Expansion and raisin bread

[John Helly]

TL;DR

What about the raisins?

Fascinated as I am of learning about cosmology, one metaphor seems particularly strained: the raisin bread metaphor for expansion. I get the notion of the dough expanding and how this accounts for the speed differential of the galaxies relative to each other; but what about the raisins? Shouldn't they be distorted into plums or dates; maybe bananas?

 

[Bandersnatch]

The raisin bread, like all analogies, suffers many ills. But this ain't one of those.
In an expanding universe structures bound by gravity - like galaxies - don't expand. So the raisins staying the same as the bread raises in the oven is actually ok.

You've referenced an empty thread template, btw.

 

[John Helly]

Sorry about the thread. I get confused by all the options and conventions yet to be discovered. Hoping a reply to this doesn't make it worse.

What is it about gravity, presumably due to visible and dark matter, that exactly negates the expansion? Is there a thread you can suggest to the math underlying this compensatory mechanism (narrower than just GR; which I'm still trying to learn)?Mahalo.

Source: https://www.physicsforums.com/threads/expansion-and-raisin-bread.1083760/

 

[Ibix]

Don't read too much into an analogy.

The expansion isn't a force pulling at galaxies and trying to stretch them - the galaxies are all moving inertially according to Newton's first law, just complicated by the curvature of spacetime. Individual galaxies and clusters are held together by their own gravity - they have stopped themselves from moving apart.

It's not radically different from two objects initially at rest in space falling towards each other because of their mutual gravitational attraction.

 

[Ibix]

You've referenced an empty thread template, btw.

I've seen this a few times. I think it might be a bug with the new thread page, or something like that.

 

[berkeman]

You've referenced an empty thread template, btw.

It's a forum bug, reference now deleted.