Question about the expanding Universe and the Andromeda Galaxy

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Discussion Overview

The discussion centers on the apparent contradiction between the expansion of the universe and the approach of the Andromeda Galaxy towards the Milky Way. Participants explore the implications of Hubble's law in this context, examining the dynamics of gravitationally-bound systems versus the overall expansion of the universe.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that recession of galaxies is primarily observed at large distances (over 100 Mpc), and that nearby galaxies, like Andromeda, can exhibit random motion that may counteract the effects of universal expansion.
  • It is suggested that Hubble's law applies as an average and does not account for peculiar velocities of galaxies, particularly in gravitationally-bound systems.
  • Several participants assert that the Andromeda Galaxy's approach speed contradicts the expected recession speed according to Hubble's law, which would suggest it should be moving away at 55 kilometers per second.
  • Some argue that the gravitational bond between the Milky Way and Andromeda overcomes the effects of expansion, leading to their approach.
  • Participants discuss the need to consider local energy density when analyzing the dynamics of the Andromeda-Milky Way system, indicating that universal expansion rates do not apply locally.
  • There is mention of the inadequacy of applying a uniform expansion rate to local systems, as local dynamics are influenced by specific gravitational interactions rather than the average expansion of the universe.
  • Some contributions emphasize that the space between the Milky Way and Andromeda is not expanding due to cosmological expansion, highlighting that gravitationally-bound systems behave differently.

Areas of Agreement / Disagreement

Participants generally agree that Hubble's law does not apply on small scales, particularly for gravitationally-bound systems like the Milky Way and Andromeda. However, there remains disagreement on the implications of this for understanding the dynamics of their interaction and the role of local versus universal expansion.

Contextual Notes

Limitations include the dependence on definitions of energy density and the unresolved nature of how local dynamics interact with universal expansion. The discussion does not resolve the complexities involved in calculating the dynamics of the Andromeda-Milky Way system.

abdossamad2003
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Hi
If the universe is expanding, then why does the Andromeda Galaxy approach us? Does not it contradict Hubble's law?
 
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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.
 
abdossamad2003 said:
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 said:
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!
 
Yes. Hubble's Law isn't expected to apply on such small scales as the distance to Andromeda.
 
abdossamad2003 said:
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:
1640976037245.png
 
abdossamad2003 said:
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.
 
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Overall, I came to the conclusion that the gravitational bond of these two galaxies overcomes expansion.
 
abdossamad2003 said:
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.
 
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  • #10
PeroK said:
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.
 
  • #11
abdossamad2003 said:
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.

abdossamad2003 said:
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.
 
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  • #12
abdossamad2003 said:
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.
 

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