# Thread: Andromeda collision & the expansion of space

1. So the Andromeda galaxy is closing on us at 400,000 km/h from 2.5 million lightyears away.

Space between the two galaxies is expanding at thousands of km per sec

How fast are the two galaxies moving in their own local space

What confuses me, is when you go back in time and the space between the galaxies was larger.
You soon reach a point when the expansion of space between them was faster than the closing speed now

2.

3. Originally Posted by PetTastic
So the Andromeda galaxy is closing on us at 400,000 km/h from 2.5 million lightyears away.

Space between the two galaxies is expanding at thousands of km per sec
The rate of expansion is about 74 km/s per megaparsec. (1) 2.5 mly is about 0.78 megaparsecs. This would make the rate of expansion at that distance would be 58 km/s.

However, my understanding is that there is no expansion within galactic clusters. You can think of this as being caused by the fact they are gravitationally bound (although that is not strictly true; it is more to do with the fact you only get expansion for a homogeneous and isotropic distribution of matter and this is only true on cosmological scales.).

How fast are the two galaxies moving in their own local space?
I would say they are closing at 400,000 km/h. I don't know if there is any other way you can define it.

(1) Hubble's law - Wikipedia, the free encyclopedia

4. Yep, sorry I said sec not hours.
266,400 km/h

Space must expand equally everywhere otherwise light would be bent in all directions, like some mad version of gravitational lensing.

5. Originally Posted by PetTastic
Yep, sorry I said sec not hours. 266,400 km/h Space must expand equally everywhere otherwise light would be bent in all directions, like some mad version of gravitational lensing.
It is bent like that. This is taken into consideration in a number of observations.

6. Gravity bends light according to general relativity. The expansion of space time by dark energy stretches or redshifts light. So a given photon is bent whenever it passes an object and stretches when it passes through expanding space time. These events can occur any number of times. DE is currently quite weak and is overcome by gravity in galaxies and in between galaxies in clusters.

7. Originally Posted by Arch2008
Gravity bends light according to general relativity. The expansion of space time by dark energy stretches or redshifts light. So a given photon is bent whenever it passes an object and stretches when it passes through expanding space time. These events can occur any number of times. DE is currently quite weak and is overcome by gravity in galaxies and in between galaxies in clusters.
Dark energy is NOT the cause of the expansion of the universe, it is responsible for the ACCELERATION of the expansion.

Your statement about the "expansion of space time by dark energy" is misleading in two ways -
1. It is space that expands, NOT space time.
2. It is not dark energy that is responsible for the expansion.

Your closing statement about DE being currently weak and being overcome by gravity is also misleading. You can apply that principle to the expansion of the universe, but not to dark energy (which is the cause of the ACCELERATION of the expansion). There has never been any question of DE having an effect mitigated by gravity in galaxies or clusters to begin with. With DE, objects might settle into equilibrium at an infinitesimally larger size than they would have done without, but it is the other forces (apart from gravity) that would mitigate this.

8. Space and time were combined into space time by Albert Einstein. So you’re saying that Al got it wrong?
DE is the energy of space time. The bigger the universe gets, the greater the expansion. DE causes the expansion, that’s why more of it causes the acceleration.
http://science1.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

9. Originally Posted by Arch2008
Space and time were combined into space time by Albert Einstein. So you’re saying that Al got it wrong?
No, I am saying that it is a common misconception to think that the expansion of the universe causes the "time" part of space-time to expand. It is the space part that expands, over proper cosmological time, which ticks at a constant rate of 1 second per second. This is why it is known as the metric expansion of space

Originally Posted by Arch2008
DE is the energy of space time. The bigger the universe gets, the greater the expansion. DE causes the expansion, that’s why more of it causes the acceleration.
Dark Energy, Dark Matter - NASA Science
No. The distinction is quite subtle, I grant you, but nowhere in that article does it say that DE causes the expansion, it only causes it to accelerate. You are making an assumption which is based on one of the possible explanations of dark energy, a hypothesis which only actually claims that a cosmological constant causes the expansion to accelerate. And that's the energy of SPACE, not space-time.

