[chris (OP)]

Dear all,

Here's a question for you to ponder on, sent in by listener Stan and posted on his behalf by me.

"Hi Chris, 

You show-run the best scientific program there is-bar none.  I listen especially avidly to the informed discussions about cosmology and astronomy. But I have not heard the distinction addressed between whether the estimated age of the universe is absolute or relative. The latest estimate, of about 13.8B years, is presumably based on our perspective here on earth, i.e. our velocity relative to the velocity of other bodies. 

But would the age of the universe be different if the observer were situated on a planet near the outer edge of the universe, traveling at speeds far greater than we are?  Shouldn't that observation show the universe is younger, and if so how much younger?  If, on the other hand, the observation is absolute from our perspective, why can relativity considerations be ignored?  And if it's relative, how can it be determinate at all, except with respect to the perspective of any one observer?

Cheers,

Stan Riveles
El Prado, NM  USA  (near Taos NM)"

[Kryptid]

It's relative. For a neutrino formed soon after the Big Bang travelling close to the speed of light, the Universe would appear significantly younger than it does to us here on Earth.

[chris (OP)]

Wow, this is bending my mind a bit...

[syhprum]

There is no outer edge or centre to the universe all points are in effect at the centre

[evan_au]

When we look out in space (as Edwin Hubble did), we see that distant galaxies are moving away from us; the more distant, the faster the recession. If we wind this backward in time, we find everything was very close together around 14 billion years ago (with a slight correction for the accelerating expansion).

Assuming space is uniform on large scales*, a resident on the planet Zorg, 10 billion light years away would look out into space, and see that distant galaxies (including ours) are moving away from her; the more distant, the faster the recession. Winding this backward in time, she would find everything was very close together around 14 billion years ago.

So I would suggest that, in their own frame of reference, the age of the universe would be similar. (But as Einstein showed, it is very hard to compare timescales in different frames of reference...)

*Homogeneity of the universe was an assumption by cosmologists to make it easier to solve the equations of general relativity. We haven't found any major deviations yet, but astronomers and cosmologists are continuing to perform a variety of tests of this fairly fundamental assumption.

[jeffreyH]

In the very early universe, once the force of gravity had separated from the other forces, the density of matter would have caused extreme time dilation. This in itself could explain inflation. It also has to be taken into account when attempting to calculate the age of the universe.

[yor_on]

Not really. What you use to define a age of the universe is how far back in time and distance you estimate the incoming light to be. To do so you use 'c' as your clock and ruler. So, no matter how 'fast' something is going relative something else it doesn't matter. If you on the other tentacle define 'c' as a variable then there will be no agreement whatsoever on the presumable age of the universe, at least not locally defined. So 'c' as a (local) constant is in agreement with the way we define local measurements to become a 'global reality' and from that you will see that the earliest light we can observe has a same 'radius' if one now can use that word for it? The other thing about it is that it won't matter for this if you move your 'center' of observation. And that is true as I see it as the solar system and galaxy is in a constant uniform 'motion' relative any other arbitrarily chosen reference point in the universe. The 'speed' of your 'center' of observation depends on what you use as your 'standing still' reference point, but we are 'moving', and the distance and age of that earliest light will stay the same.

[yor_on]

You could try Why does relativistic length contraction (Lorentz contraction) happen?  for a better answer. If your original though was that the universe should LorentzFitzgerald contract?

=

Alternatively: It doesn't really matter what 'speed' you expect yourself to have, for physics all 'relative (uniform) motion' are the same. As you easily prove by choosing another reference point for 'standing still', giving yourself a totally new 'uniform speed' inside this universe. As long as there is no 'global reference frame' for standing still, except relative your randomly picked reference frame. So, as soon as somebody find such a definition it will become a totally new ballgame.
==

Actually Stan, this last point is crucial to understand modern physics. There is no 'global' reference frame anymore, only 'relative'. The world is not my pivot of the universe, it's just one of countless reference frames I could choose to use. What we have is local definitions, and when they are found to agree with each other we reach a consensus about 'physics' over a universe. So that might be your next stepping stone :) To see what questions you can create about that. It's questions that moves physics.
=

A third way to define it then. Think of two cars in a same motion 'cruising'. For the passengers those cars are 'standing still' relative each other, for the bystander both are moving, for someone on the moon both the bystander and the cars are moving.  It's relative the observer. Time is something that is locally constant, measured in ones life span. Relative ones wristwatch it doesn't matter how 'fast' we move, we don't get any younger by it. So, when someone lifts forward a time dilation it doesn't have to do with you suddenly aging slower or faster, it has to do with a comparison, that comparison always using ones local 'wrist watch' to compare it by. In that way all definitions are local, and a 'repeatable experiment' only comes to exist when our local definitions/situations agree with each other.

so that is the 'absolute frame' that exist today. The local definition.

