Space expansion in our perspective?

[powerplayer]

So why would you want an expanding coordinate system and to say things are comoving, when it seems like it is just simple kinematics?

 

[powerplayer]

Oh and what about stretching of light? Is this from DE or DM?

 

[powerplayer]

Ah but I forgot that some galaxies are receding FTL so it can't be kinematics, correct?

 

[Ich]

So why would you want an expanding coordinate system and to say things are comoving, when it seems like it is just simple kinematics?

Because this "simple kinematics" becomes quite complicated in different coordinate systems. The description in expanding coordinates is mathematically the simplest, and it successfully makes use of the cosmological principle - which means, they become quite natural if you "add the assumption that this happens everywhere in the universe".
The drawback is that they are absolutely unsuited for describing physics on smaller scales, like galaxies or rooms.

Oh and what about stretching of light? Is this from DE or DM?

The biggest part is from motion, Doppler effect. On smaller scales, where everything can be well defined, you have Doppler effect (linear with distance) and gravitational redshift (quadratic with distance). But again, the distincition between Doppler effect and gravitational redshift becomes blurred when going to larger distances.

 

[Ich]

Ah but I forgot that some galaxies are receding FTL so it can't be kinematics, correct?

Not correct. As I said, the concept of velocity is not well defined on large scales. Additionally, the very definition of "recession velocity" is (neglecting gravitation) that of a rapidity, not a velocity.
In arbitrary coordinate systems, velocities are arbitrary, too. That has nothing to do with the usual meaning of "FTL travel", which is defined in standard SR coordinates.

 

[benk99nenm312]

Well, if you say so, but the reason it came up at all was because properties were being attributed to this fabric analogy as if it was real and could affect reality.
Hey. Two words was my contribution. And I mitigated it with an emoticon.

As I said, fabric is merely a descriptive word, an adjective to describe a slice of our 4 Dimensional Spacetime. Call it what you want, but hey, nearly every theory in existence attributes properties to this cross section of Spacetime. Relativity says that spacetime is affected by mass and energy, QFT gives empty spacetime (the vaccuum) energy and energy density. The way we describe the 'fabric' of spacetime certainly affects reality, and no more will I argue this point.

As the balloon expands, the distance between the pennies increase, yet the pennies do not increase in size. Why? Because the forces holding the atoms of the penny together utterly dwarf the forces of the balloon pulling it apart

Utterly, as of what we know, false.

Acceleration is equivalent to something repulsive, and we call that something Dark Energy.

The galaxies are moving away since the big bang kick-off. Your walls are not moving away, because nobody kicked them.

Finally we reach the correct conclusion. Thank you Ich.

 

[powerplayer]

The description in expanding coordinates is mathematically the simplest, and it successfully makes use of the cosmological principle - which means, they become quite natural if you "add the assumption that this happens everywhere in the universe".

ok, so did the term "space expansion" come from the fact that mathematically describing the motion of galaxies worked better in an expanding coordinate system?

Is there physical evidence that space is expanding or is it just a result of the math?

The concepts of distance and velocity are quite tricky at really large scales,

As I said, the concept of velocity is not well defined on large scales.

Why is this? Relitivity?

 

[Ich]

ok, so did the term "space expansion" come from the fact that mathematically describing the motion of galaxies worked better in an expanding coordinate system?

I think so. In these coordinates, if you calculate the expression for "change in proper distance", you get a very simple and suggestive formula. Like "recession = motion of space + motion through space". This took a life on its own, it seems.

Is there physical evidence that space is expanding or is it just a result of the math?

There is ample evidence for redshift being more or less proportional to distance. Which means that the universe may be described well as objects more or less at rest in an expanding coordinate system. I don't know how that pertains to evidence for "expanding space".

Why is this? Relitivity?

Yes.
For example, in special relativity you may have heard that it's tricky to http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_case:_parallel_velocities". In cosmological coordinates, you add them without correction nonetheless. That's easy and appropriate, but the result definitely has nothing to do with "velocity" in the SR sense. Especially, v<c does not apply. That's just the effect of an unusual coordinate system.
Then, in GR, matter determines geometry. You don't even have that static background on which to base your misunderstandings. Is distance changing, or is the light we took to measure distance delayed by some matter that intervened? There's not way to decide such questions.

 

[powerplayer]

Thank you Ich.

My curiosity is temporarily satisfied. :)

 

[Chronos]

Expansion is a pathetically weak 'force' compared to the other forces of nature - gravity, nuclear and electroweak. It is overwhelmed by these forces until until things become so vastly distant their effects become negligible.

 

[benk99nenm312]

Nobody knows the correct answer to this question...