Cosmology, Cluster Algebras, and $\boldsymbol{u}$

Abstract

We explore the cluster algebra structures present in the wavefunction and correlators of conformally coupled scalar fields in de Sitter space. To connect cosmological kinematics to cluster variables, we utilize the so-called $u$-variables that appear on both sides. Since $u$-variables parameterize a rigid space, it is natural to identify the ones that appear from cosmological graphs and those that arise from cluster algebras. This provides a mapping between kinematic variables and cluster variables. For $n$-site chain Feynman graphs, this gives a coordinate-invariant relation to $A_{2n-2}$-type cluster algebras, while in the case of $n$-site cycle graph momentum integrands, the relevant cluster algebra is $B_{2n-1}$, as found previously. Since kinematic variables are mapped to nonlinear combinations of cluster variables, cluster compatibility imposes strong conditions on possible symbol entries. We use this notion of compatibility to bootstrap the symbol. In the chain case, both the cosmological wavefunction and products of lower-point functions satisfy cluster compatibility, with the correlator a particular combination, so that the wavefunction and correlator share the same cluster structure. In the cycle case, however, the wavefunction appears to be uniquely selected by the cluster structure, and the correlator does not display the same compatibility as the wavefunction.