[puppypower]

When we look out in space (as Edwin Hubble did), we see that distant galaxies are moving away from us; the more distant, the faster the recession. If we wind this backward in time, we find everything was very close together around 14 billion years ago (with a slight correction for the accelerating expansion).

Assuming space is uniform on large scales*, a resident on the planet Zorg, 10 billion light years away would look out into space, and see that distant galaxies (including ours) are moving away from her; the more distant, the faster the recession. Winding this backward in time, she would find everything was very close together around 14 billion years ago.

So I would suggest that, in their own frame of reference, the age of the universe would be similar. (But as Einstein showed, it is very hard to compare timescales in different frames of reference...)

*Homogeneity of the universe was an assumption by cosmologists to make it easier to solve the equations of general relativity. We haven't found any major deviations yet, but astronomers and cosmologists are continuing to perform a variety of tests of this fairly fundamental assumption.

I think there is a mistake being made, in general, it terms of how cosmological data is being interpreted. The fastest moving objects, which are the farthest away, do not reflect the real time movement of an object.They only tell us what the object did, 10 billion year ago, They tell us nothing about object of today. Once light is emitted by an object, the light and the object no longer coordinate in time. The light becomes like a photo of a point in time, where time stops. While the material object continues to evolve, and move forward in time.

As an analogy, say I am at a pie eating contest, as the photographer. I take a picture at the very beginning of the contest, when the first bite of pie occurs. After 1/2 hour, the competition ends, so I send my opening bite picture to my editor. Although the arrival of the picture, and the end of the contest coordinate in time, for my editor, the beginning and end of the contest, do not coordinate in time relative to physical reality. My picture shows a beginning, while reality is at an ending.

The oldest objects, by being the fastest moving, tells us the universe expanded most rapidly at the beginning of the BB or big boom! The first bite of the pie, was the fastest. The pace of pie eating slows as the context goes on.The snap shot of light we receive is a photo from the beginning, which we received, late. As objects get closer, they get slower, which means the time line of physical reality shows a decelerated expansion. Light or snap shows show a different form of time-line.

Another analogy is a scrapbook containing pictures of your child, from new born all the way to 21 years old. We can take the first picture out of the scrapbook and place it side by side with the last picture, and then compare these two mixture in real time. This is not in based on the time clock of the matter from which the picture emerges. We can compare the pictures in real time, but we can't place the new born baby and the young man side by side in terms of their biological reality. That is a different clock based on matter instead of light.

When we look at the oldest objects of the universe, there are the closet to the BB center, since they did no have much time to move away very far. They are not made of light but have matter and inertia. The reason the oldest objects can appear in many places in the sky, and not all be concentrated in one place, as expected of a BB center, is due to the curving of space-time. The light bends, with the greatest degree of curvature at the very beginning, so the center appears all over the place.

[yor_on]

It's a tricky question. What we use to define a Big Bang is, once again, 'c'.
We define it as being a 'local constant'.
Your 'life span' is another 'local constant' in my terms, equivalent to 'c'.

We can exchange that for thinking of it in terms of my 'time' being slower, or faster, relative some other 'clock' I measure my local time against.

Although, if we do, choosing random 'clocks' inside this universe, a 'black hole' and some very speedy relativistic rocket, we still find ourselves in a situation where one have to decide which one then is the 'absolute time', that fits.

even worse, theoretically we now define two time rates, to ourselves, simultaneously.

[yor_on]

The truth none of us pass is this.
We live, and then we die.

We all have a invariant, locally defined, lifespan.

[ScientificSorcerer]

Instead of looking at galaxies to determine the age of the cosmos I would look at gravity waves.  If the big bang generated gravity waves via the destruction of the singularity then it could be possible to calculate the scale of these waves.  If you could measure the wavelength and amplitude of the gravity waves then you could track them back through time, then you could determine the age of the universe and the amount of energy in the big bang by determining how much mass would be required to make such waves.

[yor_on]

ScientificS. The last I've heard about primordial gravitational waves was that  "A joint analysis of data from the Planck space mission and the ground-based experiment BICEP2 has found no conclusive evidence of gravitational waves from the birth of our universe, despite earlier reports of a possible detection."

From Gravitational Waves from Early Universe Remain Elusive.

