How can Andromeda collide with the Milky Way?

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Discussion Overview

The discussion centers around the apparent contradiction between the general observation that galaxies are moving away from each other due to the expansion of the universe and the specific case of the Andromeda Galaxy's impending collision with the Milky Way. Participants explore the implications of cosmic expansion on local galactic interactions.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions how Andromeda can collide with the Milky Way if everything in the universe is moving away from everything else.
  • Another participant challenges the initial premise, stating that locally, galaxies can interact and do not necessarily follow the trend of moving away from each other.
  • A participant seeks clarification on the meaning of 'everything is moving away from everything else,' emphasizing the importance of scale in understanding cosmic expansion.
  • It is noted that while galaxies billions of light years apart are receding from each other, Andromeda's proximity (2.5 million light years) means its velocity towards the Milky Way, driven by gravitational attraction, outweighs its velocity due to cosmic expansion.
  • A later reply acknowledges personal misunderstanding influenced by popular science channels, expressing gratitude for the clarification provided.

Areas of Agreement / Disagreement

Participants express differing views on the implications of cosmic expansion for local galaxies, with some asserting that local interactions can contradict the general trend of recession. The discussion remains unresolved regarding the broader implications of these dynamics.

Contextual Notes

Participants highlight the need to consider different scales when discussing cosmic expansion and local gravitational interactions, indicating that misunderstandings may arise from oversimplified explanations.

Jur van Oerle
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If everything in the universe is moving away from everything else in the universe, how can Andromeda collide in the distant future with the Milky Way?
 
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Jur van Oerle said:
If everything in the universe is moving away from everything else in the universe...
Because this part isn't true. Locally, galaxies interact with each other.
 
So, what exactly do people mean when they say 'everything is moving away from everything else'?
 
Jur van Oerle said:
So, what exactly do people mean when they say 'everything is moving away from everything else'?
You have to keep in mind the scales. Galaxies at distances of billions of light years away are all moving away from each other at significant speeds due to the expansion of space. However the Andromeda Galaxy is only 2.5 million light years away. The magnitude of its speed away from us that is due to the expansion of space is far smaller than the magnitude of its speed toward us due to its existing momentum and gravitational attraction between the Milky Way and Andromeda.

If someone is telling you that everything includes local galaxies in this case, then they don't know what they're talking about.
 
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Likes   Reactions: CalcNerd, davenn and berkeman
Alright, that makes sense.
I think it is my own ignorance, when I watch YouTube channels like Vsauce and MinutePhysics that is what I understand from it.
Thanks for clarifying!
 
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