Question about the expanding Universe and the Andromeda Galaxy

[abdossamad2003]

Hi
If the universe is expanding, then why does the Andromeda Galaxy approach us? Does not it contradict Hubble's law?

 

[Ibix]

Recession of galaxies is only seen at large distances, over 100Mpc, if memory serves. Essentially, all galaxies have some random motion on top of the uniform expansion, and for galaxies nearer than that the typical magnitude of the random motion is comparable or greater than that of expansion. So nearby galaxies (like Andromeda, not even 1Mpc distant) can be moving away from us, towards us, or purely transversely. Uniform expansion becomes the dominant effect beyond that.

This is because the universe is not and never has been quite completely isotropic. If it were genuinely isotropic, Hubble's Law would hold at all scales, but we wouldn't be there to see it because stars, planets, and people are part of the anisotropy.

 

[DaveC426913]

Hi
If the universe is expanding, then why does the Andromeda Galaxy approach us? Does not it contradict Hubble's law?

Because
- expansion is an overall average,
- objects such as galaxies have their own peculiar velocities, independent of the overall expansion,
- expansion is overcome on small scales by gravitationally-bound objects.

 

[abdossamad2003]

Hi
If the universe is expanding, then why does the Andromeda Galaxy approach us? Does not it contradict Hubble's law?

According to Hubble's law, the Andromeda Galaxy should move away from us at a speed of about 55 kilometers per second, but on the contrary, the galaxy is approaching us at a speed of 110 kilometers per second!

 

[Ibix]

Yes. Hubble's Law isn't expected to apply on such small scales as the distance to Andromeda.

 

[DaveC426913]

According to Hubble's law, the Andromeda Galaxy should move away from us at a speed of about 55 kilometers per second, but on the contrary, the galaxy is approaching us at a speed of 110 kilometers per second!

OK, so
1. the combined peculiar approach velocity of Milky Way/Andromeda is obviously 165km/s.
2. MW and Andromeda are gravitationally bound, so Hubble Law does not apply.
3. Hubble's Law is an average.
4. You're taking Hubble's Law too literally.Also, this:

 

[PeroK]

According to Hubble's law, the Andromeda Galaxy should move away from us at a speed of about 55 kilometers per second, but on the contrary, the galaxy is approaching us at a speed of 110 kilometers per second!

Hubble's law applies to the universe as a whole and can be derived from the Friedmann equation, which depends on the energy density of the universe. If we input the average energy density across the universe we get expansion. If we analyse the energy density for the local cluster of galaxies we do not get expansion: we get gravitational attraction.

 

[abdossamad2003]

Overall, I came to the conclusion that the gravitational bond of these two galaxies overcomes expansion.

 

[PeroK]

Overall, I came to the conclusion that the gravitational bond of these two galaxies overcomes expansion.

It's more than that. The very equations that produce the universal expansion rate do not apply to a volume of the universe that has high energy density.

The dynamics of the local cluster of galaxies have to be calculated specifically. It's not necessarily the universal expansion minus local gravity.

 

[abdossamad2003]

It's more than that. The very equations that produce the universal expansion rate do not apply to a volume of the universe that has high energy density.

The dynamics of the local cluster of galaxies have to be calculated specifically. It's not necessarily the universal expansion minus local gravity.

I mean, in addition to the gravitational expansion or dark energy, the Andromeda Galaxy's calculations have to be included in the calculations.

 

[DaveC426913]

I mean, in addition to the gravitational expansion or dark energy,

The space between MW and Andromeda is not expanding due to Cosmological Expansion.
It is not a-force-that's-trying-to-move-our-galaxies-apart-yet-being-countered-by-gravity.

Likewise, the atoms in your body (or even between the Sun and the Earth) are not being inexorably pulled apart by CE.

It does not apply within gravitationally-bound systems.

the Andromeda Galaxy's calculations have to be included in the calculations.

Andromeda's motion toward us is not a property of Andromeda. It is a property of the system. From an Andromedan's PoV, we're moving toward them.

Reread PeroK's post that you quoted. Take it to heart.

 

[PeroK]

I mean, in addition to the gravitational expansion or dark energy, the Andromeda Galaxy's calculations have to be included in the calculations.

The amount of vacuum and hence dark energy in the local cluster is small compared to the average across the universe. The universal expansion rate depends on the overall energy density of the universe. The dynamics of the Andomeda-Mily Way system does not.

To illustrate this, we could consider two scenarios:

1) The current scenario where the universal expansion is $70 km/s$ per megaparsec.

2) A scenario where the universal expansion is much greater. Say, $200 km/s$ per megaparsec.

The question is whether the dynamics of the Andromeda-Milky Way system would be different in those two cases? The answer is no. This is because, as I said above, you have to calculate the dynamics of the Andromeda-Milky Way based on the local energy density. And, in both cases, the amount of dark energy in the system is the same and is independent of the overall energy density of the universe.

In other words, the collision between the Andromeda and Milky Way is not significantly affected by what the rest of the universe looks like. It's independent of the universe expansion.

This is why you cannot simply plug-in a uniform expansion rate of $70km/s$ per megaparsec. That's only the average across the universe. It's not something that happens everywhere on a local scale.

 

[PeterDonis]

Moderator's note: A discussion about Hubble's original evidence for universe expansion has been moved to a separate thread:

https://www.physicsforums.com/threads/hubble-and-the-evidence-for-universe-expansion.1010881/