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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
by 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 agoHow can the diameter be larger than 2 * the speed of light * time ?
- deleted 8y ago[deleted]
- danbruc 8y agoThe 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 agoThanks. I kind of understand enough of that to know what to search for further.
- gpderetta 8y agoIANAP, 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.