The Quantum Structure of Space and Time (293 pages)

266 The Quantum Structure of Space and Time

through periods of contraction, it stretches, there is the collision, it stretches

more and collides again. So far as that field is concerned, it is essentially seeing

an expanding or static background. To say it in a more concrete way, in a

4-dimensional theory what happens is that the field is not just coupled to the

scale factor, but also couples to the radion field, and the radion field during the

contraction phase exactly compensates for the effect of expansion, so there is

no effect, no excitation of the scalar field. Geometrically, this is because these

fields are living on the brane which is not contracting.

T. Banks You will have to show me.

P. Steinhardt I wanted to issue a challenge to those pursuing the anthropic prin-

ciple. We have now measured a number of important parameters about the uni-

verse that are important for the existence of life, but we are about to determine

more parameters that are important for life: the shape of the power spectrum,

for example, which is usually characterized by a spectral index, though it is

possible that the spectrum will actually have some bumps and wiggles in it.

We are also going to learn something about the reionization epoch. I would like

those who are pursuing the anthropic principle to give us a definite prediction

before the measurements are made. What are your anthropic expectations?

Hopefully, you will converge on a single answer. It does not good if every pro-

ponent gives me a different answer. I suspect you do not have an answer at all,

but I think it is an appropriate challenge for you to come up with one before

the measurements are made so we can see if the anthropic principle has any real

utility, and there is a chance for another success story to add to the semi-success

of the cosmological constant.

S. Weinberg I think it is an unfair challenge, because we do not know which

parameters scan. We do not know which parameters vary from multiverse to

multiverse in a smooth way. If you tell us which ones do, we migh tell you what

value to expect.

P. Steinhardt Then give me a table of possibilities, depending on your assump-

tions about which parameters scan.

N. Seiberg I am surprised that I am the only speaker, besides Kallosh, who men-

tioned the letters "LHC" at this conference. I want to pose the question: how

do you think the LHC will change the scene of our field? What fraction of

the people will continue studying the kind of physics that we have been dis-

cussing here: string theory and cosmology? What fraction will move to more

phenomenology-related topics? And also, what kind of impact can we expect

from LHC on more fundamental physics, shorter-distance physics?

R. Kallosh My understanding is that if LHC will tell us that there are super-

symmetric particles, it would mean that we have this fermionic dimension in

space and time, and it will be supergravity instead of gravity. So I would think

that we would all tend to consider cosmology in the framework of supergravity

and string theory. If they will not see supersymmetry immediately it will not