Predicting the cosmological constant with the scale-factor cutoff measure

Abstract

It is well known that anthropic selection from a landscape with a flat prior distribution of cosmological constant Λ gives a reasonable fit to observation. However, a realistic model of the multiverse has a physical volume that diverges with time, and the predicted distribution of Λ depends on how the spacetime volume is regulated. We study a simple model of the multiverse with probabilities regulated by a scale-factor cutoff, and calculate the resulting distribution, considering both positive and negative values of Λ. The results are in good agreement with observation. In particular, the scalefactor cutoff strongly suppresses the probability for values of Λ that are more than about ten times the observed value. We also discuss several qualitative features of the scale-factor cutoff, including aspects of the distributions of the curvature parameter Ω and the primordial density contrast Q.

Tree-like structure of eternal inflation: A solvable model

“…Here is a partial list: [31][26] [22]. Each proposal is based on a preferred time variable which is used to regulate the infinities implicit in an eternally inflating multiverse.…”

“…Each proposal is based on a preferred time variable which is used to regulate the infinities implicit in an eternally inflating multiverse. For example [31] uses scale-factor time which is defined in such a way that the volume of a comoving region grows exponentially. Reference [22] uses light-cone time defined so that the number of causal patches grows exponentially.…”

“…Different proposals [31][26] [22] differ in the details of the measure at small values of Hτ obs where all kinds of details are involved, but they all agree on (7.1) for large H. There is one feature of the light-cone proposal which is particularly intriguing, namely, it is precisely equivalent to the causal patch measure which is defined in terms of local observations within a single causal patch [21] [13].…”

“…The tree model is not well suited to discussing the interior of terminals, and in more detailed work on the issue [24,31,26,22] their importance has depended on the detailed choice of measure. For simplicity we will just assume that observers do not exist in terminals and accordingly we prune the tree.…”

Scanning the Fluxless G_2 Landscape

“…The key ingredient in generating such a landscape is the presence of discrete fluxes, coming from both open and closed string sectors, needed to freeze some or all compactification moduli [2], [3], [4], [5], [6]. At present, the landscape paradigm presents the only known solution [7], [8] to the cosmological constant problem [11]. Since string theory has no continuous adjustable parameters, the only possibility to implement the fine tuning of the cosmological constant is via a discrete scan over the integer flux data.…”