Age of Universe relative to what?

[goodabouthood]

People always say the Universe is 14 billions year old.

But what does that mean?

What is the time of the Universe relative to?

 

[alexg]

It means that the Big Bang took place about 13.8 billion years ago.

 

[goodabouthood]

It means that the Big Bang took place about 13.8 billion years ago.

But isn't time relative?

Did the Big Bang happen at different times in different reference frames?

When we say 13.8 billion years ago, what exactly are we referring to?

 

[phinds]

It is possible to imagine a frame of reference that is stationary relative to the CMB. If you were in such a frame of reference, and had enough snacks to last you for the duration, and a reliable watch that used our current system of duration, you would have observed that amount of time since one Plank Time following the singularity (aka the "Big Bang Event").

There are considerations that would have made this physically difficult so this is just a thought experiment.

 

[pervect]

The age of the universe is always giving in cosmological time, which is time as measured in the frame comoving with the Hubble flow, the unique frame where the universe (operationally, the cosmic microwave background radiation) is isotropic.

 

[lalbatros]

Assuming humans are bound to a co-moving frame, we could as well say that these 14 billion year are as-if measured by our common watches. This would be a time as measured in our reference frame.

However, I still wonder how time can be "continued" to periods where references clocks -like our atomic clocks- could even not be envisaged. Atomic clocks could not possibly even exist before atoms were there.

Considering that time is a measured quantity, we need a series of reference clocks bringing us back to the BB. Would it be enough to calibrate these clocks with respect ot each other?
Has such a time accounting been actually performed by astonomers?

 

[my_wan]

Relative to a comoving inertial clock like what's on your wall.

 

[phinds]

Considering that time is a measured quantity, we need a series of reference clocks bringing us back to the BB. Would it be enough to calibrate these clocks with respect ot each other?
Has such a time accounting been actually performed by astonomers?

There is no evidence that the flow of time has had any different characteristics since the Plank Time, so it seems perfectly reasonable to project backwards as we do.

 

[petm1]

The atom is the edge of my universe, all the atoms together form one edge, and this one edge was created at the same time, relative to my present. It is the atom that appears eternal to me but thanks to Einstein's calendar I can see that even the atom is temporal with a beginning and an end in time. Everything is relative to the present and each of our local clocks.

 

[Passionflower]

It is the age of an ideal co-moving clock.

 

[my_wan]

There is no evidence that the flow of time has had any different characteristics since the Plank Time, so it seems perfectly reasonable to project backwards as we do.

Not strictly true:
Time Dilation in the Light Curve of the Distant Type Ia Supernovae SN 1995K
Observation of Cosmological Time Dilation using Type Ia Supernovae as Clocks
Time Dilation from Spectral Feature Age Measurements of Type Ia Supernovae
Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications
Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves

 

[phinds]

Not strictly true:
Time Dilation in the Light Curve of the Distant Type Ia Supernovae SN 1995K
Observation of Cosmological Time Dilation using Type Ia Supernovae as Clocks
Time Dilation from Spectral Feature Age Measurements of Type Ia Supernovae
Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications
Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves

I checked the first couple of these and I completely fail to see what their points have to do with this thread. They talk about the time dilation of accelerating object ... no surprise there but what has that to do with a frame of refernce that is comoving with the CMB?

 

[my_wan]

I checked the first couple of these and I completely fail to see what their points have to do with this thread. They talk about the time dilation of accelerating object ... no surprise there but what has that to do with a frame of refernce that is comoving with the CMB?

The comoving frame seems to be what is responsible for the 'apparent' global acceleration of these objects in the universe relative to any distant comoving frame. Just because a pair of distant observers are both comoving with the CMB does not mean they escape the time dilation with respect to each other.

 

[phinds]

The comoving frame seems to be what is responsible for the 'apparent' global acceleration of these objects in the universe relative to any distant comoving frame. Just because a pair of distant observers are both comoving with the CMB does not mean they escape the time dilation with respect to each other.

