Re: If the edges of the universe curve back , is big bang bakwards

Hi Larry, that's a good question.

You're right that we can't know for sure what's going on in parts of the Universe that we can't see---and indeed we can't see a lot of it. We can know, for example, what's gone on between us and the Moon in the past second. We can know what's happened between ourselves and Pluto in the past hour. We can know what's happened between us and a wall of plasma called the "surface of last scattering", which emitted the radio waves called the Cosmic Microwave Background 14.7 billions years ago. But if the Universe has an edge which appeared last year near Andromeda, we don't know about it yet. We can only describe what's going on in our "past light cone".

So, we have to make some assumptions. We usually assume that the Universe follows some sort of physical, mathematical laws, and that these laws are the same everywhere. Importantly, we assume that the Earth isn't in some special place in the Universe---we're not at the Scenic Outlook, nor at The Exact Center, nor at The Obstructed-View Seat in Mezzanine C. We're in a perfectly normal spot.

Think about what the ancient cosmographers who drew maps of a flat Earth with waterfalls going over the edges. Notice where they put Europe---always right at the center. Why? Well, the center is the only place from which you can't see the edges. The old mapmakers put themselves in the center because it's the only place far enough from the edges.

If you want to put "edges" on the Universe somewhere, you have to make very special assumptions about the Earth. The Earth is in the special place from which the edges are invisible---zillions of other planets and galaxies must live closer to the edge than we do; they, unlike us, can see the edge of the Universe. Then you have to write some sort of law-of-gravity to govern the edges---and again, you have to engineer things such that this law looks like the flat-expanding-Universe law we see from Earth, but changes behavior dramatically (becoming a things-curve-back law) far from Earth.

It's difficult, but probably not impossible, to draw up a model universe that does this. However, unless we start seeing evidence that something like this is actually going on ... well, it'll be one of the vast class of ideas about un-observable things that might be going on somewhere we can't see them. You can imagine anything you want going on outside of our light-cone; perhaps the Universe has an edge; perhaps Andromeda has just turned into a giant leprechaun; perhaps gravity suddenly changed from "attractive" to "repulsive" on the other side of the Milky Way. But one theory is as good (or as bad) as another, unless you can think of an actual experimental consequence. Two ways you could imagine seeing "edges" on the Universe would be (a) if you can actually see the "edges" themselves, or (b) if you can learn something about the local law of gravity which tells you that it should differ elsewhere in a way that would provide an edge. An example of the former approach is reported here; an example of the latter approach is this article. But no one has found anything convincing yet.

Hope this helps,

-Ben