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# Multi-Stream Inflation: Bifurcations and Recombinations in the Multiverse 

Physics Department, McGill University, Montreal, H3A2T8, Canada 

## Yi Wang ∗ 

In this Letter, we briefly review the multi-stream inflation scenario, and discuss its implications in the string theory landscape and the inflationary multiverse. In multi-stream inflation, the inflation trajectory encounters bifurcations. If these bifurcations are in the observable stage of inflation, then interesting observational effects can take place, such as domain fences, non-Gaussianities, features and asymmetries in the CMB. On the other hand, if the bifurcation takes place in the eternal stage of inflation, it provides an alternative creation mechanism of bubbles universes in eternal inflation, as well as a mechanism to locally terminate eternal inflation, which reduces the measure of eternal inflation. 

Inflation [1] has become the leading paradigm for the very early universe. However, the detailed mechanism for inflation still remains unknown. Inspired by the pic- ture of string theory landscape [2], one could expect that the inflationary potential has very complicated structure [3]. Inflation in the string theory landscape has impor- tant implications in both observable stage of inflation and eternal inflation. 

The complicated inflationary potentials in the string theory landscape open up a great number of interest- ing observational effects during observable inflation. Re- searches investigating the complicated structure of the in- flationary potential include multi-stream inflation [4, 5], quasi-single field inflation [6], meandering inflation [7], old curvaton [8], etc. 

The string theory landscape also provides a playground for eternal inflation. Eternal inflation is an very early stage of inflation, during which the universe reproduces itself, so that inflation becomes eternal to the future. Eternal inflation, if indeed happened (for counter ar- guments see, for example [9]), can populate the string theory landscape, providing an explanation for the cos- mological constant problem in our bubble universe by anthropic arguments. 

In this Letter, we shall focus on the multi-stream infla- tion scenario. Multi-stream inflation is proposed in [4]. And in [5], it is pointed out that the bifurcations can lead to multiverse. Multi-stream inflation assumes that during inflation there exist bifurcation(s) in the inflation trajectory. For example, the bifurcations take place nat- urally in a random potential, as illustrated in Fig. 1. We briefly review multi-stream inflation in Section II. The details of some contents in Section II can be found in [4]. We discuss some new implications of multi-stream inflation for the inflationary multiverse in Section III. 

## I. INTRODUCTION 

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In this section, we discuss the possibility that the bi- furcation of multi-stream inflation happens during the observable stage of inflation. We review the production of non-Gaussianities, features and asymmetries [4] in the 

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## II. OBSERVABLE BIFURCATIONS 

