Effective curvature coupling of link-based causal set propagators in $1+1$ dimensions

Abstract

We study how a recently introduced causal-set propagator defined in terms of links responds to curvature. On sprinklings embedded in a flat 1+1-dimensional Minkowski space, this propagator is known to reproduce the massless retarded continuum Greens function on large scales. In conformally flat embeddings with constant curvature $\mathcal R$, the causal order is unchanged, but the link relationship is sensitive to the physical volume of the spanned causal diamond. We show that this volume dependence generates a leading curvature correction to the propagator. On large distances, this correction is equivalent to a continuum scalar propagator with an effective curvature coupling of the form $\xi \mathcal R$ with the coupling constant $\xi_{\rm eff}=1/6$, equivalently corresponding to an effective mass-squared parameter $m_{\rm eff}^2=\xi_{\rm eff}\mathcal R$. This coupling is not inserted by hand but emerges from the link-based path sum itself, reflecting the microscopic structure of the causal set. Numerical simulations on sprinklings embedded in $\mathrm{AdS}_{1+1}$ and $\mathrm{dS}_{1+1}$ support the predicted curvature response and indicate that the effective coupling persists as the sprinkling density is increased.