[trackpick (OP)]

Other galaxies zip away from us, sometimes faster than the speed of light. My question is: is this due entirely to space expansion or is there an additional contribution by these galaxies moving *through* the expanding space (and if so where would that have movement have originated)? I know that at least *some* of this is due to space expansion (it must be or we wouldn't be able to have objects receding at faster than the speed of light, right?), but how much?

[chris]

No; while the intervening space is growing more extensive - meaning that galaxies are retreating from us, those galaxies have their own motion too, which is superimposed upon the expansion of the Universe. In some cases the movement of the galaxies takes them away from us, but in other cases they may be moving towards us. A good example is Andromeda, which is racing towards us on a collision course that will impact in 4 billion years' time. The space between us and Andromeda will, nevertheless, have expanded in that time.

[evan_au]

(Oops! overlap with chris...)
Within our own solar system, motion of the planets is dominated by gravity.
Within our galaxy, motion of stars is dominated by gravity, but each star has its own independent speed and direction within the general flow.
Within our local galaxy group,  motion of galaxies is dominated by gravity, but each galaxy has its own independent speed and direction within the general circulation. Andromeda galaxy is heading towards the Milky Way galaxy at about 100 km/second.
See: https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision

The Hubble "constant" is currently estimated at around 73 km/s per Megaparsec.
So to see an expansion that overwhelms the random motion we see between Andromeda and our galaxy, you would need to look at least 3 Megaparsecs away (Andromeda is only 0.8 Megaparsecs away).

It is only when you look at other groups of galaxies that you start to see signs of the general expansion, so the M81 galaxy group is 3.5 Megaparsecs away, and has a recession velocity of about 300km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_groups

The Virgo Galaxy Cluster is 18 Megaparsecs away, and has a recession velocity of around 1100km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_clusters

[trackpick (OP)]

Very interesting! So if I limit my question to groups of galaxies over 3 Megaparsecs away and receding from us, does that change the answer? Do they move through space away from us in addition to space expanding (in any significant way)?

Context: a friend of mine thinks that the big bang had two effects: it was the start of space expanding and it also caused matter to fly apart similar to a conventional explosion with both of them ultimately leading to distant galaxy groups receding from us. I wasn't so sure about #2 (the explosion effect).

[evan_au]

it was the start of space expanding and it also caused matter to fly apart similar to a conventional explosion

The difference here is gravity.

When space initially expanded, the matter was a plasma (a very hot gas),
- it cooled down as space expanded (gases cool down as they expand).
- Gravity acted to pull clumps of the gas together into stars, galaxies, groups of galaxies and clusters of groups of galaxies....
- Cosmologists argue about what exactly formed the cores of today's galaxies, but they all come down to gravity in some form.

Once you have something that is bound together by gravity, like a star or a galaxy, it will hold together against the general expansion of the universe.
- At least the degree of expansion we see today
- Or expansion that we can forecast from previous trends
- Some cosmologists suggests that future expansion of the universe might be enough to overcome gravitational attraction within a galaxy - but this is a small minority view.

[trackpick (OP)]

So basically expansion of space (including dark energy) + gravity explain all the large scale behaviour we observe?

And there was no additional momentum from some kind of explosion of matter at the time of the big bang?

[jeffreyH]

We can't see inside black holes so singularities are only a mathematical anomaly. We cannot say what conditions are like beyond the event horizon. Therefore expansion is the only thing that we can postulate from the available evidence around us. We have the CMB and the redshift of light from distant galaxies which indicate an expansion. Of course scientists have far more data to work with than the layman. Any conclusions they reach are based on a great deal of research.
https://www.nasa.gov/feature/goddard/2018/improved-hubble-yardstick-gives-fresh-evidence-for-new-physics-in-the-universe

[Janus]

Very interesting! So if I limit my question to groups of galaxies over 3 Megaparsecs away and receding from us, does that change the answer? Do they move through space away from us in addition to space expanding (in any significant way)?

