# Age of Universe relative to what?

1. Jan 4, 2012

People always say the Universe is 14 billions year old.

But what does that mean?

What is the time of the Universe relative to?

2. Jan 4, 2012

### alexg

It means that the Big Bang took place about 13.8 billion years ago.

3. Jan 4, 2012

But isn't time relative?

Did the Big Bang happen at different times in different reference frames?

When we say 13.8 billion years ago, what exactly are we referring to?

4. Jan 4, 2012

### phinds

It is possible to imagine a frame of reference that is stationary relative to the CMB. If you were in such a frame of reference, and had enough snacks to last you for the duration, and a reliable watch that used our current system of duration, you would have observed that amount of time since one Plank Time following the singularity (aka the "Big Bang Event").

There are considerations that would have made this physically difficult so this is just a thought experiment.

5. Jan 4, 2012

### pervect

Staff Emeritus
The age of the universe is always giving in cosmological time, which is time as measured in the frame comoving with the hubble flow, the unique frame where the universe (operationally, the cosmic microwave background radiation) is isotropic.

6. Jan 4, 2012

### lalbatros

Assuming humans are bound to a co-moving frame, we could as well say that these 14 billion year are as-if measured by our common watches. This would be a time as measured in our reference frame.

However, I still wonder how time can be "continued" to periods where references clocks -like our atomic clocks- could even not be envisaged. Atomic clocks could not possibly even exist before atoms were there.

Considering that time is a measured quantity, we need a series of reference clocks bringing us back to the BB. Would it be enough to calibrate these clocks with respect ot each other?
Has such a time accounting been actually performed by astonomers?

7. Jan 4, 2012

### my_wan

Relative to a comoving inertial clock like whats on your wall.

8. Jan 4, 2012

### phinds

There is no evidence that the flow of time has had any different characteristics since the Plank Time, so it seems perfectly reasonable to project backwards as we do.

9. Jan 4, 2012

### petm1

The atom is the edge of my universe, all the atoms together form one edge, and this one edge was created at the same time, relative to my present. It is the atom that appears eternal to me but thanks to Einstein's calendar I can see that even the atom is temporal with a beginning and an end in time. Everything is relative to the present and each of our local clocks.

10. Jan 4, 2012

### Passionflower

It is the age of an ideal co-moving clock.

11. Jan 4, 2012

### my_wan

12. Jan 4, 2012

### phinds

I checked the first couple of these and I completely fail to see what their points have to do with this thread. They talk about the time dilation of accelerating object ... no surprise there but what has that to do with a frame of refernce that is comoving with the CMB?

13. Jan 4, 2012

### my_wan

The comoving frame seems to be what is responsible for the 'apparent' global acceleration of these objects in the universe relative to any distant comoving frame. Just because a pair of distant observers are both comoving with the CMB does not mean they escape the time dilation with respect to each other.

14. Jan 4, 2012

### phinds

I didn't realize that anyone had suggested that they would.

The objects at the edge of our observable universe are receding from us at about 3c, so relative to each other we most certainly are seeing time dilation. BUT ... a comoving frame out there would see the universe at 13.7B years old, as do we (well, almost ... we are a hair off of being comoving). I fail to see what point you are making relative to this thread, which is about the age of the universe.

15. Jan 4, 2012

### Naty1

Age of the Universe:
(Here are my notes from a very long discussion in these forums)

Do all observers agree on the age of our Universe?

Crowell:
No, they don't all agree.

But in an FRW cosmological model, there are preferred observers, who are essentially observers who detect no dipole asymmetry in the CMB. Such observers agree with one another on the amount of clock time since the Big Bang, and this is what we mean when we speak of the age of the universe in such a model.

In the real universe, a clock on the earth's surface is not a bad approximation to such a clock. The solar system isn't moving at any large fraction of c relative to the CMB, and there is not a huge amount of gravitational time dilation between the earth's surface and a point that is, say, outside the local group of galaxies.

There is not just one such frame for the whole cosmos. There is one such frame for every point in the cosmos. Global frames of reference don't exist in GR.

The existence of these preferred frames is also not a general characteristic of GR. It's just a characteristic of this particular solution of the GR field equations.

The age of the universe as usually discussed is for an observer who is at rest relative to the average motion of the matter and radiation in the universe (the "Hubble flow"), and is in the context of homogeneous models, which wouldn't include any structure such as black holes, etc. Yes, you're right, different observers can measure different ages of the universe on their clocks. You can't be "on" a black hole, but an observer hovering just outside a black hole's event horizon would say that according to her clock, the universe is very young. There is no limit on how young the universe could be according to such an observer. The same applies to an observer moving at nearly the speed of light relative to the Hubble flow.

16. Jan 4, 2012

What does CMB stand for?

So we are taking this 13.7 billion year time from the Earth's reference frame correct?

How do we even take this measurement?

17. Jan 4, 2012

### phinds

Google is a nice tool. You should learn to use it.

18. Jan 5, 2012

### TrickyDicky

Right, such an observer's clock would measure the age of the universe arbitrarily close to t=0 as it approaches arbitrarily close to c, therefore the age of the universe is totally relative and depends on the state of motion of the observer; but there are limits to this age for any observer, brought by the absolute velocity c at one side and by the "conventional" CMB velocity that actually also puts an absolute maximum limit to the age of the universe (observer's proper time) that any observer could measure (meaning there seems to be no way to go slower than the comoving frame).
The key here seems to be that in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest you reference that speed to, or otherwise how could c be absolute?

19. Jan 5, 2012

### Staff: Mentor

That is very basic SR. C is absolute not because there is an absolute rest frame but because something which is moving at c in one frame is moving at c in all frames.

20. Jan 5, 2012

### TrickyDicky

did I say anything contradicting that? Your statement is just a tautological explanation of what absolute means. Very basic indeed.