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- Thread starter Daleri Mc Rileda
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How can the age of the universe be determined when there is no absolute measure of time or distance in the universe.

Because "the age of the universe" is not an absolute measure of time. It is the elapsed time on the clocks of idealized "comoving" observers (observers who always see the universe as homogeneous and isotropic) since the Big Bang. Observers in different states of motion would measure a different age.

time expands with the expansion of space

What does this even mean?

I think you might want to spend some time working through Ned Wright's cosmology FAQ and tutorial:

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

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Because "the age of the universe" is not an absolute measure of time. It is the elapsed time on the clocks of idealized "comoving" observers (observers who always see the universe as homogeneous and isotropic) since the Big Bang. Observers in different states of motion would measure a different age.

What does this even mean?

I think you might want to spend some time working through Ned Wright's cosmology FAQ and tutorial:

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

So if the other observer calculates a different age of the universe, which is the correct age?

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f the other observer calculates a different age of the universe, which is the correct age?

What do you mean by "the correct age"?

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Which age is the actual age of the universe?What do you mean by "the correct age"?

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Bandersnatch

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Depends on who's measuring.Which age is the actual age of the universe?

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: )Depends on who's measuring.

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Which age is the actual age of the universe?

What do you mean by "the actual age"?

In case you haven't caught on yet, there is no such thing as "the correct age" or "the actual age". All there is is the age as measured by a particular observer. Different observers can measure different ages. That's all there is to it.

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I believe that it is also true that everywhere in the universe it is possible to determine an age that is associated with a co-moving (to the CMB) observer and that all such measurements/calculations no matter where taken/done will give the same answer since they compensate for the movement of actual observers.

Yes, this is correct. You don't have to be a comoving observer to calculate the age a comoving observer would measure, as long as you know your own motion relative to a comoving observer at your spatial location. We estimate that by looking at the dipole anisotropy that we observe in the CMB and calculating our motion relative to a comoving observer (who would see the CMB as having no dipole anisotropy) from that. (Note that virtually all published data for the CMB temperature distribution already subtracts out the dipole.)

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Chronos

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- #12

Drakkith

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So if the other observer calculates a different age of the universe, which is the correct age?

As other have said, both are correct. The basic idea is that most structures in the universe (planets, stars, galaxies, etc) occupy frames of reference that move relatively slowly with respect to a convenient reference frame where the CMB is extremely uniform. This makes it convenient to measure the universe's age with respect to a co-moving observer, which is just an observer anywhere in the universe who is in our "convenient" frame of reference where the CMB is uniform. We'll call this convenient frame the CMB frame. (Note that this convenient frame is not an "absolute" frame nor a "preferred" frame. It's just a convenient one)

Observers moving with respect to our CMB frame experience time dilation with respect to that frame. This means that the age of the universe as measured by these frames would be different from what the observer in the CMB frame measures. An observer moving very, very quickly with respect to the CMB frame would measure the universe as being much older than 13 billion years. You could say that the age of the universe measured from the CMB frame is the "minimum age" any observer would measure. (I think so at least)

As always, someone correct me if I'm wrong.

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Should it not be the other way round, i.e. that such observers, using their own clocks, would measure the universe to be younger than13 billion years?An observer moving very, very quickly with respect to the CMB frame would measure the universe as being much older than 13 billion years

Knowing their velocity relative to the comoving frame, they can transform their measurement to the comoving frame and get the standard 13.8 billion years for the universe.

- #14

Drakkith

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Should it not be the other way round, i.e. that such observers, using their own clocks, would measure the universe to be younger than13 billion years?

Now that you mention it, I'm not sure. I can remove that part of my post if necessary.

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Now that you mention it, I'm not sure.

AFAIK Jorrie is correct; the proper time elapsed for comoving observers is maximal, in the sense that any other timelike worldline connecting the Big Bang and a given event on a comoving worldline will have less elapsed proper time between the two events than the comoving worldline does.

- #16

Chronos

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Isotope abundance is not affected by observer reference frames, AFAIK.

- #17

Drakkith

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AFAIK Jorrie is correct

Roger.

- #18

mfb

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I don't get how different directions make any difference. Different speeds, yes. Since the CMB is everywhere, what does it even mean to have a DIRECTION relative to the CMB ?

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what does it even mean to have a DIRECTION relative to the CMB ?

It means you don't see the CMB as isotropic; you see a higher temperature in the direction you are moving, and a lower temperature in the opposite direction. We actually observe this here on Earth; the usual term is "dipole anisotropy" in the CMB. But practically all published data on the CMB corrects for this by subtracting out the dipole in order to display what the CMB would look like to a "comoving" observer at our location.

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Yes, I understand that perfectly well. What I DON't understand is how direction matter to how you perceive the age. Let me be more clear.It means you don't see the CMB as isotropic; you see a higher temperature in the direction you are moving, and a lower temperature in the opposite direction. We actually observe this here on Earth; the usual term is "dipole anisotropy" in the CMB. But practically all published data on the CMB corrects for this by subtracting out the dipole in order to display what the CMB would look like to a "comoving" observer at our location.

Let's say you have a galaxy traveling in what we will call North relative to the CMB, at a given speed. Now we have a different galaxy traveling East at exactly the same speed relative to the CMB. Each sees the CMB as an ellipsoid with different temperatures in opposite directions relative to them and each computes an age with and without compensation for the anisotropy. How is it that they would get different results?

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What I DON't understand is how direction matter to how you perceive the age.

It depends on how you are trying to evaluate the age. A comoving observer can use the redshift of the CMB (which he sees to be the same in all directions) to evaluate how much time has elapsed since the CMB was emitted; and for him, this will give the same answer as the elapsed proper time along his worldline from then to now.

But a non-comoving observer sees the CMB to have a different redshift in different directions, so if he tries to use the observed redshift to calculate an "age", he won't get a single well-defined answer. And if he corrects for the anisotropy so that he can get a single well-defined answer, what he calculates won't be the "age" along his own worldline; it will be the "age" along a comoving worldline.

For the non-comoving observer to calculate the "age" along his own worldline, he has to add an extra step after the above calculation: he has to extrapolate his own motion, relative to comoving observers, back to whatever spacelike hypersurface he wants (such as the one when the CMB was emitted), and adjust the comoving "age" he calculated above for his own motion. That will always give an answer that is smaller than the comoving "age"; but how much smaller will depend on the details of the non-comoving observer's motion, and will be different for different non-comoving observers.

- #23

mfb

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Relativity of simultaneity. If we look 5 billion years in two opposing directions, we see the universe at the same age in both directions. If our fast-moving spacecraft does the same (with "forwards" and "backwards"), it will see the different ages in different directions.Yes, I understand that perfectly well. What I DON't understand is how direction matter to how you perceive the age.

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Relativity of simultaneity.

This isn't quite as simple as it looks, because the fast-moving spacecraft can't construct an inertial frame that covers the entire universe (any more than a comoving observer can). So any simultaneity convention he adopts will have an arbitrariness to it that is not present in the usual SR case (where inertial frames pick out a particular simultaneity convention). But I agree that it will seem "natural" for him to adopt a simultaneity convention that is different from the "comoving" one.

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mfb

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