Explanation of expanding universe

  • #1

Main Question or Discussion Point

I once asked a physicist to explain what the universe expands into, and he gave me the analogy of raisins in bread dough. That the raisins represents galaxies, galactic clusters, etc...and as the universe (bread dough) expands, the galaxies (raisins) move apart. But this analogy assumes that the bread dough expands into the air around it, so I'm still unclear on what the universe is said to expand into.
Can anyone clarify? Thanks in advance!
 

Answers and Replies

  • #2
29,325
5,635
We have no evidence to support the idea that there is anything that the universe is expanding into.
 
  • #3
tom.stoer
Science Advisor
5,766
161
Mathematically it's quite simple: the universe is described by a 3-dim. expanding manifold without boundary (a 2-dim. manifold without can be visualized as an infinite sheet of paper, a surface of a balloon, a surface of a donut, ...); this mathematical concept does not require any embedding space, so the 3-dim. manifold can be described mathematically without a higher dimensional space into which it is embedded and into which it expands (a 2-dim. surface of a balloon can be described without the embedding into the 3-dim. space we ar used to).

So the only problem is that your everyday experience confused you b/c you are not able to think about these manifolds and their expansion w/o thinking about an embedding (whereas mathematically this embedding is not required).

So a 2-dim. model of our 3-dim. expanding universe would be a 2-dim. expanding sheet of paper with stars, galaxies, ... on this sheet of paper - but without the 3-dim. space into which the sheet is embedded.
 
  • Like
Likes 1 person

Related Threads on Explanation of expanding universe

  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
20
Views
3K
Replies
36
Views
13K
  • Last Post
Replies
1
Views
2K
Replies
5
Views
2K
Replies
8
Views
833
Replies
3
Views
534
  • Last Post
Replies
16
Views
3K
  • Last Post
Replies
4
Views
4K
Replies
1
Views
2K
Top