The four-dimensional null energy condition as a swampland conjecture

TL;DR

This paper proposes a new swampland conjecture that the four-dimensional null energy condition is necessary for a consistent M-theory cosmological background, supported by a top-down derivation and analysis of effective descriptions.

Contribution

It introduces the 4d NEC as a swampland conjecture, linking 4d cosmological consistency to M-theory completion, with explicit derivation from a top-down approach.

Findings

4d NEC must be obeyed for M-theory effective descriptions (except negatively curved cases)

The 4d NEC is proposed as a swampland consistency condition

Discussion of cosmological implications of the 4d NEC in M-theory context

Abstract

We analyze four-dimensional Friedmann-Lemaitre-Robertson-Walker cosmologies in type IIB, arising from a M-theory dual, and find that the null energy condition (NEC) has to be obeyed by them (except for the negatively curved case) in order for the M-theory action to have a Wilsonian effective description. However, this does not imply that the M-theory metric has to obey the 11d NEC. Thus, we propose a new swampland conjecture -- the 4d NEC is a consistency condition for any theory to have a completion within M-theory -- with an explicit derivation of it for cosmological backgrounds from a top-down perspective. We briefly discuss the cosmological consequences of such a condition derived from M-theory.