I didn't realize that anyone had suggested that they would.

The objects at the edge of our observable universe are receding from us at about 3c, so relative to each other we most certainly are seeing time dilation. BUT ... a comoving frame out there would see the universe at 13.7B years old, as do we (well, almost ... we are a hair off of being comoving). I fail to see what point you are making relative to this thread, which is about the age of the universe.

 

[Naty1]

Age of the Universe:
(Here are my notes from a very long discussion in these forums)

Do all observers agree on the age of our Universe?

Crowell:
No, they don't all agree.

But in an FRW cosmological model, there are preferred observers, who are essentially observers who detect no dipole asymmetry in the CMB. Such observers agree with one another on the amount of clock time since the Big Bang, and this is what we mean when we speak of the age of the universe in such a model.

In the real universe, a clock on the Earth's surface is not a bad approximation to such a clock. The solar system isn't moving at any large fraction of c relative to the CMB, and there is not a huge amount of gravitational time dilation between the Earth's surface and a point that is, say, outside the local group of galaxies.

There is not just one such frame for the whole cosmos. There is one such frame for every point in the cosmos. Global frames of reference don't exist in GR.

The existence of these preferred frames is also not a general characteristic of GR. It's just a characteristic of this particular solution of the GR field equations.


The age of the universe as usually discussed is for an observer who is at rest relative to the average motion of the matter and radiation in the universe (the "Hubble flow"), and is in the context of homogeneous models, which wouldn't include any structure such as black holes, etc. Yes, you're right, different observers can measure different ages of the universe on their clocks. You can't be "on" a black hole, but an observer hovering just outside a black hole's event horizon would say that according to her clock, the universe is very young. There is no limit on how young the universe could be according to such an observer. The same applies to an observer moving at nearly the speed of light relative to the Hubble flow.

 

[goodabouthood]

What does CMB stand for?

So we are taking this 13.7 billion year time from the Earth's reference frame correct?

How do we even take this measurement?

 

[phinds]

What does CMB stand for?

Google is a nice tool. You should learn to use it.

 

[TrickyDicky]

... The same applies to an observer moving at nearly the speed of light relative to the Hubble flow.

Right, such an observer's clock would measure the age of the universe arbitrarily close to t=0 as it approaches arbitrarily close to c, therefore the age of the universe is totally relative and depends on the state of motion of the observer; but there are limits to this age for any observer, brought by the absolute velocity c at one side and by the "conventional" CMB velocity that actually also puts an absolute maximum limit to the age of the universe (observer's proper time) that any observer could measure (meaning there seems to be no way to go slower than the comoving frame).
The key here seems to be that in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest you reference that speed to, or otherwise how could c be absolute?

 

[Dale]

The key here seems to be that in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest you reference that speed to, or otherwise how could c be absolute?

That is very basic SR. C is absolute not because there is an absolute rest frame but because something which is moving at c in one frame is moving at c in all frames.

 

[TrickyDicky]

That is very basic SR. C is absolute not because there is an absolute rest frame but because something which is moving at c in one frame is moving at c in all frames.

did I say anything contradicting that? Your statement is just a tautological explanation of what absolute means. Very basic indeed.

 

[Dale]

did I say anything contradicting that?

Yes:

in order to have an absolute velocity (light speed) it appears natural that there has to be some absolute rest

 

[TrickyDicky]

Yes:

That is not contradicting it at all. Would you deny that a way to define an absolute velocity is referencing it to an absolute rest. If you think about it a little you'll realize that in abstract terms one thing implies the other. That also implies that all frames agree about c, that is another way to say c is absolute.

 

[phinds]

That is not contradicting it at all. Would you deny that a way to define an absolute velocity is referencing it to an absolute rest. If you think about it a little you'll realize that in abstract terms one thing implies the other. That also implies that all frames agree about c, that is another way to say c is absolute.

You may be right, but I too felt that you had made an incorrect statement. Your wording does seem to lead to that conclusion.

