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So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

FreeFull 8y ago
The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper
- hndamien 8y ago
  Part of me thinks that black holes are the border, and we might be inside one.
kryptiskt 8y ago
The Earth's surface is finite, but without a border. A finite universe would be similar, just adding a dimension.
richardknop 8y ago
I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.
arithma 8y ago
If it was finite, does it mean it was definitely not flat?
GuB-42 8y ago
The universe is infinite by definition. It may be closed, which mean it will repeat itself indefinitely, but that's still infinite. What isn't infinite is the observable universe. But the observable universe is just the view we have, its boundaries are defined by the observer. In fact my observable universe is different from your observable universe, by an insignificant amount. When physicists talk about the universe, it usually means the observable universe. Simply because for us, that's all we have.
- ekns 8y ago
  How is the universe infinite by definition? Wikipedia for one says the following. There is no mention of infinity. "The Universe is all of space and time[a] and their contents,[10] including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown,[3] it is possible to measure the observable universe."
Fordec 8y ago
Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in. We have no clue what is beyond the explosion itself and, for intents and purposes, that expanse is infinite by current understanding. Does that expanse contain other explosions? Don't know, there hasn't really been an observed overlap so far.
- Waterluvian 8y ago
  How can the diameter be larger than 2 * the speed of light * time ?
  - deleted 8y ago
    [deleted]
  - danbruc 8y ago
    The given number of 46.5 Gly is actually the radius, not the diameter. This can be larger than the age of the universe times the speed of light because the universe expands, i.e. while the light was traveling towards us the point of emission expanded away from us. So 46.5 Gly is the distance to the point of emission today but when the light we observe today was emitted there 13.8 billion years ago the distance was much smaller. Also note that the expansion of the universe is not limited to the speed of light because it is space itself expanding and not something moving through space which in turn means that there is no limit to the speed at which the point of emission can expand away from us.
    - Waterluvian 8y ago
      Thanks. I kind of understand enough of that to know what to search for further.
- gpderetta 8y ago
  IANAP, but the big bang, name notwithstanding, is completely unlike a conventional explosion. It is not a wavefront expanding in three dimensions off a center, there is no frontier nor a center at all in our three dimensions and each point of the universe[1] is getting farther than every other point, i.e. not moving radially off a center. You could imagine that the explosion is happening on an extra dimension and our three dimensional universe is the wave front of that explosion. Except that the math doesn't actually require the extra dimension to exist nor there is any evidence that it does. edit: also the cited diameter is only for the visible universe. The universe is currently understood to be infinite. [1] my understanding this applies only to points of the universe that are not otherwise gravitationally bound with each other.
fabatka 8y ago
We can measure that the Universe is homogeneous and isotropic on sufficiently large scales (it is according to the measurements). We can also measure the 'flatness' of the Universe (it is very likely to be flat, according to measurements). I don't know the details, but I was told that with these assumptions, proving that the Universe is infinite is only a matter of mathematics.

The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper

hndamien 8y ago
Part of me thinks that black holes are the border, and we might be inside one.

kryptiskt 8y ago

The Earth's surface is finite, but without a border. A finite universe would be similar, just adding a dimension.

richardknop 8y ago

I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.

arithma 8y ago

If it was finite, does it mean it was definitely not flat?

GuB-42 8y ago

The universe is infinite by definition. It may be closed, which mean it will repeat itself indefinitely, but that's still infinite. What isn't infinite is the observable universe. But the observable universe is just the view we have, its boundaries are defined by the observer. In fact my observable universe is different from your observable universe, by an insignificant amount. When physicists talk about the universe, it usually means the observable universe. Simply because for us, that's all we have.

ekns 8y ago
How is the universe infinite by definition? Wikipedia for one says the following. There is no mention of infinity. "The Universe is all of space and time[a] and their contents,[10] including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown,[3] it is possible to measure the observable universe."

Fordec 8y ago

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in. We have no clue what is beyond the explosion itself and, for intents and purposes, that expanse is infinite by current understanding. Does that expanse contain other explosions? Don't know, there hasn't really been an observed overlap so far.

Waterluvian 8y ago
How can the diameter be larger than 2 * the speed of light * time ?
- deleted 8y ago
  [deleted]
- danbruc 8y ago
  The given number of 46.5 Gly is actually the radius, not the diameter. This can be larger than the age of the universe times the speed of light because the universe expands, i.e. while the light was traveling towards us the point of emission expanded away from us. So 46.5 Gly is the distance to the point of emission today but when the light we observe today was emitted there 13.8 billion years ago the distance was much smaller. Also note that the expansion of the universe is not limited to the speed of light because it is space itself expanding and not something moving through space which in turn means that there is no limit to the speed at which the point of emission can expand away from us.
  - Waterluvian 8y ago
    Thanks. I kind of understand enough of that to know what to search for further.
gpderetta 8y ago
IANAP, but the big bang, name notwithstanding, is completely unlike a conventional explosion. It is not a wavefront expanding in three dimensions off a center, there is no frontier nor a center at all in our three dimensions and each point of the universe[1] is getting farther than every other point, i.e. not moving radially off a center. You could imagine that the explosion is happening on an extra dimension and our three dimensional universe is the wave front of that explosion. Except that the math doesn't actually require the extra dimension to exist nor there is any evidence that it does. edit: also the cited diameter is only for the visible universe. The universe is currently understood to be infinite. [1] my understanding this applies only to points of the universe that are not otherwise gravitationally bound with each other.

fabatka 8y ago

We can measure that the Universe is homogeneous and isotropic on sufficiently large scales (it is according to the measurements). We can also measure the 'flatness' of the Universe (it is very likely to be flat, according to measurements). I don't know the details, but I was told that with these assumptions, proving that the Universe is infinite is only a matter of mathematics.