# Space expansion in our perspective?

DaveC426913
Gold Member
The word Fabric is merely informally referring to a 'slice' of the 4th Dimension. This is how I understand it.
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.

Can we stop being sarcastic to each other?
Hey. Two words was my contribution. And I mitigated it with an emoticon.

Yes. I explicitly mentioned DE and its implications. But acceleration is not expansion. Expansion is velocity, the biggest part of it caused a long time ago by inflation.
Why are galaxies accelerating then? Is it because of DE and DM?

There may be air in your room, or dark matter, or dark energy. Those things gravitate and would attract or repulse the walls. To withstand this attraction or repulsion, the wall must exert a minuscule force. But it doesn't matter whether the universe is expanding or not.
So space expansion is uniform and has nothing to do with DE and DM?

Space expansion came from the observation that light was being "shifted" to different wavelengths, correct?

I am trying to understand why there is a difference between space expansion in the distances between galaxies and the distances between the walls in my room.

Thx :)

Ich
Why are galaxies accelerating then? Is it because of DE and DM?
Acceleration is equivalent to something repulsive, and we call that something Dark Energy.
So space expansion is uniform and has nothing to do with DE and DM?
Do you know the difference between acceleration and velocity? It's exactly the same with acclerated expansion and expansion.
I am trying to understand why there is a difference between space expansion in the distances between galaxies and the distances between the walls in my room.
The galaxies are moving away since the big bang kick-off. Your walls are not moving away, because nobody kicked them.

Do you know the difference between acceleration and velocity? It's exactly the same with acclerated expansion and expansion.
I don't fully understand this answer.

So DE and DM cause accelerated space expansion, and space expansion is just actually the velocity of the galaxies from the big bang?

Ich
So DE and DM cause accelerated space expansion, and space expansion is just actually the velocity of the galaxies from the big bang?
DM causes deceleration. DE accelerates. Otherwise, yes.
The concepts of distance and velocity are quite tricky at really large scales, but that's exactly how it works when you look at smaller scales (some billion lightyears), where both concepts can be well defined. There, you have galaxies moving (their velocity roughly proportional to distance), this motion being changed (accelerated or decelerated) by their own gravity and the gravity of DM and DE.
Now add the assumption that this happens everywhere in the universe, and you get cosmology.

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

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

Ah but I forgot that some galaxies are receding FTL so it cant 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 cant 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.

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.

DaveC426913 said:
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.

Ich said:
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.

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.

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Thank you Ich.

My curiosity is temporarily satisfied. :)

Chronos