Re: infinity/creation/destruction

VoiceOfReason wrote:
'Rev Dr' Lenny Flank wrote:
alan graham wrote:

Hi you guys, just tripped over you by accident but recently a question has
been bugging me and I thought this may be the best place to ask it.
What was there before the big bang?


There was no "before" the Big Bang. Time itself began at the Big Bang.
Asking what happened "before time began" is like asking what part of
the eareth's surface is "further north than the north pole". The
question itself is meaningless.



What's at the end of the universe?


There isn't any "end to the universe". Like the surface of a sphere,
the universe is finite but has has no boundary.


Surely if there was a big bang and the universe is constantly expanding then
there is an edge, a 'bow wave',if you like.


No, there's not. Just as there is no "edge" to the surface of a
sphere.



What happens if it hits a wall
or another 'bow wave'?


You seem to have the idea that the Big Bang means that space is
expanding into some pre-existing void. It's not. Spacetime itself is
expanding. It's not expanding "into" anything.


Bzzzzt! Zzzzzt! Fzzzzllll! *clears smoke away*

Okay, this is the part that doesn't quite compute for me. Our
"universe" contains all this stuff that's expanding outwards (and
accelerating, which also stymies me, but one thing at a time.)

Let me see if I have this right. Generally, we can define our universe
as all the "stuff" we can see or detect or whatever (leave dark matter
out of it for now.)

You always need to have in mind a specific model before you
talk about these things. Let's take the standard hot big
bang model, based on ordinary GR as the basis.

Make the assumption that the universal expansions is
accelerating as seems to be the case, or at leas that
the universe is open (the expansion will never end).

Then I suggest that you call the part you've defined,
instead of `our universe', the *observable* universe, for
more clarity. It consists of all that stuff that we could
see in principle in some way.

If current theory is right, then there is stuff far enough
away that we cannot even in principle hope to see it ... so
the distinction between the observable universe and the
whole universe is a real one within the model.

The reason for this is two-fold ... the model is based on
general relativity, and in GR nothing can travel locally any
faster than the speed of light; and the universe is also
expanding. Taken together these imply that nothing can ever
reach us from regions that were farther than a certain
distance away from us near to the time of the beginning.

In the standard big bang model, what is said to be beyond
the patch of the universe we can hope to see is said to be
just more of the same stuff that is right here.

Logically, since we can never hope to observe these regions
directly, this assumption is not of course, required, and in
any case not directly testable if the standard picture is
right.

Roughly this assumption is a statement of and is often
called the cosmological principle. It was made up in order
to simplify the construction of cosmologies, and inside the
observable universe it does appear to be pretty well
satisfied by every test that's so far been made.

But you could, for example, imagine that everything outside
of our observable patch was made up of antimatter.

A model could run into some significant trouble if it tried
to say that the antimatter region overlapped into our
observable universe, though.

Let's consider stuff that is conceptually the
farthest away stuff there is, beyond which we can see/detect nothing.
Let's say it's the farthest away star (from us) that there is. Since
this thing is moving away from us, it's moving into... something? It's
moving through space?

Everything in our local observable patch that has been
detected at cosmological (very large) distances shows a
redshifted spectrum, and this in turn suggests that all of
space in our local patch is expanding.

But, when you look at stuff at cosmological distances, you
have to remember that because nothing locally moves faster
than light in the big bang model, and light moves at a
finite velocity, then you must also be looking deeply back
in time, into the history of the universe.

In the standard picture, everything outside of our patch is
expanding too.

So the stuff at the farthest distances we can see back to
has in fact by now `moved' a long way from where it was when
the light we see was emitted. So we can never hope to see
this material directly at the place where it has now reached ...
it has moved out of the observable universe already.

But, from the point of view of someone sitting right at the
place that this material has currently reached, under the
assumption of the cosmological principle, that one sees exactly
the same thing around him as we do now. In other words, it is
everything that is expanding away from everything else,
and this is happening everywhere, not just in our observable
patch.

Now it's clear that this expansion is actually a very strange kind of
motion that we are talking about in the standard hot big
bang models. It's not quite the ordinary kind of `moving
away' that you have a good feeling for from experience,
where for example, you could walk to the store dragging a
piece of string behind you and then measure it's length and
say `I walked a mile there and a mile back, so I moved
a mile away from home.'

At very large distances it is not so simple a thing to
talk about relative motion as it is at very small distances.

We have to make up special systems of coordinates
and special distance measures, and it is in terms
of these measures that we say the universe is expanding.

Do we define our universe as stuff we can
see/detect, beyond which is by definition undefined?


As I said that's usually defined as the observable universe.


If I have that wrong, is there a link to an introductory explanation
that makes sense to someone like me who didn't go beyond physics 101?


This is the best intro I've seen out there, and it's useful
for me too every once in a while.

http://www.astro.ucla.edu/~wright/cosmo_01.htm



(An aside - I once read one of Steven Hawkings' books, A Brief History
of Time, or something like that. About the point where he started
talking about six different types of quarks, I realized the force was
no longer with me...)

Start with Steven Weinberg, `The First Three Minutes.' He's
somewhat more down to earth and it's still a classic intro
to the big bang model, using a few equations and not much
more.

Hope this helps, and good luck.

David

.

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