 

[Dale]

If you think about it a little you'll realize that in abstract terms one thing implies the other.

No, it does not. Since relativity has one but not the other they clearly do not imply each other.

 

[OCR]

Your statement is just a tautological explanation of what absolute means.

What is the meaning of the word, tautological?

Wait, wait, on 5 seconds thought, I'll bet it means...

Google is a nice tool. You should learn to use it.

What does CMB stand for?

It means "cosmic microwave background radiation", well, that is if you tack an "r" on the end.

I'll post a link goodabouthood, so you can read the definition, without using tautological...

http://en.wikipedia.org/wiki/CMB



OCR

 

[TrickyDicky]

No, it does not. Since relativity has one but not the other they clearly do not imply each other.

If it has one it has the other, it is in the postulates. Maybe you are conflating light with massive objects that can never reach c.

 

[TrickyDicky]

Short SR lesson: Usually motion or velocity of massive objects is defined in reference to some other object that is considered to be momentarily at rest wrt the first, but if we choose a different object as reference we may obtain a different velocity, the velocity is relative in this sense. And the rest state by which we measure it is a conventional frame, there is nothing special or absolute about it.
The case of the velocity of light is different , regardless of the state of motion of the object we choose as reference the velocity doesn't vary. The nature of the rest frame that allows us to measure a specific invariant speed must be different than what was mentioned above.
Unless one prefers to say that this particular velocity is not a motion by any of the usual meanings of motion in physics and therefore it is not defined wrt some kind of rest frame, but that's kind of like saying that when someone sells something to someone the other one is not buying. Motion and rest are logically linked.

 

[ghwellsjr]

Short SR lesson: Usually motion or velocity of massive objects is defined in reference to some other object that is considered to be momentarily at rest wrt the first, but if we choose a different object as reference we may obtain a different velocity, the velocity is relative in this sense. And the rest state by which we measure it is a conventional frame, there is nothing special or absolute about it.
The case of the velocity of light is different , regardless of the state of motion of the object we choose as reference the velocity doesn't vary. The nature of the rest frame that allows us to measure a specific invariant speed must be different than what was mentioned above.
Unless one prefers to say that this particular velocity is not a motion by any of the usual meanings of motion in physics and therefore it is not defined wrt some kind of rest frame, but that's kind of like saying that when someone sells something to someone the other one is not buying. Motion and rest are logically linked.

You can only measure the round-trip speed of light and there is nothing special about the state of motion in which it is measured as long as it is inertial. This is covered in Einstein's first postulate, just like all other measurements and observations of physics and its laws.

The second postulate defines the one-way speed to be equal to the two-way speed. This cannot be measured but rather defines what a Frame of Reference is. Any state of motion that can be used to measure the round-trip speed of light as c can also be used as a Frame of Reference in which the propagation of light is defined to be c.

So although the one-way speed of light is absolutely defined to be c in any reference frame, that in no way implies that there is an absolute rest.

 

[Dale]

If it has one it has the other, it is in the postulates.

No, your suggestion is directly contrary to the postulates. The principle of relativity postulate says that there is no absolute rest. The light speed postulate says that c is absolute.

 

[TrickyDicky]

You can only measure the round-trip speed of light and there is nothing special about the state of motion in which it is measured as long as it is inertial. This is covered in Einstein's first postulate, just like all other measurements and observations of physics and its laws.

The second postulate defines the one-way speed to be equal to the two-way speed. This cannot be measured but rather defines what a Frame of Reference is. Any state of motion that can be used to measure the round-trip speed of light as c can also be used as a Frame of Reference in which the propagation of light is defined to be c.

So although the one-way speed of light is absolutely defined to be c in any reference frame, that in no way implies that there is an absolute rest.

What I discussed in my last posts is independent of the one way vs. round trip speed considerations or the state of motion in which is measured.
do you think that motion is logically defined wrt a rest frame or not?