B The Impact of Expansion on the Observable Universe

[GraemeSRC]

Hope nobody finds these couple of questions obnoxious.

As the expansion of space-time carries apart the galaxies and the stars which make them up, does it also increase the separation of the planets within the planetary systems of those stars? Furthermore, does it also carry apart all the atoms that make up those planets?

If I were standing on a planet near the edge of the "observable universe" as it would be determined from someone on earth, would Earth then appear to me to be at the edge of my observable universe? And would this be the same regardless of where I was in the universe?

Apart from the phenomenon of red shift, is there any other observable or measurable effect of the expansion of the universe?

 

[Nugatory]

As the expansion of space-time carries apart the galaxies and the stars which make them up, does it also increase the separation of the planets within the planetary systems of those stars? Furthermore, does it also carry apart all the atoms that make up those planets?

No to both questions. If the atoms and planets were moving freely they would drift slowly apart (very slowly! It would be a good exercise to calculate how much increase in distance you'd expect in a year between two grains of dust separated by the distance between the Earth and the moon). However there are attractive forces between them (gravity in the case of the planets, electromagnetic forces for the atoms) that hold them together and resist the tendency to drift apart. Because gravity decreases with distance, the mutual attraction between two very distant galaxies won't be enough to keep them from drifting apart - but galaxies closer to one another will stick together.

If I were standing on a planet near the edge of the "observable universe" as it would be determined from someone on earth, would Earth then appear to me to be at the edge of my observable universe?

Yes

And would this be the same regardless of where I was in the universe?

If you are somewhere that I would call "at the edge of the observable universe" then I am somewhere that you would call "the edge of the observable universe".

 

[GraemeSRC]

Thank you Nugatory.

So now I am visualising space-time as a sort of "massless fluid" that "holds" objects in position unless other greater forces act on them to cause motion in relation to it. Would this be a sensible albeit simplistic "visualisation"?

How different would this "fluid" be from the "ether" that was previously proposed, and which Einstein posited cannot be detected?

 

[phinds]

Thank you Nugatory.

So now I am visualising space-time as a sort of "massless fluid" that "holds" objects in position unless other greater forces act on them to cause motion in relation to it. Would this be a sensible albeit simplistic "visualisation"?

How different would this "fluid" be from the "ether" that was previously proposed, and which Einstein posited cannot be detected?

No, space is NOT a "fluid", it's just geometry. Things are not "held together", they simply travel on geodesics in spacetime, as described by General Relativity.

 

[Jim Lundquist]

This is the common illustration of the history of the universe, but it gives the impression that the universe burst forth from the Big Bang like a garden hose on the spray setting. Is this accurate or just a simplification? Would a more accurate be the second illustration (also a simplification), and if it were, wouldn't this process occur spherically from the origin?

 

[phinds]

This is the common illustration of the history of the universe, but it gives the impression that the universe burst forth from the Big Bang like a garden hose on the spray setting. Is this accurate or just a simplification? Would a more accurate be the second illustration (also a simplification), and if it were, wouldn't this process occur spherically from the origin?

You misunderstand what this represents. It does NOT represent an expansion from a point in space, although that totally incorrect interpretation is not only common, it is promoted by pop-science presentations. About the most accurate way you can think of it is that it represents, inversely, the matter density of the universe.

The universe started off as a hot, dense plasma of unknown extent (probably infinite) and topology and expanded to where it is today.

 

[Jim Lundquist]

You misunderstand what this represents. It does NOT represent an expansion from a point in space, although that totally incorrect interpretation is not only common, it is promoted by pop-science presentations. About the most accurate way you can think of it is that it represents, inversely, the matter density of the universe.

The universe started off as a hot, dense plasma of unknown extent (probably infinite) and topology and expanded to where it is today.

Thank you, but then let me ask this another way. Did the expansion of the universe propagate spherically as in the balloon metaphor?

 

[phinds]

Thank you, but then let me ask this another way. Did the expansion of the universe propagate spherically as in the balloon metaphor?

No, there IS NO sphere. Expansion happened everywhere at once. I know that is very hard to grasp. It's one of the things people have the most trouble with when learning actual cosmology instead of pop-sci presentations.

I recommend the article linked to in my signature, or just Google "metric expansion"

 

[Jim Lundquist]

Thank you very much for your help...I am reading your link now.

Jim

 

[GraemeSRC]

Just back from reading your links. Intriguing. One illustration shows, in three steps, a photon traveling from a distant object to an observer, as space expands over time.

The deduction given from this diagram is that the object is now 46 billion light years away as evidenced by the red shift. But surely the photon, as it arrives at the observer, is a "snapshot" of where the object WAS at the beginning of the journey, not of where it IS at the end? Surely it is only the appearance of where the object started from that has been changed by the red shift. After all, it might have ceased to exist some time after the photon left on its journey, but the observer would still see it. So don't we need to say that we are not really seeing that object 46 billion light years away, that it only appears so due to the red shift. That it was actually only 14 billion light years away in that final "snapshot", which we would need to back-calculate knowing the red shift? Or have I missed the point (again)?

 

[Bandersnatch]

So don't we need to say that we are not really seeing that object 46 billion light years away, that it only appears so due to the red shift. That it was actually only 14 billion light years away in that final "snapshot",

If we wanted to go this route, we'd have to say it was 'actually' only 44 million lyrs away at emission.
You see a snapshot of an object that was relatively close-by, whose light had to work hard against the expansion of space to get to you, which took it much longer than just the initial distance divided by the speed of light. By the time it finally got to you, the object that originally emitted the light has been carried away by expansion to much farther than the travel time times the speed of light.
So, in cosmology you have more than one distance to consider, and all have some specific meaning, neither of which maps exactly to our everyday concept of distance to what you see.
The 46 billion lyrs distance is what is called proper distance, and is where the emitter is currently - i.e. what you'd measure as the distance if you could freeze the expansion and take your time measuring it with a ruler. It's usually what's referred to as the size of the observable universe, for better or worse.