Expanding Universe

[modest]

I understand. =)

 

I was trying to question the synchronicity between clocks. How can two clocks share the same frame absolutely?

 

Comoving clocks in an expanding universe can be synchronized by agreeing on the time of a universal event. From the link you quoted,

Notice that their clocks all measure the same time because they are all controlled by the same rules of physics, and the observers can synchronize their clocks by agreeing to define t=0 to be some special moment in the life of the universe, for instance its beginning (if it had one).

The Robertson-Walker Metric

I don't know what you mean by "same frame absolutely".

 

Let me ask you, if two supernova happened 6 billion lightyears from one-another in an expanding universe then how would you determine, or how could you define, that they happened simultaneously?

 

The universe may appear to expand when looking outwardly, but what about inwardly?

 

Moving inwardly (the distance between two objects decreases over time) is collapsing. The universe is expanding. The size of some arbitrarily-drawn sphere in the universe will increase with time whether one imagines looking at the sphere from the center ('outwardly') or from the edge ('inwardly').

 

Well here it is, for instance. Nice link by the way.

Originally Posted by Jodrell Bank Centre for Astrophysics

Although SR emphasises that all observers should be treated equally, in cosmology the co-moving observers are more equal than others: anyone who presumes to move relative to them will get a distorted view of the universe in which the observer's direction of motion relative to the co-movers will pick out a special direction in space.

 

I think there is reciprocity between the "special direction in space" of any two interacting objects. Don't ask me about three, I might snap.

 

A special direction would mean that the universe is expanding a little bit slower in one direction than in the opposite direction from the point of view of some observer because the observer is moving relative to the expansion of the universe. Such an observer would have a peculiar velocity relative to the Hubble flow. Our solar system, for example, has a peculiar velocity of about 370 km/sec, so the cosmic microwave background is not exactly isotropic. It is redshifted slightly less in one direction and slightly more in the opposite direction.

The Hubble law defines a special frame of reference at any point in the Universe. An observer with a large motion with respect to the Hubble flow would measure blueshifts in front and large redshifts behind, instead of the same redshifts proportional to distance in all directions. Thus we can measure our motion relative to the Hubble flow, which is also our motion relative to the observable Universe. A comoving observer is at rest in this special frame of reference. Our Solar System is not quite comoving: we have a velocity of 370 km/sec relative to the observable Universe. The Local Group of galaxies, which includes the Milky Way, appears to be moving at 600 km/sec relative to the observable Universe.

Ned Wright's Cosmology Tutorial

~modest

[coldcreation]

Comoving clocks in an expanding universe can be synchronized by agreeing on the time of a universal event. From the link you quoted,

The Robertson-Walker Metric

 

~modest

 

Happy new year all,

 

I haven't read this thread yet, except for this post quoted above, so I may be off-topic.

 

I think I read somewhere that clocks could potentially be synchronized by agreeing on the CMB thermal spectrum (temperature), as well. So if observers all detect a 2.726 ± 0.01 K blackbody then their clocks can be synchronized accordingly; even if these observes were located in different galaxy clusters. They would have to have some knowledge of the evolution of the universe, in accord with, say, the BBT, as opposed to, say, when Jesus was born. It may not be 2010 anywhere else in the universe (lol).

 

 

 

From deep Spain: 2.726 ± 0.01 K (7:44 PM), and counting...

 

CC

[modest]

Happy new year all,

 

Happy New Year :xparty:

 

I haven't read this thread yet, except for this post quoted above, so I may be off-topic.

 

I think I read somewhere that clocks could potentially be synchronized by agreeing on the CMB thermal spectrum (temperature), as well. So if observers all detect a 2.726 ± 0.01 K blackbody then their clocks can be synchronized accordingly; even if these observes were located in different galaxy clusters. They would have to have some knowledge of the evolution of the universe, in accord with, say, the BBT, as opposed to, say, when Jesus was born. It may not be 2010 anywhere else in the universe (lol).

 

 

 

From deep Spain: 2.726 ± 0.01 K (7:44 PM), and counting...

 

CC

 

I agree

 

I hadn't thought of that, but now that you mention it I think it would be the best, and certainly most readily available, means of finding a cosmic time.

