Hello Spacelab, I'm Matthew Kleban,
I'm a theoretical physicist and an associate professor
at New York University.
And I'm here to answer your questions.
The first question is:
If the multiverse theory is true
and other universes truly exist,
let's say we had the technology to escape our universe
and go into inter-universal space...
since time only exists because of the big bang
and there are other "times" in different universes
same as our own, then what about the area
in between the universes, does time exist there?
That's a great question, but before I answer it
let me say just a little about time and the big bang.
So first of all, the big bang is an instant
about 14 billion years ago,
where the known laws of physics break down
in what is called a singularity.
And what that means is that
we don't know what happened really at that time,
or before that time.
And there are several alternatives.
So one of the alternatives, and the one that
multiverse theorists predict is that
time, in fact, did not begin at the big bang,
and we really can ask what happened before it.
In the multiverse theory,
the big bang is the formation of a bubble
in another state, apparent state.
So you can imagine that most of the universe
is filled with a liquid, let's say, let's take beer.
So you have a giant, volume of beer,
and in beer every now and then a bubble will appear.
And inside each bubble is a universe.
And this whole structure:
the beer, the bubbles, everything,
that's the multiverse.
But for each bubble for the universe inside each bubble,
the big bang is just the moment that the bubble appeared.
It's not a singularity.
The temperature and the density
at the time the bubble appeared is very high,
but it's finite, it's not infinite.
And we can use not the very well known laws of physics,
but some slightly speculative laws of physics
to ask, to make predictions about that time,
about the time the bubble formed.
And in particular, we can ask about what's outside
and that's a reasonable question.
Now, about time, there's one time which exists everywhere
in the multiverse,
and it exists in the bubbles,
and it exists in the region in between the bubbles,
but you have to bear in mind
that ever since Einstein's theory of relativity,
we've learned that time is a much more slippery
and flexible thing than what we use to think.
Time is something that can run at different rates
in different places.
For instance, if you're far down in the gravitational field
like deep in a mine, far below the surface of the Earth,
time will run more slowly,
fewer seconds will tick off on a clock
than if you're high up,
like up on top of a mountain,
or out of the gravitational field of the Earth entirely.
And that's something that's been tested
experimentally with great accuracy.
It's true.
So it applies to the multiverse too.
And it means that in different places in the multiverse
time is gonna run at very different speeds.
So there's one universal time,
but it's running at very different rates in different places.
The second question is:
With the theory or fact that the universe is expanding
how does science determine the edge of the universe?
And once the energy or forces,
which drives the expansion is spent
and the universe begins to shrink, what is left in it's place?
The same void or nothingness before expansion? Thank you.
That's another great question
and it's one that many people ask about.
And the standard answer is that
the universe actually doesn't have an edge at all.
Now there's two ways that that could be true.
One of them is that the universe could be infinetely big.
And what that means is that
no matter how far you travel,
no matter how far you go in any direction,
you never come to an edge.
There really is no edge, it just goes on and on forever.
You might wonder how something that's already
infinite big could possibly expand or contract,
but actually there's a pretty good answer to that.
For the whole thing to expand,
just means that every part of it is expanding.
And what that means is that
if you have 2 distant objects, say 2 galaxies that are
far apart from each other,
they're moving apart because the space
in between them is expanding.
So if you look around you,
you're living in one of those galaxies,
and you look around, what you see is that
all the galaxies you can see are receding away from you.
And the speed with which they are receding away from you
depends on how far away they are.
The further away they are, the more space there is between you,
and each part of that space is expanding
so the faster that distant galaxy is moving away from you.
So that's how an infinitely big universe can expand or contract.
The same thing can work in reverse.
So that's one way that the universe could not have an edge,
and there's actually another way,
which is that the universe could be curved.
And this is very hard for us to picture,
because our brains just aren't very good at
visualizing curvature in 3-dimensions.
But you have to imagine is that
we are 2-dimensional creatures
and we're just not aware of the third dimension,
doesn't exist for us,
we have no idea that it's there.
But in fact, we live on the surface of a sphere,
the surface of a ball.
So we can only move around in 2 dimensions:
forward and back, left and right,
but not up and down,
and we don't know that there's is a third dimension.
So is this is a 2 dimensional analogy, it's not perfect,
but if you can just manage to bump it up by one dimension
so now we're 3 dimensional creatures
and we can't imagine that our 3 dimensions could be curved,
but mathematically it's possible.
And that's the second way in which there might
not be an edge to the universe.
But having said that,
sometimes there is, in some theories,
there is an edge to the universe.
And in particular, in the multiverse theories,
the universe is inside a bubble that's itself
is embedded in this parent state, this parent vacuum,
we sometimes call it.
And in those theories there's an edge,
it's the edge of this bubble.
And what we can say scientifically
is if that's true, the edge of the universe
is at least ten times further away than we can see.
Than the furthest thing we can see.
And probably, it's much further away than that.
How do we know that?
Because if the edge was closer than that
we'd see some traces of it in cosmological data,
and we don't see any of those traces.
So if the universe has an edge, it's really far away.
But it's something that we could possibly
detect in the future as cosmological observations get better.
And as for what the universe expands into
or what it leaves behind, if it contracts,
if it's infinitely big then it really is everything,
and it doesn't expand into anything
or leave anything behind as it contracts
cause it's already infinitely big.
And if it curves back on itself
you can think of it as embedded in a higher
dimensional or four spatial dimensional space,
but you really don't need to.
You can think of it as a self contained thing
that, again, just expands or contracts without having
any space around it.
But in the multiverse theories,
where the universe is a bubble,
it really does have something rather, this parent state.
And so you might wonder, what is that parent state?
And the answer is, it's something
very unlike what we see around us.
It's a boiling, violently expanding,
rapidly changing state where the laws of physics
would look very different.
Every now and then new bubbles appear
so new universes are created.
Sometimes they may disappear,
so all sorts of stuff is happening out there.
It would be a very inhospitable environment
for a human being to find themselves in.
But it is something that we can understand
using these speculative laws of physics.
They make predictions about the physical characteristics
of that state, and so it's a very good question,
and it's something that we're investigating right now
to learn more about it.
Thanks very much for the great questions.
If you've got any other questions for the next expert
please leave them in the comments below.
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