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The Cosmological Bootstrap: Weight-Shifting Operators and Scalar Seeds (1910.14051v2)

Published 30 Oct 2019 in hep-th, astro-ph.CO, gr-qc, and hep-ph

Abstract: A key insight of the bootstrap approach to cosmological correlations is the fact that all correlators of slow-roll inflation can be reduced to a unique building block---the four-point function of conformally coupled scalars, arising from the exchange of a massive scalar. Correlators corresponding to the exchange of particles with spin are then obtained by applying a spin-raising operator to the scalar-exchange solution. Similarly, the correlators of massless external fields can be derived by acting with a suitable weight-raising operator. In this paper, we present a systematic and highly streamlined derivation of these operators (and their generalizations) using tools of conformal field theory. Our results greatly simplify the theoretical foundations of the cosmological bootstrap program.

Citations (164)

Summary

The Cosmological Bootstrap: Weight-Shifting Operators and Scalar Seeds

This paper focuses on advancing the cosmological bootstrap approach, drawing on principles analogous to the S-matrix bootstrap used in particle physics. It explores how all correlators related to slow-roll inflation can be traced back to a fundamental building block: the four-point function of conformally coupled scalars mediated by a massive scalar exchange. The authors aim to provide a streamlined derivation of spin-raising and weight-shifting operators that transform this singular scalar correlator into all other desired correlators, thereby simplifying the cosmological bootstrap framework.

Key Insights

  1. Bootstrap Philosophy: The central idea in bootstrap methods is to construct correlation functions directly in terms of symmetry principles and singularity constraints rather than through explicit bulk dynamics. Inflationary models enable the application of this approach, which is aligned with the symmetries of de Sitter space.
  2. Four-Point Function: Starting from the four-point function of conformally coupled scalars, the paper showcases how solutions for particle exchanges with spin can be obtained by utilizing spin-raising operators. Similarly, solutions for massless external fields are derived using weight-shifting operators.
  3. Weight-Shifting Operators: The derivation of the necessary differential operators harnesses conformal field theory tools, specifically embedding space techniques. These operators effectively shift the weights of external fields and spins of exchanged fields to generate new correlators, facilitating the computation of more complex inflationary correlators with massless fields.
  4. Inflationary Correlators: By applying these weight-shifting operators to perturbed dimensions of exchange solutions and taking the soft momentum limit, the paper develops a general framework to compute inflationary three-point functions. This includes describing bispectra sourced by massive fields with arbitrary spin.
  5. Partially Massless Fields: The paper introduces intriguing considerations about partially massless fields, demonstrating how their presence in the spectrum may be identified through distinct patterns in inflationary bispectra. These fields have unique characteristics and may contribute nontrivial Coulomb-like potentials despite not having propagating longitudinal modes.

Results and Implications

This paper provides practical tools for handling complex inflationary correlation functions and offers clear methodologies to generate spinning and massless solutions from scalar seeds. The implications of this research are multifaceted:

  • Practical Tools: The refined operators enable calculations that were previously cumbersome, significantly easing the exploration of high-spin exchanges in cosmological models.
  • Spectroscopy of Massive Fields: The ability to explore the angular patterns and non-analytic features in inflationary bispectra gives theoretical tools for the paper of the universe's early massive fields, potentially uncovering new physics tied to primordial non-Gaussianities.
  • Theoretical Foundations: This work supports the alignment of de Sitter and conformal symmetries in inflationary cosmology, reinforcing the theoretical basis of using conformal techniques in momentum space.

Future Directions

Further research could delve into higher-point functions involving massless fields and conserved tensors using this expanded operator framework. The potential applications of this methodology extend to refining other areas, such as conformal field theory expansions and improving understanding of symmetry constraints in complex inflationary scenarios. Additionally, exploring possible interactions and consistent theories of partially massless fields remains an open area of inquiry, which could lead to new insights into the field theory in curved spacetimes.