We need a different reason for the expansion of the universe in the first place, when there was a hell of a lot of matter and very little space... no? Why did gravity not win, back then?

We cannot say that DE causes the expansion. It isn't strong enough to have done so, which is why the expansion was decelerating for 7-8 billion years before the effects of DE started to become measurable and turn that deceleration into an acceleration. If gravity was winning for all that time, why did it lose in the first place?

10. I am still a bit confused on the basic issue of space expanding equally everywhere or not.

To keep galaxies stationary in their own local space & not exceeding the speed of light, it must be the space between them that expanding.
Therefor the cosmological redshift is caused by the expansion of space not the movement of the galaxies.

I always assumes cosmological redshifts were measured inside our local cluster as normal, but redshifts from local motion dominates?
If there is a cosmological redshift then space is expanding.

11. There is no measurable cosmological redshift within our local cluster, as our local cluster is not measurably expanding - it is bound by gravity! Also, across the distances involved the amount of cosmological redshift would be so small as to be unmeasurable, as it would be many magnitudes smaller than the redshift (or blueshift) caused by the peculiar motions of the objects in question. Gravity wins, around here.

So the universe is not expanding equally everywhere. It is not expanding between here and the moon, because the moon is bound by the gravity of the Earth.. and so on... up to distances larger than our local cluster. It is only in the voids between the clusters of galaxies that the universe expands, in the regions of the universe that are the least dense.

12. Well if you think about it; it would be the natural order of things to expand as things change or grow. Would that not be correct to assume.
Andromeda is a very beautiful gallaxy which by it's color scale could be teaming with life of some sort, well hopefully anyways. Some of the images which I have seen appear to be gallaxies which may have collided but this would never happen here in a 100 million lifetimes of man. So in the meantime lets just let Andromeda lie out there all nice and peaceful and maybe in one of those 100 million lifetimes someone will get to go visit it. Pull up Hubble telescope images in your bing image searches and check out the images which you see. This is where the image of Andromeda come from and many many more.

13. Originally Posted by warthog213
Some of the images which I have seen appear to be gallaxies which may have collided but this would never happen here in a 100 million lifetimes of man.
Pretty close. Andromeda will collide with our galaxy in about 4 billion years, which is about 50 million lifetimes.

14. Originally Posted by SpeedFreek
... So the universe is not expanding equally everywhere. It is not expanding between here and the moon, because the moon is bound by the gravity of the Earth.. .
one little correction, the moon is now and has been moving away from our earth
if your arguement hinges on the space between us and the moon not expanding
then

15. Originally Posted by sculptor
one little correction, the moon is now and has been moving away from our earth if your arguement hinges on the space between us and the moon not expanding then we need to address that?
That is due to tidal effects: Curious About Astronomy: Is the Moon moving away from the Earth? When was this discovered?

16. Strange, we were talking about the entire universe which Einstein clarified as space time. I did not imply that seconds were becoming longer, so thanks for clarifying that point to yourself.
DE was causing the universe to expand against gravity and then about 6 billion years ago when the mass of everything in the universe became diffuse enough, the expansion began to accelerate.
No one knows what happened at t=0, however the negative energy component of the scalar force in inflation theory and DE both quack like a duck. Perhaps you would like to share what you think causes the expansion of the universe?

17. Originally Posted by Arch2008
Strange, we were talking about the entire universe which Einstein clarified as space time. I did not imply that seconds were becoming longer, so thanks for clarifying that point to yourself.
Anyone who states that space-time is expanding is implying that seconds are becoming longer, which is why I clarified this issue earlier. Only the space part of space-time expands.

