# Age of the universe claims

1. Jul 3, 2006

### MeJennifer

I like to discuss the in my opinion absurdity of the age of the universe claims that are made. How can there be such a thing as one age in for people who have an understanding of SR and GR?

For an object that travelled at light speed during the big bang explosion the age of the universe is zero. For other objects is may be longer or shorter than from the perspective of earth.

So what's up with those absolute numbers? Looks pretty meaningless to me.

2. Jul 3, 2006

### Garth

Hi MeJennifer and welcome to these Forums - keep asking the questions, you obviously know something about S&GR already!

The age of the universe is calculated in the frame of reference in which the cosmological principle holds, which is a slice, or foliation of space-time in which the universe should appear homogeneous and isotropic (on large enough scales). This defines a particular time axis and according to the standard $\Lambda$CDM model the universe is 13.7 Gyrs. old.

This time standard also requires fundamental particles to have constant mass over cosmological history - it is the age of the universe as told by an atomic clock.

Garth.

3. Jul 3, 2006

### MeJennifer

Well it did not take too long for the views on the age and size of the universe in an Einsteinain way to be undermined.

Too inconvenient, too counter intuitive?
It seems what was needed by the ever intelligent scientists was just another measure of absolute space and time now that the Cartesian world-view was squashed by herr Einstein and who really cares about Mach's theories anyway.

So this "homogeneous" area which presumably was like the good old big bang and shortly thereafter era, was it so homogeneous when it expanded? No variance of speed inside? So time and space was homogeneous during and just after the big bang?
If not, then this whole theory falls flat on its behind in my opinion.

Last edited: Jul 3, 2006
4. Jul 3, 2006

### LURCH

I have a similar missgiving about cosmological age measurements, so I'll just add it to this thread (with your permission, Jennifer). My confusion is based on time dilation not caused by relative motion, but by gravity. If all mass in the univesre was once much closer together than it is today, then time in the past must have been much slower than it is now. To any cosmologists occupying the early universe this effect would not have been observable, as it would have been roughly uniform throughout the universe. But to observers today looking into the distant past, the difference should be observable and measurable.

Has it ever been observed or measured? And how is it compensated for in modeling?

5. Jul 3, 2006

### Garth

How many more times does this misconception have to be put right on PF?

"TIME CANNOT GO SLOWER" such a sentence is a contradiction in terms.

Time ALWAYS 'passes' or 'flows' at the tautological rate of one second per second.

What SR motion and GR gravitational time dilation is about is when the clock or process rate of one frame of reference is observed from another frame of reference. In GR gravitational time dilation the two frames of reference are at different gravitational potentials.

If a clock is lowered into a strong gravitational field from a tall tower and then recovered it will record a shorter passage of time than one that stayed at the top. This is because its word line was 'shorter' lower down due to the curvature of space-time - think of the curved funnel analogy.

The time dilation you are talking about is observable and measurable, as the Hubble cosmological red shift.

I hope this helps.

Garth

6. Jul 3, 2006

### pervect

Staff Emeritus
Huh?

I have no idea what your objection is.

For a dramatic visual demonstration of how our galaxy appears at relativistic velocities, see for instance

http://www.exo.net/~pauld/stars/PD_images_relativ.html

Note that this is from a published source, i.e.
In search of the "Starbow": The appearance of the starfield from a relativistic spaceship
John M. McKinley and Paul Doherty
American Journal of Physics 47(4),April 1979 pp 309-316

You appear to have some understanding of relativity, based on the fact that you understand that age is relative. I don't understand why you don't understand that the universe appears isotropic only in one special frame, and appears distinctly non-isotropic if one is moving at relativistic velocities.

For a less dramatic but more technical discussion, see Ned Wright's cosmology tutorial.

http://www.astro.ucla.edu/~wright/cosmo_01.htm

It is well known that the universe appears isotropic only from someone at rest with respect to the Hubble flow.

Because of the importance of the Hubble flow, cosmologists base their coordinate system on it. Hence, "cosmological time".

