What is the Difference Between Motion and Expansion in the Hubble Law?

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Homework Help Overview

The discussion revolves around the Hubble Law and the concepts of motion and expansion in the context of galaxies. Participants are exploring the implications of the law on the behavior of galaxies, particularly regarding their size and movement relative to one another.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants are examining the statements made by two students about whether galaxies are expanding or moving, questioning the definitions of gravitationally bound systems, and discussing the implications of the Hubble Law on galaxy interactions.

Discussion Status

The discussion includes various viewpoints, with some participants agreeing with the idea that galaxies are gravitationally bound and questioning the notion of expansion leading to an increase in size. Others are considering the intrinsic motions of galaxies and the potential for collisions despite the overall expansion of space.

Contextual Notes

There is an emphasis on not providing direct answers to homework questions, with participants encouraged to engage in discussion and critical thinking about the concepts involved. Some participants note the importance of distinguishing between motion and expansion in their analysis.

itsirk2008
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Can anyone help me with this question? Or at least lead me in the right direction?

Two students are having a discussion after studying the Hubble Law.

Student 1: According to the Hubble Law space is expanding. Therefore our galaxy is bigger today than it was in the past and will keep getting bigger.

Student 2: No. Our galaxy is a gravitationally bound system. So it is not getting bigger but it is moving at greater speeds as it ages.

Which student do you agree with? Discuss why you agree or disagree with either or both of them.
 
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I need help with this one also...

Student 1: According to the Hubble Law space is expanding. This means that other galaxies are moving away from us. So our galaxy will never collide with the Andromeda Galaxy.

Student 2: All galaxies have their own intrinsic motions. Since the Hubble flow is constant, all those galaxies that are moving towards us will collide with our galaxy sooner or later.

Which student do you agree with? Discuss which parts of their statements you agree or disagree with and explain your reasoning.
 
We don't spoonfeed here, but we're happy to help. What are your thoughts?
 
Was not looking for spoonfeeding...that won't get me anywhere in the long run?

As far as student # 1, I agree with the statement that according to Hubble Law, the space is expanding. But expanding does not necessarily mean bigger.
 
I also know from my reading that the space between our galaxies is expanding; the farther a galaxy is, the faster it is going.
 
Need homework help - Hubble Law

Can anyone help me with this question? Or at least lead me in the right direction?

Two students are having a discussion after studying the Hubble Law.

Student 1: According to the Hubble Law space is expanding. Therefore our galaxy is bigger today than it was in the past and will keep getting bigger.

Student 2: No. Our galaxy is a gravitationally bound system. So it is not getting bigger but it is moving at greater speeds as it ages.

Which student do you agree with? Discuss why you agree or disagree with either or both of them.
 
...and I agree with student # 2 when he/she says that our galaxy is a gravitationally bound system.
 
1] Galaxies are indeed gravitationally bound.

As for #2. The velocity of Andromeda wrt to Milky Way is actually well-known and pretty much nails the answer.
 


itsirk2008 said:
Can anyone help me with this question? Or at least lead me in the right direction?

Two students are having a discussion after studying the Hubble Law.

Student 1: According to the Hubble Law space is expanding. Therefore our galaxy is bigger today than it was in the past and will keep getting bigger.

Student 2: No. Our galaxy is a gravitationally bound system. So it is not getting bigger but it is moving at greater speeds as it ages.

Which student do you agree with? Discuss why you agree or disagree with either or both of them.

I would like to help you understand Hubble law better. But our rule is we don't give anybody answers to homework. Also there is a homework help section of the forum where people are skilled in getting you to think and come up with the answer yourself. They could handle this. Personally I can't figure out how I can help you without giving away the answer.

BTW I notice from the way the problem is worded that you don't have to choose one and say that one is right and the other is wrong. You can, if you want, disagree with both. The key thing will be your discussion. It looks like a multiple choice question where you pick the right answer, but it is really a discussion question. That is my take on it. You better go to Homework Help section with this.

Try this:
https://www.physicsforums.com/forumdisplay.php?f=153
 
  • #10
Threads merged and moved to homework.
 
  • #11
What might help you is if you did some research on the differences between motion and expansion. Expansion, as in Hubble's law, refers to space time whereas the motion within that expansion (galaxies etc...) is bound by c.


hope this helps
 

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