Originally Posted by Arch2008
DE was causing the universe to expand against gravity and then about 6 billion years ago when the mass of everything in the universe became diffuse enough, the expansion began to accelerate.
NO!! Why do you keep saying that?!

DE does not cause the universe to expand. About 6 billion years ago DE started to cause the expansion of the universe to accelerate. Before that, the expansion was slowing down, as gravity was winning the fight with whatever caused the expansion in the first place (which CANNOT be DE as it wasn't strong enough to have a measurable effect on the expansion for 7-8 billion years!)

Originally Posted by Arch2008
No one knows what happened at t=0, however the negative energy component of the scalar force in inflation theory and DE both quack like a duck. Perhaps you would like to share what you think causes the expansion of the universe?
We don't know.

18. Originally Posted by Strange
Originally Posted by sculptor
one little correction, the moon is now and has been moving away from our earth if your arguement hinges on the space between us and the moon not expanding then we need to address that?
That is due to tidal effects: Curious About Astronomy: Is the Moon moving away from the Earth? When was this discovered?
it is gravety which ties the moon to the earth
and
gravity that is moving the moon away doe to sharing of kinetic energy
("mr spok when will the spin of the earth and orbit of the moon be at equilibrium?")
(and the median distance at that time would be....?")

thanx strange

19. Originally Posted by sculptor
it is gravety which ties the moon to the earth
and
gravity that is moving the moon away doe to sharing of kinetic energy
("mr spok when will the spin of the earth and orbit of the moon be at equilibrium?")
(and the median distance at that time would be....?")

thanx strange
Isn't the fact that the moon is moving away do to its velocity in orbit being greater than the gravitational force Earth generates to hold it?

20. No, it is moving away from earth because angular momentum is being transferred from the earth's rotation to the moon's orbit by tides on earth.

21. Strange, in the early universe inertia and DE caused the universe to expand. Inertia caused objects that were moving apart to continue to do so but DE caused space time itself to expand. DE did not suddenly pop in out of nowhere 7 billion years ago. It is the background energy of space time. Einstein introduced it into his equations as the cosmological constant. The more space time expands the more DE you have and the greater the expansion. We do know this.
http://en.wikipedia.org/wiki/Dark_energy
http://en.wikipedia.org/wiki/Metric_expansion_of_space

22. Originally Posted by Arch2008
Strange, in the early universe inertia and DE caused the universe to expand. Inertia caused objects that were moving apart to continue to do so but DE caused space time itself to expand. DE did not suddenly pop in out of nowhere 7 billion years ago. It is the background energy of space time. Einstein introduced it into his equations as the cosmological constant. The more space time expands the more DE you have and the greater the expansion. We do know this.
http://en.wikipedia.org/wiki/Dark_energy
http://en.wikipedia.org/wiki/Metric_expansion_of_space
I'm not sure why you are addressing your responses to me as I am not the one pointing out where you are wrong. (Although, obviously, you are. Or, at least, very confused. Pay attention to SpeedFreek; he knows what he is talking about.)

23. Originally Posted by Arch2008
in the early universe inertia and DE caused the universe to expand. Inertia caused objects that were moving apart to continue to do so but DE caused space time itself to expand. DE did not suddenly pop in out of nowhere 7 billion years ago. It is the background energy of space time. Einstein introduced it into his equations as the cosmological constant. The more space time expands the more DE you have and the greater the expansion. We do know this.
Dark energy - Wikipedia, the free encyclopedia
Metric expansion of space - Wikipedia, the free encyclopedia
This is a very complicated subject, and your words do not accurately reflect the meaning of the contents of those links. There are subtleties that you are seemingly unaware of, so when you say "We do know this", well, actually, we don't!

Firstly, we do not know whether DE has been there all along, like a cosmological constant. It might be an emergent or dynamic phenomena like quintessence.
Quintessence (physics) - Wikipedia, the free encyclopedia

Secondly, even if DE was there all along, acting like a cosmological constant, then for the first few billion years of the history of the universe it was too weak to cause any expansion, WHICH IS WHY THE RATE OF EXPANSION WAS DECELERATING!