Like our own local group, those groups are bound together by gravity and are following orbits around their common center of gravity.    So while that group might be receding solely due the expansion of the universe, the individual galaxies within it have individual velocities with respect to the group as a whole.  Any measurement of a given galaxy's velocity would be a combination of the group's recession velocity and the galaxies velocity inside the group.

[geordief]

Am I OT to ask what are the possible causes of expansion /inflation in the first place?

I suspect it is a very complicated answer (and there was not an explosion at the centre as has been repeated many times).

Perhaps there was an earlier thread that dealt with expansion and inflation per se?

[Bill S]

… a friend of mine thinks that the big bang had two effects: it was the start of space expanding and it also caused matter to fly apart similar to a conventional explosion…

Interesting thought, but what does it mean?  If the BB brought space (the Universe) into existence and caused it to expand; surely the energy responsible for that would impact on all of the Universe in the same way.  Your friend would need to postulate another cause for independent “outward” motion of matter through space.  (I think)

[geordief]

Is (part of) the  theory that at "first" there may have been a  particle/wave  which combined a particle with its anti particle?

When these separated into their constituent parts they moved apart  and somehow there were still connected leading to further interactions...

Like pulling apart a piece of dough into two halves,there are bits left in the middle.

just faffing ,maybe 

[PmbPhy]

Other galaxies zip away from us, sometimes faster than the speed of light. My question is: is this due entirely to space expansion or is there an additional contribution by these galaxies moving *through* the expanding space (and if so where would that have movement have originated)?

There are a few points to address here.

1)  is this due entirely to space expansion

Yes.

2)  galaxies moving *through* the expanding space

No

3) would that have movement have originated

Please rephrase

Elaboration:
1) In these discussions its best to use visual aids. The analogy used for this visualization is that of either an expanding sheet or an expanding sphere. The first represents the scenario due to flat, open space and the other due to curved/closed space.

Flat: picture a sheet of rubber which is being stretched uniformly. For purposes of illustration we paint a grid of on the sheet and place ants on it. The grid lines intersect at points. Those points don't move through the sheet. The ants do. As the sheet is stretched it represents the expansion of space. The points can represents places where galaxies are and the ants other galaxies which have a motion "through" space and in addition to the motion of the expansion of the sheet.  Notice that as we expand the sheet the distance between points increases but no point is moving relative to the sheet.

Curved: Same thing but think of a balloon expanding. A grid cannot be used this time other than locally. But we can still have ants moving. If we were to draw lines on it to resemble a soccer ball then the we can also visualize those points moving apart.

Without that analogy I myself wouldn't be able to visualize it. Analogies are such wonderful tools.

Notice how I disagreed with chris. He said yes and I said no but we described the same thing.

The ants moving represent the additional motion galaxies have like chris explained. There need not be such additional motion.

Also note that when our galaxies collide there won't necessarily be any collisions of matter. Individual stars systems are light years apart and as such collisions of stars etc. will be quite rare if at all,

[trackpick (OP)]

Ok, thanks everyone! I think I have all my questions I've asked so far answered at this point.

This is all kind of mind blowing to me and thinking about implications I have a follow-up question:

Does space expansion give objects actual momentum (in the physics sense)? As a thought experiment, let's say that space stopped expanding one fine day: would galaxies continue to recede or would that just stop? I suspect that answer is the latter: if you take space expansion away, presumably all the matter in the universe would immediately start moving together again due to the force of gravity.

[guest45734]

I have a follow-up question:

Does space expansion give objects actual momentum (in the physics sense)? As a thought experiment, let's say that space stopped expanding one fine day: would galaxies continue to recede or would that just stop? I suspect that answer is the latter: if you take space expansion away, presumably all the matter in the universe would immediately start moving together again due to the force of gravity.

