Cosmology^255
6.4 Discussion
G. ’t Hooft I would like to have an answer to the question: Is this notion of the
landscape the same as we have heard earlier today? Because here it seems
that you can travel on a spacelike orbit. Let us imagine that we can move
in this universe on a spacelike orbit, just to watch around us. The question
is: would we be able (even in principle) then to enter into another part of the
universe where the standard model is a different one and, if so, what would that
transition look like? Would we pass through a membrane, or would we pass
through the horizon of a black hole? What is it that separates these different
kinds of universes? In a picture Linde had these bubbles connected by little
throats, and I wonder if you would travel through such a throat what would
the transition be like? Is there a moment when you would say “Hey, now we
have 4 generations!”, or 25 generations, or something that changes?
A. Linde Let me answer the first question, “What I am going to see at the bound-
ary?” That depends on the boundary. Usually, you have boundaries as follows:
you have one minimum and another minimum and between the minima you
know that you must go up the hill. Typically then, these two minima are di-
vided by domain walls. The sizes of two sides are exponentially large. If these
domains are both de Sitter, you are in a hopeless situation, because if you will
be traveling here this part will be running away from you because of de Sitter
expansion. If you are sitting in Minkowski space, which is just by chance, then
you will have a possibility to travel here for some time, and after that you would
see the wall and if you are young and stupid still at that time, then you will
go through the wall and die because your particles will not exist in that part of
the universe.
G. ’t Hooft The question was not really whether one could travel there on a time-
like geodesic, but suppose - in our imagination - one goes over a space-like
geodesic, or a space-like orbit, or even back in time. At some point you should
see a transition when you go from one universe to the next. So if you go back
in time or on a space-like orbit, then, as you say, you would go through some
membrane or something. I want to know what that thing is like.
A. Linde Are you are asking about the process, for example, that happens if I am
sitting at this point, and quantum fluctuations just push me to some place?
S. Shenker I think that what ’t Hooft is asking is the following. Suppose you pick
some time variable, does not matter which. Look at a fixed-time space slice and
then go along. What happens when you jump from one bubble to the next?
A. Linde That is the thing that I answered, exactly. The image which I showed
you is the result of a computer simulation of one particular time slice, a space-
like hypersurface.
G. ’t Hooft I just want to have you pin down somehow: What would this mem-
brane look like? A membrane separating two different kinds of universes: would
