# Space expansion in our perspective?

So if space is expanding everywhere, does this mean the space in my room is expanding?

I'm assuming that its expanding so slow because of the small scale, but is there still a measurable amount of space expansion occurring in my room in any time interval?

If i were to wait a few billion years, would the space in my room be noticeably bigger or would i be expanding at the same rate and not notice a difference?

P.S. (I like explanations and details.)

Thx :)

Ich
I'm assuming that its expanding so slow because of the small scale, but is there still a measurable amount of space expansion occurring in my room in any time interval?
No.
There is observational evidence that galaxy clusters are moving away from each other. They do so with a velocity proportional to their distance.
One can choose an expanding coordinate system such that all those comoving clusters have constant coordinate values. Since they all are in relative motion, their proper distance increases nonetheless. This increase of distance while keeping constant coordinate positions is dubbed "Space expansion". If you'd use a different coordinate system, you would call the same thing "velocity".
The walls of your room are not comoving. They are where they are, without relative motion. The distance between them is not increasing.
In the expanding space paradigm, you'd say they have a inward peculiar velocity that cancels the expansion. That's just a complicated way to say that they don't move.

Our planet will remain the same size. So will our galaxy. It is the space between galaxies that are expanding.
cheers, BT

In the expanding space paradigm, you'd say they have a inward peculiar velocity that cancels the expansion.

So space is expanding locally, but it is being canceled by inward peculiar velocity? What is inward peculiar velocity?

Ich
In expanding coordinates, one often says that http://en.wikipedia.org/wiki/Comoving_distance" [Broken]things are not moving, that the increasing distance is due to expansion of space. Peculiar velocity is what's left when you subtract that part. It's everything that does not behave exactly like a homogeneous expanding universe.
So things without relative motion (zero change in distance), like the walls of your room, would be described in a funny way: you subtract the supposed motion due to expansion from zero, and are left with a negative value. You call that an inward peculiar motion.

This kind of description is matematically well defined, and it's useful for modeling the universe. But it's a real nuisance if applied to things that are simply not expanding, it makes you think all kind of weird stuff is going on, when simply those things have no relative velocity.

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Isnt the answer simply that atoms and objects are bound together by force?

DaveC426913
Gold Member
This is how I illustrate it:

The expanding universe is an expanding balloon. Galaxies are shown as pennies glued all over the balloon.

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.

Thanks for the great replies. :)

So the space in my room is expanding, however the 4 fundamental forces can resist the force of expansion.

Galaxies that are far enough apart don't have the required gravitational force to resist the expansion and therefore are accelerating apart.

Is this correct thinking?

Ich
Isnt the answer simply that atoms and objects are bound together by force?
DaveC426913 said:
the forces holding the atoms of the penny together utterly dwarf the forces of the balloon pulling it apart.
powerplayer said:
So the space in my room is expanding, however the 4 fundamental forces can resist the force of expansion.
No.

There is no physical property of empty space called expansion. Expanding space is locally exactly the same as nonexpanding space. The difference is in the description, not in space.
If the walls of your room don't have relative velocity (which I hope is true), they are not expanding. That's it. There's no force needed to prevent "expansion".

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.

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This is how I illustrate it:

The expanding universe is an expanding balloon. Galaxies are shown as pennies glued all over the balloon.

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.

Space is not expanding uniformly.

Ich said:
There is no physical property of empty space called expansion. Expanding space is locally exactly the same as nonexpanding space. The difference is in the description, not in space.
If the walls of your room don't have relative velocity (which I hope is true), they are not expanding. That's it. There's no force needed to prevent "expansion".

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.

I simply can't make sense of what you are saying. Expanding space is locally exactly the same as nonexpanding space? How does that work?

Where does relative velocity enter the realm of space expansion? Our best guess is that dark energy has something to do with the possible negative energy density of the universe. The acceleration of galaxies away from us is interpreted as the effect of this negative energy density force overcoming the attractive force of gravity.

DaveC426913
Gold Member
Space is not expanding uniformly.
I did not imply it was.

I simply can't make sense of what you are saying. Expanding space is locally exactly the same as nonexpanding space? How does that work?
Space is not doing anything. The idea of the "fabric" of space and what it might be doing - as anything other than an analogy - is misbegotten.

I did not imply it was.

Space is not doing anything. The idea of the "fabric" of space and what it might be doing - as anything other than an analogy - is misbegotten.

Misbegotten? The idea of the fabric of spacetime is nothing more than an analogy? I'd love to see the proof from which you extract that.

Ich
Expanding space is locally exactly the same as nonexpanding space? How does that work?
There is nothing, absolutely nothing, in space that could betray "expansion". The concept of velocity does not apply to space. Objects can move, space can't.
Spacetime can be curved, and this curvature can be measured locally. Spacetime curvature is not expansion. Flat, empty space may be "expanding". It's just a matter of the coordinates you use.
Where does relative velocity enter the realm of space expansion?
Well, those galaxy clusters are moving away, aren't they? And their velocity is proportional to distance, at least in our vicinity (say, some GLy out).
You will encounter people that tell you "they are not actually moving, it's rather space expanding" or so. Ask them how they can tell expansion from motion. If there's a galaxy some 100 MLy away, what measurement establishes that this galaxy is not moving away?
They are actually moving away from us, and that motion is called expansion.
The acceleration of galaxies away from us is interpreted as the effect of this negative energy density force overcoming the attractive force of gravity.
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.

