3 ms·

Important to note, that this number refers to the “observable universe”. The number might be much higher, especially given inflation theory, but those galaxies

Important to note, that this number refers to the “observable universe”. The number might be much higher, especially given inflation theory, but those galaxies currently lay outside our observable universe due to speed of light constraints. Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you leave the screen at the top and appear on the bottom). In that case there might be fewer actual galaxies than what we observe. https://en.m.wikipedia.org/wiki/Observable_universe https://en.m.wikipedia.org/wiki/Observable_universe

dclowd9901 6y ago
> Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you leave the screen at the top and appear on the bottom). Would this mean the universe would be spherical?
- XorNot 6y ago
  Nope! Lots of possible finite but unbounded shapes - i.e. a torus shaped universe would have no observable edge, and also wouldn't be obviously wrapping around (you are at no risk of seeing light from yourself headed back at you from the other direction). You can then twist those geometries up into all sorts of knots and strange shapes to get all manner of universes where there is no edge, but light paths don't obviously loop back around.
- antonvs 6y ago
  The standard FLRW metric[1] that's used to model the universe is "not necessarily simply connected," which means it allows for shapes such as toruses, multiple connected toruses, and so on. There's one proposal for something called the Poincaré dodecahedral space - see "Is the universe a dodecahedron?"[2] But a sphere, or rather, a higher-dimensional hyperspherical equivalent, is the simplest possibility for that kind of scenario, and the FLRW metric allows that. This article, "The Shapes of Space", has some nice explanations: http://www.ic.uff.br/~aconci/poincare.pdf http://www.ic.uff.br/~aconci/poincare.pdf [1] https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr... [2] https://physicsworld.com/a/is-the-universe-a-dodecahedron/ https://physicsworld.com/a/is-the-universe-a-dodecahedron/

dclowd9901 6y ago

> Another interesting possibility is a finite but unbounded universe which means we might see duplicate galaxies from light circumnavigating (think of an old video game where you leave the screen at the top and appear on the bottom). Would this mean the universe would be spherical?

XorNot 6y ago
Nope! Lots of possible finite but unbounded shapes - i.e. a torus shaped universe would have no observable edge, and also wouldn't be obviously wrapping around (you are at no risk of seeing light from yourself headed back at you from the other direction). You can then twist those geometries up into all sorts of knots and strange shapes to get all manner of universes where there is no edge, but light paths don't obviously loop back around.
antonvs 6y ago
The standard FLRW metric[1] that's used to model the universe is "not necessarily simply connected," which means it allows for shapes such as toruses, multiple connected toruses, and so on. There's one proposal for something called the Poincaré dodecahedral space - see "Is the universe a dodecahedron?"[2] But a sphere, or rather, a higher-dimensional hyperspherical equivalent, is the simplest possibility for that kind of scenario, and the FLRW metric allows that. This article, "The Shapes of Space", has some nice explanations: http://www.ic.uff.br/~aconci/poincare.pdf http://www.ic.uff.br/~aconci/poincare.pdf [1] https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr... [2] https://physicsworld.com/a/is-the-universe-a-dodecahedron/ https://physicsworld.com/a/is-the-universe-a-dodecahedron/