Space does not give objects momentum. Objects have momentum because of their mass and relative velocities. Dark Energy causes space to expand and Galaxies(mass) to recede away from each other (Dark energy may be due to quantum fluctuations). Mass causes gravity and space time curvature, (gravity MAY be due to the absorption of quantum fluctuations)

Speculating: Quantum fluctuations drive the expansion and contraction of space. If Dark energy was to cease to exist and space stopped expanding, quantum fluctuations causing dark energy would also cease to exist. The expansion of space is due to quantum fluctuations that are not being absorbed by mass, and if there is no quantum fluctuations then I suspect space would not exist.

Forces are transmitted by virtual particles which move through space. Virtual photons cause the magnetic field. Virtual particles can also be viewed as quantum fluctuations.

[Bill S]

  As a thought experiment, let's say that space stopped expanding one fine day: would galaxies continue to recede or would that just stop?

Possibly the simplest way to look at this is to say: Now, the galaxy groups are not moving relative to space (apart from gravitational motion), so if space stopped expanding, what would suddenly cause them to start moving?

In other words; the galaxies would not stop; they are already "stopped".

[saspinski]

Does space expansion give objects actual momentum (in the physics sense)? .

Suppose that the distance of a galaxy 1 billion of ly far away was measured by a supernova type 1a exploding there. And whose redshift matches that distance according to the Hubble law.

I would say that its momentum relative to us is zero, in the coordinates of the expanding spacetime. It is non zero in our local, (almost) flat spacetime coordinates.

[jeffreyH]

This all depends on the assumption that there is an internal force driving the expansion. It may be external to our universe and pulling everything apart. If a force is pulling everything apart then it would be indistinguishable from gravity. It would also mean that the force IS imparting momentum to the galaxies. This is limited by the speed of light and time dilation.

[trackpick (OP)]

This all depends on the assumption that there is an internal force driving the expansion. It may be external to our universe and pulling everything apart. If a force is pulling everything apart then it would be indistinguishable from gravity. It would also mean that the force IS imparting momentum to the galaxies. This is limited by the speed of light and time dilation.

I'm trying to understand what you're saying but failing.
- How does it matter if such a force is internal or external?
- If it is a force and it's creating momentum, then how could that lead to us observing galaxies receding at close to the speed of light (and hypothesizing that there are galaxies receding faster than the speed of light)? Doesn't special relativity prohibit this?

[guest45734]

- How does it matter if such a force is internal or external?
- If it is a force and it's creating momentum, then how could that lead to us observing galaxies receding at close to the speed of light (and hypothesizing that there are galaxies receding faster than the speed of light)? Doesn't special relativity prohibit this?

The information arriving to us from distant galaxies arrives at light speed, and is blue or red shifted depending on if a galaxy is moving away from us or towards us, the light we view from distant galaxies can take a long time to get here. The light arriving from them travels through an expanding space, at max light speed. The galaxies them selves can be viewed as free falling away from us due to the EXPANSION of space between our galaxy and those moving away from us. They feel no acceleration, and have been free falling away from us at an increasing speed relative to us for a long time. The edge of the VISIBLE universe is moving away from us at approx 3c . 

Gravity can be viewed as the CONTRACTION of space, or more simply the earth sucks ie absorbs space around it causing the appearance of space time curvature.

There is no place in the universe that does not have either matter or quantum fluctuations either random or otherwise. The virtual particles that transmit the forces in the universe(gravity, dark energy, magnetism etc) are quantum fluctuations. Dark energy is a quantum fluctuation, gravity is theorized to be caused by a virtual graviton it also is a quantum fluctuation.

Momentum is only felt when an object is accelerated or decelerated, it is not felt in free fall.

Warp factor 3 as in as in star trek allows the enterprise to exceed the speed of light. It is based on causing a warp in space time around the enterprise that causes an expansion and contraction of space that allows the ship to free fall through space without feeling acceleration.

[Bill S]

If a force is pulling everything apart then it would be indistinguishable from gravity.

Sounds like Donald Hamilton's "falling galaxies theory". 
http://novan.info/cosmol.htm

Run away!!