universe is unbounded (globally flat), but I think the latest evidence suggests that is at least approximately true).

Last edited: Jun 9, 2010
9. Jun 9, 2010

### lufbrajames

I've been trying to avoid this phrase but i dont have anything else to say

That doesn't make sense

(which isn't exactly an argument in physics)

Space may be infinite but from what I understand there is a finite number of galaxies in the universe. So if you plot coordinates of each of the galaxies surely you will be able to pin point a centre? Which is where, I would have thought that time travels at its fastest?

Jim

10. Jun 9, 2010

### stevmg

Has that ever been established... that there are a finite number of stars???

Also, to find a geometric center would require a common frame of reference and there is none.

11. Jun 9, 2010

### lufbrajames

I'm not quite sure I understand what a common frame of reference is. But if you were inside a balloon, given enough measurement you could work out where the centre is, relative to you, but that's not important, it's the fact that there is a centre that's important.

Jim

12. Jun 9, 2010

### Mike_Fontenot

Not if our universe is globally flat. If you make that assumption, the universe is infinite in extent, with an infinite number of galaxies. The average DENSITY is finite. And the portion of the universe that we can see (at the current time) is finite. But there are an infinite number of galaxies which are so far away that their light hasn't yet had time to reach us. That's why you hear the phrase "the visible universe" used in estimates of the number of galaxies, etc.

13. Jun 9, 2010

### bcrowell

Staff Emeritus
The current data aren't good enough to determine whether the universe is spatially infinite or spatially finite. If it's spatially infinite, then according to the models the number of galaxies is infinite, and likewise for the finite case.

In the balloon metaphor, there is no space inside the balloon. The surface of the balloon is all the space there is.

14. Jun 9, 2010

### Passionflower

So are you saying in GR it is impossible to have a globally flat universe with an finite number of galaxies?

15. Jun 9, 2010

### Staff: Mentor

For the third time now, there is no such point.

Let's say that you have a baloon (a positively curved 2D space). Now, let's say that you draw a bunch of dots on the baloon surface and inflate the baloon. Each dot will get further away from every other dot (as measured along the surface), but there is no point on the 2D surface of the baloon that is the center of the expansion.

In this analogy the universe (baloon) is a curved 2D space embedded in a flat 3D space. The center of the expansion exists only in the flat 3D space, and not within in the curved 2D space. So there is no point in the universe which is the center of expansion in this analogy.

Last edited: Jun 9, 2010
16. Jun 9, 2010

### lufbrajames

Thanks for everyones input, I can't say that I'm not a little confused.

So assuming that is correct (but for 3D for our universe) if you set off in any direction you'd get back to where you started? What evidence/theory has lead us to believe this? It seems quite odd.

Jim

17. Jun 9, 2010

### Staff: Mentor

If it turns out that the universe is finite, has positive curvature, and a specific topology (which I don't know the name for), yes.

The theory is general relativity and there is quite a bit of evidence to support it such as the precession of Mercury, gravitational lensing, gravitational time dilation, and the Shapiro delay. The specific solution used in cosmology is the FLRW metric and it is closely related to the big bang model and all of the cosmological evidence supporting that including the Hubble constant and the cosmic microwave background radiation.

18. Jun 9, 2010

### Passionflower

In all fairness though, all these cases are Ricci flat cases so they can obviously not be used as proof for cases that are not Ricci flat such as the FLRW metric.

Yes, but unless we propose dark matter and/or dark energy these theories are under stress to say the least.

19. Jun 10, 2010

### Staff: Mentor

I thought I was careful to list them as support for GR, not the FLRW metric. I apologize if my wording was unclear on that point.

20. Jun 10, 2010

### Passionflower

No need for apologize, after re-reading I see now what you mean, my apologies, it appears that I was premature.