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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 galaxi

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 either. However, if you include time, then at some point in the distant future there will be a last galaxy because old stars will all burn out or be sucked into black holes. And further in the future protons will decay, so there will be no baryonic matter left, plus black holes will eventually evaporate. Much sooner than that though, the continued expansion of spacetime means that galaxies will in time disappear over the expanding cosmological horizon, and future lifeforms will know nothing of the universe outside their own aging galaxies. See: https://en.wikipedia.org/wiki/Future_of_an_expanding_universe https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...

drKarl 10y ago
I wonder if there would be a Restaurant at the End of the Universe
Razengan 10y ago
> 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..
- dsfuoi 10y ago
  The surface of the ballon represents a 2d space, where you cannot look "up-down" in the 3rd dimension, just like in our 3d space you cannot look "up-down" in the 4th dimension.
- zuminator 10y ago
  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.

drKarl 10y ago

I wonder if there would be a Restaurant at the End of the Universe

Razengan 10y ago

> 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..

dsfuoi 10y ago
The surface of the ballon represents a 2d space, where you cannot look "up-down" in the 3rd dimension, just like in our 3d space you cannot look "up-down" in the 4th dimension.
zuminator 10y ago
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.