Age of the universe -- absolute or relative?

**Frank3** · Apr25-05, 09:22 AM

I have a question.

Consider twin physicists Al and Bert who devise a new method (far better than measuring cosmic microwave background) to determine the age of the universe. They do so to an extraordinarily high degree of precision – stating it in seconds to many, many decimal places. Having done so, they synchronize two clocks to the determined age of the universe and allow them to continue ticking off time from that point on.

After winning the Nobel prize for this work, Al uses his prize money to build a near light speed space ship and he heads off, with his clock, to travel the cosmos at high speed, accelerating and changing directions along the way, coming in close proximity to black holes and other high gravity bodies, and eventually returning to earth. Of course, the time elapsed for Al will be less than the time elapsed for the earthbound Bert, so when they compare clocks they show different times.

So, my question: Now how old is the universe?

I suppose my larger question is whether the age of the universe is an absolute or relative measure. And beyond that, is it meaningful to think about the measure of elapsed time since the beginning of the universe in the relatively flat space-time we inhabit in the same way as elapsed time in the earliest moments after the big bang when all space-time was more highly curved?

**eNathan** · Apr25-05, 10:17 AM

Originally Posted by Frank3

I have a question.

Consider twin physicists Al and Bert who devise a new method (far better than measuring cosmic microwave background) to determine the age of the universe. They do so to an extraordinarily high degree of precision – stating it in seconds to many, many decimal places. Having done so, they synchronize two clocks to the determined age of the universe and allow them to continue ticking off time from that point on.

After winning the Nobel prize for this work, Al uses his prize money to build a near light speed space ship and he heads off, with his clock, to travel the cosmos at high speed, accelerating and changing directions along the way, coming in close proximity to black holes and other high gravity bodies, and eventually returning to earth. Of course, the time elapsed for Al will be less than the time elapsed for the earthbound Bert, so when they compare clocks they show different times.

So, my question: Now how old is the universe?

I suppose my larger question is whether the age of the universe is an absolute or relative measure. And beyond that, is it meaningful to think about the measure of elapsed time since the beginning of the universe in the relatively flat space-time we inhabit in the same way as elapsed time in the earliest moments after the big bang when all space-time was more highly curved?

Your right, in different parts of the universe (near black holes namely) time runs slower. Thus, we cannot measure how elasped time in which the unverse has been in existance from every reference frame in the universe. I can establish one method of measuring this elapsed time. If you find out the amount of elapsed time in different (almost every

) part of the universe, add them tother, and divide it by how many variables you added, you will have the "average time". This is the simple equation to find the average of something. I suppose that if we know how much matter is in the entire universe, how long the universe has been around from our reference frame (on earth) and how much spacetime is warped on earth, a mathematician can calculate the average age of the univese.

But in general, the elapsed time with which the universe has been around is all relative to the spacetime curvature in any givin point.

My theory is that time runs faster in thin space -- space expands

-eNathan

**HallsofIvy** · Apr25-05, 10:44 AM

Originally Posted by Frank3

So, my question: Now how old is the universe?

By whose clock? The point is that time is not absolute. For any question about time, you must specify in which frame of reference it is measured in order for the question to make sense.

**Frank3** · Apr25-05, 11:34 AM

HallsofIvy -

OK, that's my understanding (that elapsed timeis not absolute) and one of the points of my question. So when we hear estimates that the universe is 13.8 billion years old, for example, should this include "as measured from our earth frame of reference"?

Any thoughts on my other underlying question: When we hear about events that occurred during the first second or the first few minutes, or first 300,000 years of the universe, how do we "understand" that relative to the measurement of elapsed time in our current earth frame of reference?

**Mortimer** · Apr25-05, 01:22 PM

I guess that in this case it would be necessary to make the distinction between proper time and "coordinate time". Proper time is the time as measured by a clock in motion or in a gravity field, while coordinate time is the time as measured by a stationary observer at infinity where there is no gravity.

A similar distinction needs to be made in general relativity when calculating the velocity of an object that falls towards the Schwarzschild radius of a black hole. Here we talk about the coordinate velocity, which is defined as coordinate distance per coordinate time while for the falling object itself the velocity needs to be expressed as metric distance (which is larger than the coordinate distance) per proper time (which is slower than the coordinate time).

So when calculating the "coordinate age" of the universe one needs to theorize an observer who has been there all the time stationary and at infinity (which is close enough to our current position on the surface of earth). The "proper age" of the universe must then always be younger. If we talk about the first second of the universe we then talk about the first coordinate second.

**JesseM** · Apr25-05, 11:00 PM

Originally Posted by Frank3

OK, that's my understanding (that elapsed timeis not absolute) and one of the points of my question. So when we hear estimates that the universe is 13.8 billion years old, for example, should this include "as measured from our earth frame of reference"?

In general relativity, a "reference frame" is only a local concept, but you can also do something called a "foliation" on a curved 4D spacetime, slicing it into a stack of curved 3D hypersurfaces. In idealized cosmological models it is possible to slice spacetime in such a way that the distribution of matter is perfectly homogeneous in each slice--if you choose this foliation, each slice is called a "hypersurface of homogeneity", and all particles in a given hypersurface of homogeneity will have experienced the same amount of proper time since the big bang, so this gives a global time parameter associated with this foliation. In the actual universe, matter is not going to be perfectly homogenously distributed no matter what foliation you choose, but I think the idea is to choose the one where it's as close to homogenous as possible. I'm pretty sure the average rest frame of the cosmic microwave background radiation would be as good an approximation as any, in which case you could consider the age of the universe to be the proper time experienced by a particle that's been at rest relative to the CMBR since the big bang (and I think the earth is pretty close to being at rest relative to the CMBR).

**HannonRJ** · Apr26-05, 12:51 PM

After about 65 years of thinking about this subject, I have concluded that "the universe" had no beginning and will have no end. Its observable nature have have changed over the eons, but it is "immortal".