 

It would be interesting, and I'm trying to figure out how, to convert between years and CMB temp...

 

... :confused: ...

 

Ok, the best I could find is equation 66 here. It relates temperature to redshift as,

[math]T_0 = \frac{T_1}{1+z}[/math]

this would be the same as,

[math]T_{CMB} = 2.728(1+z)[/math]

which makes sense because 1+z is the ratio of scale factors which is the factor by which radiation is stretched. Except for small redshift, the only way I know to accurately solve z as a function of time is with a cosmology calculator where "light travel time" on the calculator is "years ago" in the table below and the resulting redshift is added to one and multiplied by 2.728 to give CMB time. So, I guess I'll see what time 'significant' things happened in our expanding universe by our cosmic clock...

 

all values very approximate,

 

[font="Courier New"]
Event                 Years Ago  Redshift   CMB Time 
--------------------  ---------  --------  ---------
First Civilization ->     10000     0.000    2.728 K     
Lucy --------------->  3.2 Myrs     0.000    2.728 K
(Australopithecus)   
Was Born
Dinosaurs Died ----->   65 Myrs     0.005    2.742 K
First Large -------->   0.5 Gyr     0.037    2.829 K
Animals Fossilize  
Solar System-------->   4.5 Gyr     0.431    3.904 K
Formed
First Galaxies------>  13.2 Gyr    10.253   30.698 K
Formed? 
CMBR Formed--------->  13.6 Gyr      1500     4000 K 
[/font]

 

What's fun is that the clock is ticking down towards zero—the heat death of the universe—a kind of cosmic how-much-useful-time-do-you-have-left clock :hihi:

 

~modest (across the pond, 2.726 ± 0.01 K -- 5:19 pm and counting down...)

[Southtown]

Comoving clocks in an expanding universe can be synchronized by agreeing on the time of a universal event.

 

I don't know what you mean by "same frame absolutely".

Agreeing on any one event will not guarantee that time passes the same for all frames.

 

Let me ask you, if two supernova happened 6 billion lightyears from one-another in an expanding universe then how would you determine, or how could you define, that they happened simultaneously?

Relative to a third perspective? I'd say the answer depends on who asks.

 

Moving inwardly (the distance between two objects decreases over time) is collapsing. The universe is expanding. The size of some arbitrarily-drawn sphere in the universe will increase with time whether one imagines looking at the sphere from the center ('outwardly') or from the edge ('inwardly').

That sounds like something I'd agree with, I think.

 

A special direction would mean that the universe is expanding a little bit slower in one direction than in the opposite direction from the point of view of some observer because the observer is moving relative to the expansion of the universe. Such an observer would have a peculiar velocity relative to the Hubble flow. Our solar system, for example, has a peculiar velocity of about 370 km/sec, so the cosmic microwave background is not exactly isotropic. It is redshifted slightly less in one direction and slightly more in the opposite direction.

Ned Wright's Cosmology Tutorial

~modest

Yes. Everything will exhibit bias of existence because its own existence is defined relatively by its interaction with other entities. Now I shall check your link, sorry.

[Southtown]

But the first law mandates reciprocal uncertainty relative to c. CMB>

[Southtown]

Happy new year all,

 

I haven't read this thread yet, except for this post quoted above, so I may be off-topic.

 

I think I read somewhere that clocks could potentially be synchronized by agreeing on the CMB thermal spectrum (temperature), as well.

 

CC

MUAHAHAHAHHAHAHAAAA >:} Honestly didn't see that first...

 

I wish I wasn't Tijuana...

 

Woah Oh Radio

[modest]

Comoving clocks in an expanding universe can be synchronized by agreeing on the time of a universal event...

Agreeing on any one event will not guarantee that time passes the same for all frames.

 

Correct. But, the laws of physics do that. All of the observers we are talking about are in the same physical situation. The are all at rest relative to their visible universe. Their clocks therefore run at the same rate in comoving coordinates. As the link we've been quoting says,

The simplest universe consistent with SR is one which appears isotropic (the same in all directions) to a set of privileged observers, called co-moving observers because each observer sees the others as moving along with the overall cosmic expansion. (Actually these observers are just a figure of speech - the important thing is that there are sites from which the universe would appear isotropic if there were anyone there to observe).

