All the stars and galaxies we can currently see – the *observable *universe – were compressed into a volume the size of a grapefruit at that time. However, the entire universe was much, much larger, possibly infinite.
The universe is expanding at greater than the speed of light right now. However the rate of expansion per unit distance is much smaller now that it was during the Inflationary Epoch. Only very, very distant objects are currently receding from us at greater than the speed of light, whereas during the Inflationary Epoch even very close objects were receding that fast.
The early universe was a hot plasma that was opaque to visible light. Light could not propagate any significant distance until about 300,000 years after the Big Bang.
Don’t think of the Big Bang as an explosion. The universe popped into existence everywhere all at once. At the time, it was very hot and very dense. Over time it has become less hot and less dense, but it’s not exploding outwards.
The first sentence is true. The second sentence is true iff the Universe is finite. If it’s infinite, then you’d never end up back at your starting point.
(to nitpick myself: It could, in principle, be finite in one or two dimensions, and infinite in the others, but I don’t think anybody puts much stock into that possibility)
Learjeff, there is in fact a possibility that we are seeing most of it, and there have been tests done to see if that can be proven. Basically, the cosmic microwave background radiation that we see was all emitted from a spherical shell centered on us. If the actual size of the Universe is smaller (in any dimension) than that spherical shell, then there would be circles of points on that shell that were actually the same points. If there are two such circles that are actually the same circle, then the pattern of the CMB around those circles should be the same, on different parts of the sky. Various groups have looked for such matching circles, but alas, have not found anything definitive: Some groups have claimed to have found something, but their results are all quite statistically weak.
Now, it’s possible that what they’ve found is in fact the real deal, and the statistical weakness of the result is just bad luck. And it’s also possible that the Universe is bigger than the size of that spherical shell, but only a little bigger. In either case, it would be true that we do see most of the Universe.
When I first heard this a bunch of years ago, it wasn’t obvious why this is so. Then someone explained that if it was finite in the past but infinite now, then there would be a moment in time when the universe went from being finite to being infinite.
I’m assuming that you’re joking… but the welder’s helmet would help in the early universe to about the same degree that an umbrella will help against asteroid impacts.
