Cosmology 239
view we have already answered the prettiest part of the problem, and now we
have this more difficult, chemical, condensed matter problem of reproducing
the nature of the world we see. I am not saying this is necessarily the case, but
it would be an equally valid analogy to the one that you are suggesting.
E. Rabinovici I want to point out that in field theories that are scale-invariant
and finite, the vacuum energy is the same if scale invariance is spontaneously
broken or not, and in particular it can be zero. I appreciate that you view this
spontaneous breaking of the scale invariance as fine-tuning, therefore it is not
on your list, but I beg to differ.
J. Polchinski I think that your selection principle is what I call the static solution
principle where you insist that the Lagrangian be such as to give you a static
solution. I think that is not a selection principle, and in particular, if you think
about it, it is like the dynamical solutions where to know that you would have
a static solution you would have to look at your system for all of time. It is
not something that can constrain what state the system is at the beginning of
time. So it really is the same problem as in other dynamical ideas. That also
applies to self-tuning which uses the same strategy.
V. Rubakov I wanted to make a comment on your argument against the dynam-
ical relaxation mechanisms. Actually the fact that the cosmological constant
is so small might tell us that the cosmological evolution is quite different from
what we think. This was called by Graham Ross “dkjj-vu Universe”, mean-
ing that there could be a state of the Universe some time in the past which
was very similar to the Universe we are now living in, and at that time one or
another relaxation mechanism worked. Or, even more, maybe there is strong
non-locality and some relaxation mechanism works at very large length scales,
while beyond our small part of the Universe, the Universe is just empty. Then
all this criticism will not work.
A. Polyakov A couple of comments, nothing to do with philosophy. There are
two mechanisms which are probably worth having in mind. They may work
eventually. First, as far as the strong CP problem is concerned, there is at
least one model in which the solution of the problem comes from the infrared
corrections and is completely analogous to the behavior of the theta-angle in the
quantum Hall effect. Namely, the behavior is that you take an arbitrary bare 0 of
the order of 1 and, as you go to the infrared, it tends to zero. Which means that
it actually predicts that if you go back to higher energy, the renormalization
group flow enhances the CP violation. I certainly do not have any realistic
model of field theory with this feature, but some highly non-trivial Yang-Mills
theories have it, so it is worth keeping in mind. Although it actually plays for
the anthropic principle, which I do not want.
The second thing is that there is also another model, I think, in which we can
at least see a tendency of the cosmological constant to be screened by the renor-
malization group flow also. And it is very natural, because the cosmological