7. Jul 3, 2006

### MeJennifer

So?
How do you conclude from that that you can determine the age of the universe by taking that as the ultimate frame of reference?
Did I miss anyone demonstrating a plausible theory that indicates that that all too isotropic stuff was as isotropic during or just after the big bang?
Think about it in reverse time, suppose all this isotropic stuff goes back to the size of a pindrop with all the energy extremes that come with it. would you consider that pindrop to be the ultimate in homogeneity? And would you consider that time and space is uniformly defined in the region?

8. Jul 4, 2006

### Garth

Yes - though there are anisotropies created by quantum fluctuations at the ~10-5 level. These are observed in the Comic Microwave Background (CMB) by satellites such as WMAP.

Garth

9. Jul 4, 2006

### pervect

Staff Emeritus
I personally wouldn't want to push the models back much further than the time of the formation of the elements.

This is placed at sometime under 1000 seconds
http://www.astro.ucla.edu/~wright/BBNS.html

An error of 1000 seconds over 15 billion years doesn't strike me as being anything to strike up an argument over.

10. Jul 4, 2006

### Garth

In the standard model you have to push the isotropy all the way back to the close of the Inflation epoch at 10-33 sec.

Otherwise any smoothing process would not have enough time to work on bayonic matter before the Surface of Last Scattering at ~ 300 Million yrs. after BB.

However DM halo formation would be underway well before that.

Garth

11. Jul 4, 2006

### hellfire

The question how far the validity of the models can or must be pushed is interesting. In my opintion the FRW model must be valid even before inflation, otherwise it is not possible to guarantee the existence of an initial, causally connected, homogeneous patch, which is stretched by inflation.

12. Jul 4, 2006

### Garth

Yes, indeed, however in the standard model the universe need not have been very homogeneous before Inflation. Inflation would then smooth everything out by inflating it by a power of 1060 or so.

Garth

13. Jul 4, 2006

### hellfire

I am not sure you are right, Garth. How can you ensure thermal equilibrium if there is no causally connected initial state?

Last edited: Jul 4, 2006
14. Jul 4, 2006

### Garth

In the standard model the pre-inflation 'seed' from which our entire observable universe (and more) expanded was causally connected.

That is why inflation has to be invoked, to solve the horizon problem (so the present entire observable universe was once within a causally connected space).

This together with the density and smoothness problems and the non-detection of a magnetic monopole (if they exist), were problems of a decelerating universe that Inflation resolved.

But note these problems would not have existed in the first place if there is no deceleration in the expansion history of the universe.

Garth

15. Jul 4, 2006

### pervect

Staff Emeritus
I thought of another approach of how to set the limits:

We have been able to study energies up to 1Tev in the laboratory. So it seems reasonable to claim that we can understand the universe at any age in which its energy is less than 1Tev in terms of physics we know, as we have seen this sort of physics in the laboratory.

This would be a scale factor a(t) which is 4*10^15 times smaller than it currently is if my calculations are right

(10^12 electron volts) / (k * 2.7 kelvin) in Google calculator

I'm not sure what this translates to in terms of time, offhand - I usually use Ned wright's calculator, but it doesn't include radiation pressure, so it won't give the right value of time for z=4e15

16. Jul 4, 2006

### Garth

1012 eV ~ 1016 0K ~ 3 x10-13 sec.

Garth

Last edited: Jul 4, 2006
17. Jul 5, 2006

### LURCH

I wasn't aware it had been answered before. Could you provide a link?

Like the difference between the present universe and that of the distant past. That is exactly the basis of the question. The gravitational curvature of today's space is shallower than the that of the early cosmos (if indeed the early cosmos was more densely populated), so time at a great distance should appear slowed, when compared to time in our current frame of refference. If an astronomer looking through a telescope could see a ticking clock 10 billion light-years away, and compare it with a clock sitting on a table next to the telescope, all esle being equal, the clock in the past should appear to tick more slowly, because it is inside a steeper gravity well than the clock in the present.

Taking that annalogy, the present would be the tall tower, and the distant past is the stronger gravitaional field into which we look when we look out to great distances.

Thanks. It does help, a little. Every discussion on the matter at least helps to clarify the question.

Last edited: Jul 5, 2006