Sunday, August 09, 2009

Life on moduli space?

New paper! Although I'm skeptical about the utility of the anthropic principle (see previous discussion on this blog), I couldn't resist pondering a basic question raised by the large number of string vacua, the vast majority of which seem to be supersymmetric.

Is there something special about exact supersymmetry that precludes complex life? It's a simple question, but difficult to answer.

Life on moduli space?

While the number of landscape vacua in string theory is vast, the number of supermoduli vacua which lead to distinct low energy physics is even larger, perhaps infinitely so. From the anthropic perspective it is therefore important to understand whether complex life is possible on moduli space -- i.e., in low energy effective theories with 1. exact supersymmetry and 2. some massless multiplets (moduli). Unless life is essentially impossible on moduli space as a consequence of these characteristics, anthropic reasoning in string theory suggests that the overwhelming majority of sentient beings would observe 1-2. We investigate whether 1 and 2 are by themselves automatically inimical to life and conclude, tentatively, that they are not. In particular, we describe moduli scenarios in which complex life seems possible.

From the paper:

Assuming our current understanding of string theory is correct, the number of distinct vacua with unbroken supersymmetry and exact low-energy moduli (supermoduli) is infinitely larger even than the vast number of string landscape vacua in which supersymmetry is broken and the cosmological constant nonzero \cite{BDG,DK}. For example, in Calabi-Yau compactifications, the continuous parameters determining the shape of the compact space are themselves moduli and result in an infinite set of physically distinct vacua. Indeed, the highly supersymmetric vacua may be on stronger theoretical footing than their non-supersymmetric counterparts \cite{BDG}.

If complex life is possible on even a tiny fraction of points on supermoduli space, it would be difficult to understand, within an anthropic framework, why we do not ourselves observe unbroken supersymmetry and massless moduli fields.

There is thus ample motivation to investigate whether complex life can exist on moduli space -- specifically, in low energy effective theories with 1. exact supersymmetry and 2. some massless multiplets (moduli).

... Below we list some minimal requirements for complex life. In fact, we do not know whether any of these conditions are necessary or sufficient for life, although it seems they are more likely to be necessary (especially B.) than sufficient. These requirements primarily place constraints on low energy physics. As we discuss below, they do not seem to exclude moduli vacua, at least not in any obvious way.

A. structure formation

B. deviation from thermal equilibrium (long lived sources of free energy)

C. stable matter, complex chemistry

Because inflationary dynamics are typically determined by high energy physics, it seems reasonable to assume that the specific properties of any inflationary epoch (including the spectrum of density perturbations) are independent of the low energy properties of a particular vacuum. Therefore, the requirement of an inflationary epoch neither favors nor disfavors properties 1-2. ...

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