# I The Age of Our Universe

1. Dec 29, 2016

### Jacob Gawel

At the big bang, 'we' (an inside observer) were travelling at great speeds. Does this mean to say that time was 'slower' and thus the value (distance traveled in time) of time within 1 second, experienced by an inside observer, was much larger than our modern, perceived, value of 1 second?
If so, how is this phenomenon accounted for in our predictions of the age of the universe?

2. Dec 29, 2016

### Orodruin

Staff Emeritus
No, this is not correct. Comoving observers have no speed relative to the background medium.

3. Dec 29, 2016

### Jacob Gawel

4. Dec 29, 2016

### phinds

The concept of a comoving observer is very basic to cosmology. How about you do some research and come back with a specific question if there is something you don't understand.

5. Dec 29, 2016

### Orodruin

Staff Emeritus
Which part of the statement was unclear? It is impossible to explain further if you do not specify which part you did not understand.

6. Dec 29, 2016

### Staff: Mentor

This may be a source of confusion. The adjective "inside" doesn't apply here. All observers are inside the universe, and there are no outside observers in modern physics.

7. Dec 30, 2016

### Ibix

@Jacob Gawel - there are a number of misconceptions in your post. You are applying special relativistic concepts such as time dilation to a scenario where special relativity does not apply. You are treating speed as an absolute quantity, which isn't correct even in Newtonian physics. And you are not allowing for the complexity of defining speed in a curved spacetime. The end result, unfortunately, is that your question makes no sense. To explain why, however, would mean teaching you general relativity - and I don't have a couple of years free to do that right now.

Suffice it to say that there is a clear meaning to the statement that the universe is 13.9 billion years old, which is that a clock that sees the universe as isotropic and has existed since the Big Bang would show 13.9 billion years having passed. Other clocks could show other things, but it would be harder to defend them as "the age of the universe".

8. Dec 30, 2016

### Jacob Gawel

By the term 'inside observer', I was referring to an observer who is travelling at the same 'speed' as the expansion of the universe at the big bang, as opposed to an observer in a rest frame. I now know that this, essentially, is incorrect and I will do my research on 'comoving observers'.

9. Dec 30, 2016

### Staff: Mentor

Several speculative and off topic posts have been removed. Please don't try to post speculative or off topic ideas in response to this thread.

10. Dec 30, 2016

### PAllen

A starting misconception is that there is any such thing as a rest frame. In cosmology, not only is this concept impossible, it is even impossible to define anything resembling an inertial frame of SR covering more than a (cosmologically) small region.

11. Dec 30, 2016

### vanhees71

Coarse-graining over a large scale of space leads to the description of the spacetime as a space with maximally symmetric spaces for a socalled "comoving observer". I.e., there exists a reference frame (physically determined as the frame according to which the temperature of the cosmic microwave background radiation is isotropic), for which an observer, at rest relative to this frame, considers space at any fixed (coordinate) time as homogeneous and isotropic. This leads to the Friedmann-Lemaitre-Robertson-Walker solution of the Einstein field equation with an energy-momentum tensor consisting of a universe filled with homogeneous and isotropic matter and radiation:
$$\mathrm{d} s^2=\mathrm{d} t^2 -a^2(t) \left [\frac{\mathrm{d} r^2}{1-kr^2} + r^2 (\mathrm{d} \vartheta^2 +\sin^2 \vartheta \mathrm{d} \varphi^2) \right].$$
with $k \in \{-1,0,1\}$. In this sense there's indeed a kind of "rest frame", physically specified by the homogeneity and istropy of the CMBR wrt. to this frame.

12. Dec 30, 2016

### Chris Miller

Misconceptions notwithstanding, I've been thinking along a similar vein. Having learned here that there is no universal now (or when or while or center) it would seem to me that there is no universal age. E.g., to an observer under massive gravitational forces (or traveling near c relative to earth's [and the above-mentioned "rest"] frame of reference) it's going to be a different age and size/shape than it is to us. And it's probably safe to assume that gravitational forces were huge following the Big Bang, which would have dilated time, which brings me back to the poster's question.

13. Dec 30, 2016

### PAllen

Yes, but this coordinate system has little in common with an SR rest frame. Its closest analog in SR is coordinates completely unlike Minkowski coordinate; specifically, Milne coordinates are the closest flat spacetime analog; and in these (Milne) coordinates, world lines of constant coordinate position have unambiguous relative velocity.

