# Sourced Scalar Fluctuations in Bouncing Cosmology

## TL;DR

This paper investigates sourced scalar and tensor fluctuations in bouncing cosmology, showing they are nearly scale-invariant and chiral, with potential detectability by gravitational wave observatories, but inconsistent with current CMB data.

## Contribution

It provides a detailed calculation of sourced fluctuations in bouncing cosmology, including second-order curvature perturbations and their spectra, highlighting the effects of gauge field coupling.

## Key findings

- Spectra are nearly scale-invariant and slightly blue.
- Tensor-to-scalar ratio is approximately 1/9.
- Model predictions are incompatible with current CMB observations but may be detectable by LIGO/LISA.

## Abstract

We calculate the scalar power spectrum generated by sourced fluctuations due to coupling between the scalar field, which holds most of the energy density of the universe, and a gauge field for a general FLRW metric. For this purpose we calculate the curvature perturbation to second order in the presence of gauge fields, and show that the gauge fields behave like an additional potential term. We then apply the analysis to the case of slow-contraction. Due to the interaction between the scalar field and gauge fields additional 'sourced' tensor and scalar spectra are generated. The resulting spectra are chiral, slightly blue and arbitrarily close to scale invariance. The only difference between the tensor and scalar spectra is the coupling constant with an ${\mathcal O}(1)$ numerical coefficient, and some momentum space polarization vectors. As a result the tilt of the spectra are the same. For the nearly scale invariant case, the momentum integration gives the same leading contribution. Hence, $r\simeq 1/9$ where the deviation from this value is controlled by the deviation from scale invariance, and is not in agreement with CMB observations. Deviating considerably from near scale invariance, and considering a bluer tilt with $n_T>0.12$, the model cannot account for CMB observations, but can be detected by LIGO and/or LISA in the future.