# Why doesn't the anthropic principle select for N=2 SUSY compactifications with an exactly zero cosmological constant?

The party line of the anthropic camp goes something like this. There are at least $10^{500}$ flux compactifications breaking SUSY out there with all sorts of values for the cosmological constant. Life takes a lot of time to evolve, and this is incompatible with a universe which dilutes away into de Sitter space too soon. The cosmological constant has to be fine-tuned to the order of $10^{123}$. Without SUSY, the zero point energy contribution from bosons and fermions would not cancel naturally.

However, superstring theory also admits N=2 SUGRA compactifications which have to have an exactly zero cosmological constant. Surely some of them can support life? I know there are a lot more flux compactifications out there compared to hyperKahler compactifications, but does the ratio exceed $10^{123}$? What probability measure should we use over compactifications anyway? Trying to compute from eternal inflation leads to the measure problem.

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Maybe we are in an N=2 universe. arxiv.org/abs/hep-th/0109168 physics.stackexchange.com/questions/27421/… – Mitchell Porter Jul 23 '12 at 9:59
Eternal inflation is the wrong measure, you should use causal patch inflation--- what's the probability of producing a given string vacuum from a small deSitter patch. This is not going to be like the eternal inflation nonsense, since it doesn't have the outside volume. – Ron Maimon Jul 23 '12 at 19:55
Yes, but the real answer is that the Anthropic Principle is just a notion that some folks have. There is no real evidence for it at all. Don't forget, the fact that we exist to ask the question guarantees that this universe has parameters that allow life. – Paul J. Gans Dec 22 '12 at 2:33

## 1 Answer

N=2 compactifications are a lot more constrained than compactifications with no SUSY. For life to be likely to evolve, there needs to be more than just fine tuning of the cosmological constant. So many other parameters also need to be adjusted to maximize the chances of life evolving. There are a lot more metastable vacua with a wide range (landscape) of different parameters. On the other hand, the N=2 parameters aren't a priori maximized for the evolution of life.

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Note that since we already know that life evolved, we don't need parameters that make this likely; they only have to make it possible. We could live in a universe where life has a really low (but nonzero) chance to evolve but where we were lucky anyway. – Lagerbaer Dec 22 '12 at 16:43
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