On the (Small) Number of Atoms in the Universe • Hacker News Board

Post by zuminator » Sun, May 01, 2016, 1:49 PM UTC

Razengan wrote: post #160

zuminator wrote: post #139

Earlier quoted context omitted.

The typical way to picture the universe is like the surface of an expanding balloon, a 2-manifold which happens to be embedded in 3-space. If you picture galaxies as spots on the balloon, there's no "last" galaxy, they're all roughly equidistant from their neighbors. Analogously, our Euclidean universe is thought of (in terms of noncompact spatial dimensions) as an expanding 3-manifold. There's no last galaxy there e…

> If you picture galaxies as spots on the balloon, there's no "last" galaxy What about looking "up" and "down?" i.e. into the inside of balloon or away from its surface? I have a hard time wrapping my head around the balloon surface analogy, because galaxies seem to be in all directions of each other..

The analogy fails there because our 3manifold space isn't necessarily "embedded" into a higher dimensional construct the way the surface of a sphere is embedded into our 3 dimensional universe. So there isn't necessarily a hyperdimensional up or down to consider.

Or, alternatively, if you think of time as a dimension, then we can call "up" the future and "down" the past. In that case, if you look down inside the past, you'll see that in that analogy the center of the universe corresponds with the big bang! And all the galaxies are equidistant from that point/moment in spacetime.