14. Dec 30, 2016

### PAllen

It might be helpful to provide unambiguous examples of what the OP may be getting at:

1) Suppose there is some non-comoving observer (that sees anisotropy). If they compare their clock to clocks of comoving galaxies as they pass them, will their measure of age of universe be different? Of course. In this sense, as already noted, the age (hypothetically) measured by comoving observers (they all get the same thing, by isotropy and homeneity) is just a definition of 'age of the universe'. However, it seems a far more useful definition than any other.

2) Suppose there is a specific non-comoving observer that travels from one galaxy to another, setting their clock to agree with the comoving clock of that starting galaxy. When they arrive at a different galaxy, how will their clock compare to the comoving clock of the destination galaxy? In general, their clock will be behind the destination comoving clock when they arrive.

15. Dec 30, 2016

### Ibix

Gravity isn't a force. And the basic problem with your premise is: time is dilated with respect to what? With a black hole I can park a clock close to it and one far away from it and exchange light signals to compare clock rates. How am I going to exchange signals with a clock 13.9 billion years ago? I can't. So there isn't really a meaningful way to ask whether clocks ticked at the same rate or not.

13.9 billion years is what a clock would read if it was at rest in the comoving frame. It's the length of the longest time-like "straight line" I can draw on a map of 4d spacetime that has one end at me now and one end at the Big Bang.

As I said before, other clocks could read other things. But they're harder to defend as measuring the age of the universe.

16. Dec 30, 2016

### Chris Miller

Just googled "What is gravity?" and got lots of links, almost all beginning with "gravity is a force..." E.g.:

grav·i·ty
ˈɡravədē/
noun
1. 1.
PHYSICS
the force that attracts a body toward the center of the earth, or toward any other physical body having mass...

But whatever the correct semantics, it impacts time. Mightn't whatever material changes were occurring after the Bang be considered a clock? Might one ask "With respect to what?" re the size of the early universe as well?

17. Dec 30, 2016

### PAllen

It's very hard to figure out what you are getting at. If you are possibly wondering whether there could be some observer that would measure the universe to be older than the standard definition, the answer is no. Any hypothetical clock in the universe that existed from the Big Bang to some 'now', that follows some world line different from a comoving clock, will show a younger age for the universe compared to the now coincident comoving clock.

18. Dec 30, 2016

### Chris Miller

Thank you, PAllen, this is all I, and I believe the poster, were trying to establish.

19. Dec 30, 2016

### Staff: Mentor

That definition soooooooo doesn't apply at cosmological scales.

20. Dec 31, 2016

### Battlemage!

A dictionary is probably a poor place to look for a physics definition.

21. Jan 3, 2017

### Hugh Parker

Age of the universe?

I have read that the Big Bang occurred almost 14 billion years ago. Astronomers report they have captured images of galaxies 19 billion light years away, but nothing exceeds light speed. The universe appears to be expanding. Perhaps some of the "normal matter" universe is no longer detectable. Dark matter and dark energy might possibly extend beyond what we can detect. So, what is a good estimate of the age of all of the universe?

22. Jan 3, 2017

### stoomart

An expanding universe that is 14 billion years old is much larger than 14 billion light years across.

23. Jan 4, 2017

### StandardsGuy

The Big Bang is said to be the beginning of time. Present theories have an inflation expansion that was greater that the speed of light. Some things like gravity didn't start immediately. No one really knows when time started, or whether it is a constant rate. So the answer is we don't know.

24. Jan 4, 2017

### phinds

That's actually more correctly stated as "the Big Bang Theory, the currently accepted theory of cosmology, does not include a creation event and really only starts about one Plank time after the singularity and so has nothing to say about whether or not time existed prior to the singularity".

That is a recession speed not a proper velocity, so is irrelevant to this thread.

Citation? I don't think this is correct. Gravity was present as part of the 4 combined forces.

The assumption is that time has always passed at one second per second since the beginning of the Big Bang Theory. I refer you to Stephen Weinberg's "The First Three MInutes"

25. Jan 4, 2017

### Staff: Mentor

Actually, even this isn't quite correct. A better statement would be: "The Big Bang Theory, the currently accepted theory of cosmology, does not include a creation event. The well-established part of the model starts with the universe in a hot, dense, rapidly expanding state, called the Big Bang; what existed prior to that state is not known for sure, but the best current contender is some version of an inflationary phase." The end of inflation was not "one Planck time after the singularity", since there might not even have been a singularity and inflation lasted longer than one Planck time anyway.