Scalar perturbations from inflation in the presence of gauge fields (2404.19694v3)

Abstract: We study how Abelian-gauge-field production during inflation affects scalar perturbations in the case when the gauge field interacts with the inflaton directly (by means of generic kinetic and axial couplings) and via gravity. The homogeneous background solution is defined by self-consistently taking into account the backreaction of the gauge field on the evolution of the inflaton and the scale factor. For the perturbations on top of this background, all possible scalar contributions coming from the inflaton, the metric, and the gauge field are considered. We derive a second-order differential equation for the curvature perturbation, $\zeta$, capturing the impact of the gauge field, both on the background dynamics and on the evolution of scalar perturbations. The latter is described by a source term in the $\zeta$ equation, which is quadratic in the gauge-field operators and leads to non-Gaussianities in the curvature perturbations. We derive general expressions for the induced scalar power spectrum and bispectrum. Finally, we apply our formalism to the well-known case of axion inflation without backreaction. Numerical results show that, in this example, the effect of including metric perturbations is small for values of the gauge-field production parameter $\xi> 3$. This is in agreement with the previous results in the literature. However, in the region of smaller values, $\xi\lesssim 2$, our new results exhibit order-of-unity deviations when compared to previous results.

• The paper introduces a formalism that integrates gauge fields with the inflaton, resulting in a second-order differential equation for curvature perturbations.
• It reveals that gauge field interactions lead to non-Gaussian features, modifying the scalar power spectrum and bispectrum critical for inflationary model constraints.
• Numerical simulations support the theoretical predictions, demonstrating measurable impacts on CMB and large-scale structure observables based on gauge field parameters.

Overview of "Scalar perturbations from inflation in the presence of gauge fields"

The paper by R. Durrer, et al., explores the impact of gauge field production during inflation on scalar perturbations. The authors derive a comprehensive formalism to account for how these gauge fields, interacting through kinetic and axial couplings, influence the curvature perturbations observed in the cosmic microwave background (CMB) and large-scale structures (LSS) in the Universe.

Key Contributions

1. Incorporation of Gauge Fields in Inflationary Perturbations: The paper investigates Abelian gauge fields integrated with the inflaton field, considering both kinetic and axial couplings. The backreaction of these gauge fields on inflationary dynamics and scalar perturbations is systematically considered for consistent treatment across various models.
2. Analytical Formulation and New Insights: The authors provide an analytical formulation resulting in a second-order differential equation for the curvature perturbation, ζ, which incorporates the gauge field's effects. This equation signals the emergence of non-Gaussianities through a source term quadratic in the gauge-field operators, adding a layer of complexity to standard inflationary predictions.
3. Power Spectrum and Bispectrum Derivations: The paper derives expressions for the scalar power spectrum and bispectrum as influenced by gauge field production, offering new insights into the shape and evolution of primordial fluctuations. These expressions are pivotal for constraining inflation models against observational data and enhance comprehension of PNG from inflation.
4. Numerical and Theoretical Implications: Numerical simulations validate the theory, showing alteration in perturbative spectra dependent on the parameters chosen for gauge field production. For instance, deviations from previously accepted results are highlighted for gauge-field production parameters, ξ, especially for smaller values.

Implications and Speculations

The inclusion of gauge fields in inflationary frameworks enhances the complexity of cosmological perturbations and potentially leads to signatures observable in the CMB and LSS data. As experiments refine the measurement of non-Gaussianities, these results could significantly narrow down viable inflation models. Future investigations will likely iterate upon these foundational results to incorporate more nuanced backreaction effects and potential interactions in multifield inflationary models.

Future Directions

The research discusses potential avenues for future exploration, particularly in models with strong backreaction where inflaton dynamics and modulations exhibit non-linear behavior. These scenarios might present additional observable features, such as oscillatory patterns in the scalar power spectra, relevant for upcoming probes into the early Universe.

The meticulous formalism presented lays vital groundwork upon which more intricate scenarios and additional particles can be introduced. This research represents a step forward in bridging microscale inflaton interactions and their macroscale cosmic imprints, fostering a richer understanding of the dynamics during inflation.