Title: A Boostless Bootstrap and Emergent Lorentz Symmetry
Scattering amplitudes are very cumbersome to compute with traditional methods, especially when spinning particles are in the spectrum. However, the bootstrap approach, that bypasses Lagrangians and gauge redundancies, has made the computation of S-matrix elements far more efficient and illuminating. The idea of the bootstrap approach is to work directly with observables and constrain their form using fundamental physical principles such as symmetries, locality and unitarity. In this talk, we will introduce a “boostless bootstrap” where we allow interactions between massless particles to break Lorentz boosts and bootstrap the corresponding amplitudes. We will give a pedagogical introduction to “on-shell methods” and will show that unitarity demands that Lorentz boost symmetry and the equivalence principle emerge when a spin-2 particle is included in the spectrum. We will explain that our motivation for considering such a set-up is the cosmological bootstrap programme where cosmological correlators are bootstrapped without employing the traditional in-in formalism. Our long-term goal is to bootstrap inflationary correlators, which are the focus of many upcoming cosmology missions, and we will explain why first bootstrapping amplitudes without Lorentz boosts is a sensible starting point. Along the way, we will discuss how our methods will be adapted for cosmology.