 

The existence of three or more such observers places very strong constraints on the possible structure of space-time.

For a start, there is a "natural" time coordinate: the time as measured by each co-moving observer, equipped with a standard clock. Notice that their clocks all measure the same time because they are all controlled by the same rules of physics

, and the observers can synchronize their clocks by agreeing to define t=0 to be some special moment in the life of the universe, for instance its beginning (if it had one). The existence of a cosmic time is an enormous simplification; without it, there is no sensible way of separating space from time in the 4-D continuum of space-time. Next one can show that in such a universe there could be a co-moving observer at every point, all of whom see the universe as isotropic, i.e. 3 co-moving observers imply an infinite number of co-moving observers.

Robertson-Walker Metric

 

Let me ask you, if two supernova happened 6 billion lightyears from one-another in an expanding universe then how would you determine, or how could you define, that they happened simultaneously?

Relative to a third perspective? I'd say the answer depends on who asks.

If you use comoving coordinates then everybody will agree that the two supernova are simultaneous when they both happen at the same cosmic time. As ColdCreation pointed out, an observer at each supernova will measure the CMB at the same temperature as the other. The universe is always cooling and two different observers who measure the same temperature for the universe would have measured it simultaneously (in comoving coordinates).

 

Moving inwardly (the distance between two objects decreases over time) is collapsing. The universe is expanding. The size of some arbitrarily-drawn sphere in the universe will increase with time whether one imagines looking at the sphere from the center ('outwardly') or from the edge ('inwardly').

That sounds like something I'd agree with, I think.

:)

But the first law mandates reciprocal uncertainty relative to c. CMB>

B)

I wish I wasn't Tijuana...

 

Woah Oh Radio

Wouldn't that go "I wish I was in Tijuana..." ;)

 

Don't go around tonight. Well, it's bound to take your life. There's a bathroom on the right.

 

~modest

[Southtown]

Correct. But, the laws of physics do that. All of the observers we are talking about are in the same physical situation. The are all at rest relative to their visible universe.

I'd personally call it a bit of a stretch to presume that separate laws always coincide the same way in all instances of interaction.

 

Wouldn't that go "I wish I was in Tijuana..."

That's what I thought afterward, also. lol

[modest]

Correct. But, the laws of physics do that. All of the observers we are talking about are in the same physical situation. The are all at rest relative to their visible universe.

I'd personally call it a bit of a stretch to presume that separate laws always coincide the same way in all instances of interaction.

 

The principle required for comoving observers to keep the same cosmic time is the cosmological principle,

 

The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the Universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the Universe which we can see is a fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the Universe is knowable and is playing fair with scientists

 

~William Keel

 

Cosmological principle - Wikipedia, the free encyclopedia

 

I think its a fair assumption.

 

~modest

[Time_Travel]

Is the Universe Expanding Uniformly in all directions.....

 

If yes then why isn't there uniformity in distribution of matter.

[HydrogenBond]

An interesting theoretical effect that could explain the expansion of the universe is a gravity siphon.

 

Say you wanted to empty a pool. A siphon can be used. The water begins at the pool level. Before the water can reach the ground, it first has to go up, against gravity, to get over the walls of the pool. Then the water can fall, to the final ground level. As the level of the pool drops, the water has to be lifted even higher to get over the pool wall, before it can fall.

 

If we had a 10 ft pool of water with a 1 foot rim, at first the siphon will need to lift the water 1 ft. By the end, the siphon will lift the water 11 feet.

 

A gravity siphon would use gravity instead of water. What we need a two gravity potentials higher to lower, with the siphon having to go over a pool rim. As the gravity potential lowers, the rim of the universe pools stays the same. The gravity siphon has to lift higher and higher (expansion).

 

In the universe the ground level (lowest gravity potential) is a black hole. The rest of the matter is at higher potential. The black hole ,in the center of a galaxy, is not only pulling matter to this lowest potential, but it also gives off powerful emissions that can disrupt star formation. This increases the gravity potential of the stella matter (expands). This would be connected to the aspect of the gravity siphon this is going over the pool top.