So, DE did not cause the universe to expand in the first place - something else did! Whatever that something was, it imparted the inertia you mentioned.

So, we have the universe expanding via inertia, gravity slowing down that expansion and DE possibly sitting in the background having no measurable effect until such a time as the gravitational density of the universe was low enough for DE to start accelerating the expansion, or DE emerging and starting to accelerate the expansion. Of course, if DE was in the background all along it would make a contribution to the rate of expansion, but that contribution was negated by gravity whilst the universe was decelerating (perhaps without DE the universe would have decelerated quicker, granted, but in no way can we say DE caused the expansion of the universe, when the expansion was decelerating).

Whatever imparted the inertia in the first place is the cause of the expansion. It cannot have been DE that imparted that inertia, as (if DE existed then) it was too weak to have any effect on the expansion for billions of years to come.

It is therefore incorrect to state that DE causes the universe to expand. It certainly causes the recent acceleration of the expansion, but it is not the cause of all that inertia in the first place. The universe does not expand due to DE, it expands due to something else and DE only accelerates that expansion.

24. Sorry for the follow on post, but I missed this aspect of things..

Originally Posted by Arch2008
Inertia caused objects that were moving apart to continue to do so but DE caused space time itself to expand.
There is no difference between these two statements, in the way you seem to think. Space-time "itself" is simply the framework we use to describe separations between events - it is the objects that are "moving apart" that defines the space-time between them. Those objects were not moving apart though space, it was the space in between them that was expanding.

And, for once and for all.. space-TIME does NOT expand!

Perhaps you should read DrRocket's description of the expansion of the universe to see what I mean...

GR treats the universe over all time as a single entity – spacetime. This can also be done in Newtonian mechanics, so there is nothing really new about spacetime. What distinguishes GR is that spacetime is not just affine 4-space, but in fact is a Lorentzian 4-manifold of undetermined topology, with a curvature tensor that is also unknown but is determined by the distribution of mass/energy via a stress-energy tensor defined by a very complex set of partial differential equations. These equations, the Einstein field equations can only be explicitly solved in a few simple circumstances. Gravity is the result of curvature of spacetime.

In general because of curvature neither space nor time have any global meaning. However, if one makes the assumption that spacetime is homogeneous and isotropic, then spacetime decomposes as a 1-parameter foliation by space-like 3-dimensional hyperplanes of constant curvature. The parameter serves as a surrogate for time and the hyperplanes as a surrogate for space. The hyperplanes inherit a true Riemannian metric from spacetime and expansion of space means that the distance between points increases as the value of the time-like parameter increases.
...The distance between points on those 3-dimensional hyperplanes that represent space, increases as the value of the time-like parameter increases...
...The distance between points in "space" increases, as "time" ticks on.
..."Space" expands, as "time" passes...

But space-time does not expand. It is meaningless to say the time part expands.

Imagine that the universe was not expanding - imagine a "static" universe (the reason Einstein introduced the cosmological constant in the first place). Would you then say space-time was "static"? No you wouldn't, because the time part ticks inexorably on at a rate of 1 second per second.

Originally Posted by Arch2008
DE did not suddenly pop in out of nowhere 7 billion years ago.
It might have done something similar, about 10 billion years ago, if dark energy turns out to be quintessence rather than a cosmological constant.

Originally Posted by Arch2008
It is the background energy of space time. Einstein introduced it into his equations as the cosmological constant. The more space time expands the more DE you have and the greater the expansion.
If DE works as a cosmological constant, then yes. If DE works as quintessence, then no. Oh, and one more thing - space time does not expand, only space does! Oh, and another thing - the expansion was not becoming greater whilst the universe was decelerating - the expansion was lessening at that time rather than increasing...

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