# What do we mean by space is expanding?

I know this question had often been asked on Physics.SE as well as this site but I don't get them. What do mean when we say that space is expanding? I mean that space is no physical entity that can expand. Space is just (apparently) "nothingness". Please solve my confusion.

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Does something like this help explain it for you? astronomy.stackexchange.com/questions/305/… Also, try thinking of it as space stretching, or more space being created between things. – Carl Jun 11 '14 at 9:11

Indeed space is not a physical entity (as far as we know). Saying space expands is another way to say that galaxies recede away from each other at a rate proportional to their distance.

The "expanding space" picture is unfortunately a source of endless confusion and misconceptions, as can be seen on the page of this very question: mpv mentioned that "The real expansion of space would manifest itself in the following way: you have 2 rockets in empty space, stationary with respect to each other. None of them starts its engines, but yet, they start moving away from each other."

This is plain wrong, the "expansion of space" is not some magical force that pulls objects away from each other. If you mean empty space as in no dark energy, no dark matter and no visible matter then the two rockets would move towards each other due to their mutual gravitational attraction (if you neglect their mass then they would not move at all). In the general case what will dictate the motion of the rockets is their gravitational attraction, the matter nearby (visible and dark) and dark energy (whatever that might be), the combination of which might pull them closer or further away from each other. The expansion of space is not an additional force. Galaxies don't keep moving away from each other because some undetected substance is created between them, but because they have an initial velocity which they acquired at the time of the big bang: without anything to stop them they keep moving away.

Joan.bdm said: "As I said in a large scale all objects recede from us, if it wasn't for the space expansion it would mean we are at the center of the universe."

The second part of the above sentence is wrong as well. As long as all galaxies recede from one point at a rate proportional to their distance, it follows that from ANY point all galaxies will be seen to recede at a rate proportional to their distance. This is easily seen with a little drawing (each * corresponds to a galaxy):

Before

After

If all galaxies are seen to recede at a rate proportional to their distance from the point of view of the galaxy at the center, then from any galaxy all other galaxies will be also seen to recede at a rate proportional to their distance.

I don't think we have any satisfying explanation as to why galaxies move away from each other at a rate proportional to their distance in the first place. The "expansion of space" is not an explanation, it is an analogy. A related and interesting read: Expanding Space: the Root of all Evil?

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But if it is insufficient, why do so many physicists agree to it and why is it also a part of the Lambda CDM model? – Yashbhatt Jun 12 '14 at 3:12
@Lee If you have some objections to my answer, please add them as a comment to my answer to keep the discussion structured. My explanation is not adding some new force on top of dark energy, but it is actually the manifestation of dark energy (lambda term in the Einstein equations). We both talk about the same thing. – mpv Jun 12 '14 at 8:30
@mpv I would have added it as a comment, but I can't comment your answer until I have "50 reputation". Unfortunately the way you phrased your answer, someone who doesn't know better will come to believe that a creation or stretching of space causes the rockets to move away. You can have so-called expansion of space without dark energy (constant or decelerated expansion), in which case the rockets would not move away from each other. In your example with the rockets space is not completely empty since you keep dark energy. Also dark energy is not necessarily homogeneous on small scales. – lee Jun 12 '14 at 11:08
@Yashbhatt Many (most I hope) physicists do not agree with the idea that space is a physical entity or substance actually being created or stretched between galaxies. The expansion of space is a mathematical model, a description of the fact galaxies move away from each other at a rate proportional to their distance, inside the framework of the theory of general relativity. This mathematical description is part of the ΛCDM model, but the idea that some physical entity between galaxies is being stretched or created is not. – lee Jun 12 '14 at 11:30
@lee Then what is expansion of space? – Yashbhatt Jun 12 '14 at 12:45

The expansion of space means that objects in cosmological distances are receding away from each other.

You may ask: is it the same thing like if there were 2 rockets in empty space (stationary with respect to each other) and then one of them starts its engines and accelerates away?

The answer is no. The above described case is not expansion of space. It is a simple movement with respect to an inertial frame. Notice that the accelerating rocket would be able to detect its motion with an accelerometer.

The real expasion of space would manifest itself in the following way: you have 2 rockets in empty space, stationary with respect to each other. None of them starts its engines, but yet, they start moving away from each other. As if dragged by some invisible force, but not feeling any acceleration (the accelerometers would indicate zero).

This is very similar to the case of inhomogenous gravitational field. Tidal forces would also increase the separation between the 2 rockets without any readout on accelerometers. However in the case of inhomogenout gravitational field there is usually a nearby gravitating body and the 2 rockets would be in free fall towards the body. But in expanding space there is no such body towards which objects would gravitate. The space is expanding because the spacetime metric is changing.

All this means that in expanding universe you cannot find one global inertial frame. Distant objects initially at rest would start moving away from each other immediately. This is not how an inertial frame behaves.

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"The real expasion of space would manifest itself in the following way: you have 2 rockets in empty space, stationary with respect to each other. None of them starts its engines, but yet, they start moving away from each other. As if dragged by some invisible force, but not feeling any acceleration (the accelerometers would indicate zero)." Have we ever tried that? – Yashbhatt Jun 11 '14 at 17:36
You can't try that currently: Hubble law is dominant at Mpc scales – Py-ser Jun 12 '14 at 3:17
@Yashbhatt We cannot do a direct experiment for that due to vast distances. We conclude that from the observations and also this is what we get from certain solutions of the Einstein equations for gravity. We don't know for sure: it is just a plausible explanation that so far is consistent with observations and it makes sense in the current theoretical framework. – mpv Jun 12 '14 at 8:16
@mpv But doesn't expansion of space seem like an ad hoc hypothesis to prevent anything from reaching light speed? What evidence do we have that it's not galaxies moving but space exapnding? – Yashbhatt Jun 12 '14 at 8:24
@Yashbhatt Yes, the links were provided in one of my previous comments. physics.stackexchange.com/questions/38779/… and also publish.csiro.au/?act=view_file&file_id=AS03040.pdf – mpv Jun 12 '14 at 17:39

All we know is this :

Distant objects are receding from us at a rate proportional to their distance from us.

All versions of "space is expanding" are philosophical variants of this.

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Actually everything is receding from everything, there is no central point. – Joan.bdm Jun 11 '14 at 11:51
I know that but space is not a real thing. So, if I am standing and a cat is running away from me, what do I say? That the cat is stationary, but the earth is expanding? How can we say that space expands for sure when we can't differentiate between objects moving away and expanding space? – Yashbhatt Jun 11 '14 at 12:58
As I said in a large scale all objects recede from us, if it wasn't for the space expansion it would mean we are at the center of the universe. – Joan.bdm Jun 11 '14 at 13:19
@Joan.bdm Then what is wrong with about the example about two galaxies I mentioned? – Yashbhatt Jun 11 '14 at 13:50
Of course all this depends on the point of sight, but the most realistic theory is the expansion. If you see a galaxy receding from you you could also imagine both galaxies are static but yours is becoming smaller (that would also provoke the same effects but it's impossible). It makes sense to think the universe expands since it's the only thing that exist, nothing else could stop it. The weird thing it that it is accelerating it's expansion velocity. – Joan.bdm Jun 12 '14 at 6:29