<p>I've searched quite a lot but found no satisfying definition, so please:</p>

<p>How do we actually define space when talking of "space expansion"? Isn't it just a metric, a dimension we measure, instead of an entity? How to define the "fabric" that is increasing? I can only understand it in one of these 2 ways:</p>

<p>1) The "distance unit" (size of the meter) is increasing, meaning light is also "dragged" with the increase. Because we define 1 meter as the difference in spatial position between 2 objects which light takes 1⁄299792458 seconds to cover. So, it means in this case, light is stretched, to cover the same distance in the same time. But also, each wavefront of the light beam covers the meter in the same time as before, it's still 1 meter for all of them. So it means the beam on the whole, even if longer, also travels faster to exactly compensate for the elongation ("longer" and "faster" are relative to the "old" meter). So I don't see any explanation for redshift appearing. Also, if the meter would be increasing, I can't understand how they say that distant regions of space are "growing appart" "faster than light" - since the distance unit is defined in relation to light in the fist place.</p>

<p>2) The number of "distance units" (number of meters) is increasing - which seems to mean actually movement instead of metric expansion. The objects have more and more meters between them, meaning light takes more and more time to travel between them (because we define one meter in relation to light). But it seems not true, because they say big regions of space expand faster than light - and objects can't actually move (rate of meters increasing) apart faster than light, do they? Or are there maybe 2 ways of moving - the one which is relative to the mystery dark matter/energy filling the "void" and so it's a moving bound by light speed, then the other kind of moving, which includes the effects of dark energy, and is unbound by light speed?</p>

<p>Or, to rephrase without the level of abstraction of the meter: </p>

<p>Is the space expansion dragging light?</p>

<p>1) If yes, why do we have cosmological redshift and why do we have space expanding faster than light?</p>

<p>2) If no, it’s interesting to see light having a special treatment. Also, this case seems very close to “movement”.</p>

CC BY-SA 4.0
2020-05-09T16:41:36.913Z
The second is closer, but things aren't moving - think of it as space being created between space, so you don't move but just get further away. But a metre is still a metre.
adrianmcmenamin
CC BY-SA 4.0
2020-05-10T11:53:55.650Z
Thank you Adrian. 
I would also like a definition of “going away” without moving :) though, besides the analogy. 
I guess we could do without the level of abstraction of the meter and rephrase like: 
Is the space expansion dragging light?
1) If yes, why do we have cosmological redshift?
2) If no, it’s interesting to see light having a special treatment. Also, this case seems very close to “movement”, whatever the definition of that is. (Sorry I’m eager for definitions but there are too many analogies around there.)
radu_cloud
<expansion><space><redshift><dark-energy>
Definition of space - to cope with "space expansion"
radu_cloud
# Definition of space - to cope with "space expansion"

(Asked by: username_0 on 2020-05-08)

I've searched quite a lot but found no satisfying definition, so please:

How do we actually define space when talking of "space expansion"? Isn't it just a metric, a dimension we measure, instead of an entity? How to define the "fabric" that is increasing? I can only understand it in one of these 2 ways:

1) The "distance unit" (size of the meter) is increasing, meaning light is also "dragged" with the increase. Because we define 1 meter as the difference in spatial position between 2 objects which light takes 1⁄299792458 seconds to cover. So, it means in this case, light is stretched, to cover the same distance in the same time. But also, each wavefront of the light beam covers the meter in the same time as before, it's still 1 meter for all of them. So it means the beam on the whole, even if longer, also travels faster to exactly compensate for the elongation ("longer" and "faster" are relative to the "old" meter). So I don't see any explanation for redshift appearing. Also, if the meter would be increasing, I can't understand how they say that distant regions of space are "growing appart" "faster than light" - since the distance unit is defined in relation to light in the fist place.

2) The number of "distance units" (number of meters) is increasing - which seems to mean actually movement instead of metric expansion. The objects have more and more meters between them, meaning light takes more and more time to travel between them (because we define one meter in relation to light). But it seems not true, because they say big regions of space expand faster than light - and objects can't actually move (rate of meters increasing) apart faster than light, do they? Or are there maybe 2 ways of moving - the one which is relative to the mystery dark matter/energy filling the "void" and so it's a moving bound by light speed, then the other kind of moving, which includes the effects of dark energy, and is unbound by light speed?

Or, to rephrase without the level of abstraction of the meter:

Is the space expansion dragging light?

1) If yes, why do we have cosmological redshift and why do we have space expanding faster than light?

2) If no, it’s interesting to see light having a special treatment. Also, this case seems very close to “movement”.

**Tags:** expansion, space, redshift, dark-energy

> **Comments:**
> **username_1**: The second is closer, but things aren't moving - think of it as space being created between space, so you don't move but just get further away. But a metre is still a metre.  
> **username_0**: Thank you Adrian.  I would also like a definition of “going away” without moving :) though, besides the analogy.  I guess we could do without the level of abstraction of the meter and rephrase like:  Is the space expansion dragging light? 1) If yes, why do we have cosmological redshift? 2) If no, it’s interesting to see light having a special treatment. Also, this case seems very close to “movement”, whatever the definition of that is. (Sorry I’m eager for definitions but there are too many analogies around there.)  

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## 0 answers