This link might help understand what Ich is saying or at least clarifies this confusion that has affected not only lay people but experts too (BTW I found the link in this forum)
http://arxiv.org/abs/0803.2701

DaveC426913
Gold Member
Misbegotten? The idea of the fabric of spacetime is nothing more than an analogy? I'd love to see the proof from which you extract that.

The onus is on you to show the idea has substance and is not merely an analogy.

I believe we must all recognize we are discussing the interpretation of observation. I put no such onus on myself, I merely contend that we cannot simply dismiss such concepts as "the fabric of spacetime". The onus is in fact on you to disprove it, as you are the one who claims it misbegotten.

Yes, galaxies are receding from us. It is the idea that they are receding at an ever faster pace that requires further analysis. If I throw a baseball in space, it recedes away from me at a constant velocity. The galaxies, as of what I last heard, are accelerating away. The acceleration is why, it would seem to me, you cannot simply say that expanding space is the same as nonexpanding space. Maybe locally they are too similar to discern from one another, but the very idea of expansion implies more than a constant velocity of recession.

And this is where the possibility of DE comes in. Different interpretations are always out there.

DaveC426913
Gold Member
I merely contend that we cannot simply dismiss such concepts as "the fabric of spacetime". The onus is in fact on you to disprove it, as you are the one who claims it misbegotten.
It can't be dismissed because it was never present. This 'model of space as a fabric' would first have to be more than merely an analogy.

So, my question then becomes "What do you mean when you talk about this 'fabric of space'? Can you please point me to some literature where this model is used in anything but an analagous way?"

It can't be dismissed because it was never present. This 'model of space as a fabric' would first have to be more than merely an analogy.

So, my question then becomes "What do you mean when you talk about this 'fabric of space'? Can you please point me to some literature where this model is used in anything but an analagous way?"

The way I've understood this is that the "fabric" is just merely the unification of space and time dimensions into Spacetime. To say this model was never presented... i'm confused. The literature is all around us...

DaveC426913
Gold Member
The way I've understood this is that the "fabric" is just merely the unification of space and time dimensions into Spacetime. To say this model was never presented... i'm confused. The literature is all around us...

It is an analogy, to help us understand such things as curvature of space. But analogies break down; you cannot infer from an analogy back to real life.

Likewise, the expanding balloon analogy of the universe is an analogy; it helps us visualize how we can appear to be at the centre of our universe when in fact we are not in any such privileged position. But you cannot infer from the analogy such things as 'OK, if the balloon is a sphere, then it has a centre, right?' No. The analogy breaks down. The universe isn't really a balloon.

Likewise, there is no actual property of space that is expanding and carrying things with it. The only things that can move are matter and energy. We measure expansion of the universe by measuring the increasing distances between objects. The space between the objects has nothing to do with it.

It is an analogy, to help us understand such things as curvature of space. But analogies break down; you cannot infer from an analogy back to real life.

Likewise, the expanding balloon analogy of the universe is an analogy; it helps us visualize how we can appear to be at the centre of our universe when in fact we are not in any such privileged position. But you cannot infer from the analogy such things as 'OK, if the balloon is a sphere, then it has a centre, right?' No. The analogy breaks down. The universe isn't really a balloon.

Likewise, there is no actual property of space that is expanding and carrying things with it. The only things that can move are matter and energy. We measure expansion of the universe by measuring the increasing distances between objects. The space between the objects has nothing to do with it.

Ok.. then by all means rewrite the books on Relativity. What do you propose then, instead of the dimensions of space and time? As humans we defined ourselves to be moving through dimensions. It's not an analogy of anything..

Your last sentence is quite a bold statement, the truth is that we don't necesarily know, that is just what most agree on. That's the concept of dark energy.

DaveC426913
Gold Member
Ok.. then by all means rewrite the books on Relativity. What do you propose then, instead of the dimensions of space and time? As humans we defined ourselves to be moving through dimensions. It's not an analogy of anything..

When did we start talking about dimensions? We were talking about fabric. Stay awake. :grumpy:

It is an analogy, to help us understand such things as curvature of space. But analogies break down; you cannot infer from an analogy back to real life.

It is really not so easy. Universe exchanges energy with spacetime.

I'm quite awake thanks, there's a fine line between discussion and argument. How would you define the fabric of spacetime?

DaveC426913
Gold Member
I'm quite awake thanks, there's a fine line between discussion and argument.
Pulling a giant glaring straw man is certainly more argument than discussion; it came right out of left field. For a dozen posts we're talking about fabric, then you accuse me of rewriting laws of physics regarding dimensions. Reread your post 20.

How would you define the fabric of spacetime?
?? You know how I define it; it is an analogy.

Just so you don't think I'm the only one, peruse https://www.physicsforums.com/showthread.php?t=391606", to satisfy yourself that knowledgeable people do not take this fabric as more than a tool for explanation.

framedragger eloquently says "The existence of dimensions doesn't imply that they form an 'interwoven fabric'. That's a great way to describe spacetime without math, but it's not physical."

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The word Fabric is merely informally referring to a 'slice' of the 4th Dimension. This is how I understand it. The word fabric is where I believe you get your analogy. From what it sounded like, to me you seemed to be challenging the existence of Spacetime itself. That is why I asked you to define it. Can we stop being sarcastic to each other?