On the importance of heavy fields during inflation (1201.4848v3)

Abstract: We study the dynamics of two-field models of inflation characterized by a hierarchy of masses between curvature and isocurvature modes. When the hierarchy is large, a low energy effective field theory (EFT) exists in which only curvature modes participate in the dynamics of perturbations. In this EFT heavy fields continue to have a significant role in the low energy dynamics, as their interaction with curvature modes reduces their speed of sound whenever the multi-field trajectory is subject to a sharp turn in target space. Here we analyze under which general conditions this EFT remains a reliable description for the linear evolution of curvature modes. We find that the main condition consists on demanding that the rate of change of the turn's angular velocity stays suppressed with respect to the masses of heavy modes. This adiabaticity condition allows the EFT to accurately describe a large variety of situations in which the multi-field trajectory is subject to sharp turns. To test this, we analyze several models with turns and show that, indeed, the power spectra obtained for both the original two-field theory and its single-field EFT are identical when the adiabaticity condition is satisfied. In particular, when turns are sharp and sudden, they are found to generate large features in the power spectrum, accurately reproduced by the EFT.

On the Importance of Heavy Fields During Inflation

The paper, "On the Importance of Heavy Fields During Inflation," authored by SebastiEspedes, Vicente Atal, and Gonzalo A. Palma, addresses the dynamics of inflation within the framework of two-field models that exhibit a significant mass hierarchy between curvature and isocurvature modes. The investigation highlights the critical role heavy fields play in the low-energy regime of effective field theories (EFT) during inflation, despite being integrated out.

Summary of Key Findings

The authors explore the conditions under which a low-energy EFT can reliably describe the inflationary dynamics when curvature and isocurvature modes have a substantial mass separation. The primary condition for the EFT's validity revolves around the concept of adiabaticity, which demands that the rate of change of the angular velocity characterizing the 'turn' of the inflationary trajectory remains suppressed compared to the masses of the heavy modes. This condition is vital to avoid exciting the heavy degrees of freedom, which would result in deviations from the effective single-field dynamics.

Numerical Analysis and Models

The paper provides a numerical analysis conducted on various two-field inflationary models. It demonstrates that when the adiabaticity condition is satisfied, the power spectra derived from the original two-field and corresponding single-field EFT are identical. This finding holds particular importance for cases involving sudden, sharp turns in the trajectory, where the interaction of heavy fields can cause noticeable perturbations observable in the primordial power spectrum, such as large features.

Implications and Future Research

The work emphasizes the subtleties involved in dissecting multi-field inflation theories, particularly in scenarios where heavy fields dynamically impact observable quantities like the power spectrum. The insights gained from analyzing the effects of 'turning' trajectories within these models can influence our understanding of the primordial universe and assist in the development of more accurate models for predicting cosmic microwave background (CMB) anisotropies.

From a theoretical perspective, the findings advocate for a re-evaluation of the conditions under which heavy fields can be consistently integrated out without losing significant dynamic contributions. The paper invites further exploration into the influence of UV degrees of freedom on inflationary dynamics, with particular attention to refining the conditions where EFT descriptions remain robust in the presence of sharp turns.

Conclusion

The paper advances our comprehension of multi-field inflation, particularly concerning the conditions that allow for a coherent low-energy EFT when dealing with isocurvature modes. By delineating the constraints necessary to maintain the adiabaticity of inflationary dynamics in the face of interacting heavy fields, the paper provides a groundwork for further investigations into such phenomena. Future research could extend these considerations to higher-order interactions, thereby offering a more complete picture of the role of heavy fields in the early universe.