When it comes to variable speeds of 'c' there is one last alternative. In that one no constant exist, over a 'whole universe'. Because looking at it this way 'time' is what the universe defines for you at whatever place you are. That also plays havoc with any ideas of any other constants. How can we ever agree on any experiment being equivalent, aka repeatable, if there is no way to define a time. And of course, it's not only the time, it's the ruler we use too that then becomes a variable. Against it we have experiments defining 'c'. although strictly locally, and to that with a local clock and ruler measuring it. That's what we use to define physics, local constants that we expect to be the same everywhere and at all times. It's simpler, and it makes sense to me.
=

If I was to add to gravitational waves, which I do :) I would point out that there is no center to our universe. it's homogeneous and isotropic over large distances, so whatever we see from Earth should be what we would see if we could move to the 'fringe' of our observable universe, a whole new 'sphere' of light looking the same. If one now think of the CMB (cosmic background radiation) we see from here, should one expect it to look otherwise, observed from there? So, this 'Big Bang/inflation' we're thinking of has no 'global' origin, to me it seems just as 'local' as our constants are. And so, us not finding any 'B mode' polarizations may make sense from that perspective, if now all origins are 'local'. At least it's worth pondering about.

[puppypower]

If you look at a black hole, light is not able to escape, because space-time is so curved by gravity, that light is essentially stuck in an curved space-time orbit/path and can't propagate outward. Even if we know the pressure and reactions of the BH are causing energy to be emitted from a center of the black hole, the light/energy becomes distributed throughout the perimeters. You cannot tell the point of origin, from the light, since the light appears to be everywhere.

If you consider the singularity assumption of the BB, we have essentially maximum curved space-time, for quite some time from time=0, where light and energy cannot escape, but rather distributes itself in a tight orbit. From the light, we can't infer a center, even if all the light came from a center. The uniformity of the cosmic microwaves would be expected from an origin of near infinite curving of space-time. This is why the oldest objects, do not appear, all in one place, as inferred by the orbiting light from the original big bang, even though a singularity origin requires it needs to be from the same origin.

[yor_on]

Don't know PP.  "Specifically, the BICEP/Keck experiments found evidence for a "curly" pattern of polarized light called B-modes. These patterns would have been imprinted on the CMB light as the gravitational waves slightly squeezed and stretched the fabric of space. Polarization describes a particular property of light. Usually, the electric and magnetic fields carried by light vibrate at all orientations equally, but when they vibrate preferentially in a certain direction, the light is polarized. "

You could have spagettification, aka tidalforces inside a black hole, but how they would act with polarizations I'm not sure. Possibly chaotically, meaning that it could be as you suggest. But to get from that to the way we think our universe is expanding is complicated. You would have to explain how a accelerating expansion then can take place 'everywhere,' as if this universe was a extension of a expanding bubbles surface. Your idea seems to be that a 'center' should be possible although impossible to infer from 'B modes'?

[yor_on]

What I meant by my own example of a isotropy and a homogeneous universe is that if you now set a point from where you observe the cosmic background radiation, then move this point of observation meter by meter in some direction, the infalling light should tell you the exact same story, from its earliest start and forward in time. If it didn't it would become very strange as we then would be wrong in a lot of assumptions we build both physics and astronomy on.
=

Btw: this is more than a assumption as the earth, solar system, galaxies all move. So we do change our point of observation over time, but the story we keep getting from that light stays the same

[PhysBang]

Two quick notes:

First, if we look at recession velocities in the recent cosmological past, we find that things farther away are receding faster the farther away they are. But we do not see this happen in a linear fashion the farthest away that we can observe. Scientists use the specific way that recession velocity relates to distance to measure many different parameters of the universe. It is not the case that we are looking all the way back to the early universe and just seeing things move faster in the early history of the universe.

Second, that the universe may have begun with a singularity does not mean that the universe has or had a center. Even infinitely large spaces can collapse into a singularity and singularities can open in to infinitely large spaces (the temporal reverse of the first procedure).

[yor_on]

I actually, thinking of it again, meant one thing more by my example. Presuming that a expansion/inflation has no center and that it 'expands' equally in each point. If I move my point of observation a countless amount of times, different directions etc, studying the polarization of incoming light from the CMB. If I now find no defined pattern to it, is that more surprising than finding one?
=

What I think myself is that finding one should be the real enigma. There's two ways to look at this universe.

1. this universe has a expansion
2. those expansions are the universe.

If 2 then it seems extremely weird, to me, to find a ordered pattern of polarizations, that will correlate to the polarizations at all other points of observation. Better point out that it is the CMB I'm thinking of here, as some sort of 'imprint' on the sky.

[PmbPhy]

When the age of the universe is stated it refers to the time as measured by clocks at rest in the zero momentum frame of the bulk matter of the universe. But otherwise such an time is relative.