Do relativistic effects make the age of the universe moot?

[Andy DS]

How can we know the age of the universe, with time dilation due to the mass and velocity of matter which condensed after the big bang. If time passes at different speeds depending where it is perceived, does this not make the age of the universe somewhat moot.

 

[Ibix]

The age usually quoted is the proper time since the singularity for a co-moving observer, one who sees the CMB as isotropic. This turns out to be the maximum time that can have been experienced by observers at a given event.

 

[PeroK]

The age of the universe is generally given in comoving coordinates, involving cosmological time:

https://en.wikipedia.org/wiki/Cosmic_time

If you want to quote the age of the universe in the reference frame of, say, a particle being accelerated at CERN, then that gives a different, but not very useful, time.

 

[phyzguy]

Because different observers perceive time passing at different rates does not make the question of how much time has passed moot. It just means that you have to specify some details about your observer. Typically one quotes this for a "co-moving observer" who is moving along with the expansion of the universe. Also, most of the matter in the universe is moving at relatively slow speeds and in regions of relatively low gravitational potentials so that these corrections are small. Your question is like saying, "Because clocks read different times in different cities on Earth, there is no way to know what time it is."

 

[PeroK]

Your question is like saying, "Because clocks read different times in different cities on Earth, there is no way to know what time it is."

I think that is very different, as those are just different offsets. We all agree about the passage of time.

 

[Vanadium 50]

We can't find The One True Age of The Universe, because there is no One True Age of The Universe.

So what?

We can still define useful quantities, like @Ibix did with the age of a comoving observer. Or we could point out that the difference between all the times one is likely to use is small. The difference between the Earth's frame and a comoving frame is about 30000 years over the lifetime of the universe. The uncertainty on the age of the universe is about 40 million years - a thousand times larger.

 

[Andy DS]

"Your question is like saying, "Because clocks read different times in different cities on Earth, there is no way to know what time it is.""

No phyzguy, my queasion was more like because a clock on Earth and one near a black hole run at different rates, can this effect the answer.

 

[russ_watters]

"Your question is like saying, "Because clocks read different times in different cities on Earth, there is no way to know what time it is.""

No phyzguy, my queasion was more like because a clock on Earth and one near a black hole run at different rates, can this effect the answer.

Ok, but what's the problem? I don't see why this makes the question/answer moot.

 

[phinds]

No phyzguy, my queasion was more like because a clock on Earth and one near a black hole run at different rates, can this effect the answer.

But if we want to know the age of the universe, the observed passage of time near a black hole is irrelevant.

 

[kimbyd]

How can we know the age of the universe, with time dilation due to the mass and velocity of matter which condensed after the big bang. If time passes at different speeds depending where it is perceived, does this not make the age of the universe somewhat moot.

I thought I'd add my two cents to the pile.

Time dilation shortens the apparent age of the universe. That is, if you are moving quickly compared to the galaxies in our universe, observers in galaxies will see your clock ticking slowly. Which means that you will observe a shorter age.

And there is no bound to how short the observed age could possibly be. If you could travel at a high enough speed, you could in principle see less than a second pass since the big bang (this isn't a practical possibility because matter would slow you down through friction, but that's a different question).

But there is a limit to how long the observed time could be. And the maximum observed age is the age as observed by a somebody who sees the cosmic microwave background having the same average temperature in every direction. This is known as a "co-moving observer", as others have mentioned: they travel along with the expansion.

We aren't exactly co-moving observers, but we're close. From memory, our velocity with respect to the background is something like 600km/s. Which leads to a current time dilation factor of about 0.0002%. The actual time dilation from the big bang for the matter that makes us up wouldn't be exactly this (because it's moved around a bunch), but it'd be close. That error is much smaller than our current measurement error (around 0.2% or so, a hundred times greater), so it's generally ignored.

 

[PeterDonis]

the maximum observed age is the age as observed by a somebody who sees the cosmic microwave background having the same average temperature in every direction

And that maximum age is the age that is being referred to when the term "the age of the universe" is used.

 

[Ibix]

But if we want to know the age of the universe, the observed passage of time near a black hole is irrelevant.

More precisely, when we talk about "the age of the universe" we've just chosen not to talk about the passage of time near a black hole, although that would be different.

For @Andy DS, an analogy is speeds. The speed limit on most roads around here is 30mph. But the Earth is doing about 20km/s around the Sun - how can we stay below 30mph when we're doing 20km/s just sitting here? The point, of course, is that by "30mph" we actually mean "30mph compared to the surface of the Earth where I am". Nobody bothers saying the bit in italics because people just understand it (possibly due to never questioning whether it could mean anything else, one suspects), but the qualification is important because it is also true that I'm currently doing 20km/s around the Sun, and many other speeds with respect to many other reference points.

That doesn't mean that it's impossible to define my speed. It just means that "my speed" needs a bit of qualification - my speed compared to what. Similarly with the age of the universe - strictly we should say "the age of the universe as measured by an observer who sees (and has always seen) the CMB as isotropic". But, like the ground frame in the speed limits, the isotropic-CMB observer is an obvious (obvious to cosmologists, anyway) type of observer to use to define age. Most people are either aware of which type of observer is being used in unqualified statements about the age, or aren't aware of or interested in the subtleties. Welcome to the first group.

 

[Andy DS]

I see where all the replies are pointing and many thanks.
I was thinking that during cosmic inflation there may have had some relativistic effects but if we are saying this doesn't have an effect on what we know about the age of the universe because we are all riding along on the same wave I think I understand now.

 

[Ibix]

all riding along on the same wave I think I understand now.

As long as you only mean the bit I've bolded figuratively, then yes. Inflation doesn't change co-moving observers into non-comoving ones or vice versa, would be a more precise way of putting it.

 

[kimbyd]

I see where all the replies are pointing and many thanks.
I was thinking that during cosmic inflation there may have had some relativistic effects but if we are saying this doesn't have an effect on what we know about the age of the universe because we are all riding along on the same wave I think I understand now.

Yup, that's correct!