# What do we exactly mean by space is expanding?

1. Jun 11, 2014

### Yashbhatt

I know this question had often been asked on PF 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.

2. Jun 11, 2014

### Mordred

the increase of space is simply an increase of geometric volume, that volume is filled with the contents of the universe.

You are right that space is not a physical entity, you oft read misnomers that imply that space has some physical property, however space itself does not. It has neither a physical property nor energy in and of itself.

3. Jun 11, 2014

### Yashbhatt

But does it have the space to expand? Like if a balloon(yes, I am using the misleading balloon analogy) is expanding,
it is expanding in space? Then, what is space expanding into?

4. Jun 11, 2014

### Mordred

this is one of the best balloon analogies I've come across, it is written by another forum member Phind's. He covers that question in better detail than I

http://www.phinds.com/balloonanalogy/ : A thorough write up on the balloon analogy used to describe expansion

5. Jun 11, 2014

### eltodesukane

Space expanding is the same thing as matter contracting.
So instead of space expanding, just think matter contracting.
All we can measure is that the ratio galaxy_size/intergalactic_distance is getting smaller.

6. Jun 11, 2014

### phinds

No, it is not. This has been discussed here on the forum many times.

"Expanding space" only happens outside galactic clusters. What would "matter contracting" mean? Why would it be different from "space shrinking"? Space does not expand or shrink and neither does matter. Things on the size of galactic clusters and larger just move farther apart withing the geometric framework that we call "space".

7. Jun 11, 2014

### Yashbhatt

Consider this example. Let there be two spaceships between Milky Way and some other galaxy. So, if space is expanding, then the two space ships should start moving away from each other even if they are set at a fixed distance from each other and are stationary with respect to each other. Will they move away?

8. Jun 11, 2014

### Mordred

Ok lets assume you have two stationary objects in a non gravitationally bound region. (expansion doesn't affect gravitationally bound objects)

then the answer is yes the volume of space between them will increase. So they will have an increase in distance between them, however they will not have gained any inertia, they will both remain stationary. The distance between them will simply increase. Expansion does not impart inertia

9. Jun 11, 2014

### Yashbhatt

I assume by gravitationally bound you mean strongly bound objects like binary stars, galaxies etc. Otherwise everything is gravitationally bound as there are no limits to gravitational force.

10. Jun 11, 2014

### phinds

No, that's not quite right. Certainly, you are correct in saying that there are no limits to gravitational force, but that does NOT imply that all objects in the universe are gravitationally bound to each other.

The earth and the moon are gravitationally bound. The Milky Way and the Andromeda Galaxy are gravitationally bound. The Milky Way and the Bullet Cluster are NOT gravitationally bound. I think Your requirement of "strongly" bound is correct if you consider that your definition of what constitutes strongly bound is overly restrictive. Personally, I don't know if "strongly" bound even HAS a solid definition but "gravitationally bound" does ... it means, roughly, things that don't fly apart from each other.

11. Jun 11, 2014

### Jorrie

I think one must be a little careful with this. Yashbhatt asked the question for two objects stationary relative to each other, which could be viewed as the Tethered Galaxy Problem of Tamara Davis et al.

According to Mordred's (apparent) definition, one or both of the tethered ships must have had inertia relative to the Hubble flow, which is then diminished as they later joint the Hubble flow. In the epoch of non-acceleration of expansion, the said inertia could even have gone up (as they fall "through" each other), before joining the Hubble flow on the opposite side.

12. Jun 11, 2014

### Yashbhatt

I don't understand the last part. Receding objects can show red shift and approaching objects can show blue shift?

13. Jun 11, 2014

### Mordred

yes motion has an influence on observations called Doppler effect. as objects approach there is a shift in frequency in light and sound waves (blueshift) as they recede there is a corresponding redshift

http://en.wikipedia.org/wiki/Doppler_effect

however redshift(blueshift) due to expansion not motion as per se. is called cosmological redshift
this article will explain the three types of redshifts in cosmology usage.

http://cosmology101.wikidot.com/redshift-and-expansion

14. Jun 11, 2014

### Jorrie

They say the opposite of what you say, but qualified for the 'tethered case'. The detail explanation is in section III (p. 5) of the referenced paper. Essentially, a distant galaxy that would presently be "untethered" from our Galaxy could have a huge peculiar velocity towards us, with the resulting Doppler blueshift overwhelming the normal cosmological redshift.

15. Jun 11, 2014

### Mordred

actually I didn't even consider the tethered scenario, my main point is no momentum is imparted upon the galaxies due to expansion. in this article its covered by the statement.

"space is expanding and is determined by the acceleration of the expansion of the universe -- not a force or drag associated with the expansion itself"

still its a good point to be aware of the tethered scenarios

by the way Jorrie I tried posting the lightcone calc on the forum recently and it came up as an error what is the latest version link or is version 7 still the latest?

Last edited: Jun 11, 2014
16. Jun 12, 2014

### Mordred

Jorrie off topic is version 7 the latest version of the lightcone calc?

I'm going to test the one off your signature

$${\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}$$ $${\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&0.0004&0.0006&45.332&0.042&0.057&3.15&66.18\\ \hline 0.003&339.773&0.0025&0.0040&44.184&0.130&0.179&3.07&32.87\\ \hline 0.009&105.913&0.0153&0.0235&42.012&0.397&0.552&2.92&16.90\\ \hline 0.030&33.015&0.0902&0.1363&38.052&1.153&1.652&2.64&8.45\\ \hline 0.097&10.291&0.5223&0.7851&30.918&3.004&4.606&2.15&3.83\\ \hline 0.312&3.208&2.9777&4.3736&18.248&5.688&10.827&1.27&1.30\\ \hline 1.000&1.000&13.7872&14.3999&0.000&0.000&16.472&0.00&0.00\\ \hline 3.208&0.312&32.8849&17.1849&11.118&35.666&17.225&0.77&2.08\\ \hline 7.580&0.132&47.7251&17.2911&14.219&107.786&17.291&0.99&6.23\\ \hline 17.911&0.056&62.5981&17.2993&15.536&278.256&17.299&1.08&16.08\\ \hline 42.321&0.024&77.4737&17.2998&16.093&681.061&17.300&1.12&39.37\\ \hline 100.000&0.010&92.3494&17.2999&16.328&1632.838&17.300&1.13&94.38\\ \hline \end{array}}$$

kk its working now

17. Jun 12, 2014

### Jorrie

Yup, we stopped tinkering at version 7. ;)

18. Jun 12, 2014

It is an anachronism.

There are two ways to describe an expanding Universe (equally valid in General Relativity due to diffeomorphism).

1) Keep the coordinates of space fixed and the coordinates of particles change with time.
2) Let the space coordinates expand with time so the particles coordinates remain the same.

They are equivalent but in the 2nd example it is easier to talk about "expanding space" while in the first about "galaxies flying away from each other".

When you change coordinates in General Relativity you also change the gravitational fields. (Just like in the coordanates that move with a falling elevator there is no gravity). So in the 2nd one the particles feel no "force" while in the first one they experience a gravitational force between particles.

So does space really "expand"? It depends on your coordinate system!

19. Jun 12, 2014

### phinds

Your item 1 and your statement about it in the next sentence are contradictory. Galaxies flying away from each other IS what we see experimentally but "coordinates of particles change with time" is NOT what we see. Coordinates of GALAXIES (that are not in galactic clusters) change with time relative to each other, yes, but particles, no.

20. Jun 12, 2014