# Generation of Isocurvature from Curvature Inhomogeneities on Super-Horizon Scales

**Authors:** Albert Stebbins  
**arXiv:** [2311.17379](https://arxiv.org/abs/2311.17379)  
**Date:** 2023-11-29  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2311.17379v1/

## Key points

- Isocurvature modes are variations in the ratio of different components
- Isocurvature modes can be produced and are proportional to curvature inhomogeneities
- Mechanism allowing for differing yields of dark matter in causally disconnected regions is required
- Events in widely separated regions of the universe share correlated information due to fluctuations in cosmological flow caused by curvature inhomogeneities
- Fluctuations deviate from expected value and lead to non-zero covariance between expansion rates at causally disconnected points
- Isocurvature modes can have effects on scales larger than the horizon (super-horizon covariance)
- Uncertainties in baryonic physics result in a power spectrum, with limits beyond what dark matter freeze-in models can achieve
- Overlap with an independent study conducted by Holst Hu & Jenks (2023)
- Importance of understanding isocurvature modes and their implications for cosmology, particularly regarding dark matter freeze-in models.

## Summary

This paper explores the generation of isocurvature inhomogeneities on super-horizon scales from curvature inhomogeneities. The authors demonstrate that these isocurvature modes, which refer to variations in the ratio of different components (e.g., dark matter to photons), can be produced and are proportional to the curvature inhomogeneities. The key requirement for this process is a mechanism that allows for differing yields of dark matter in causally disconnected regions of the universe. The paper highlights that while events occurring in widely separated regions of the universe are causally disconnected, they still share correlated information. This correlation arises due to fluctuations in the rate of expansion (θ) of cosmological flow caused by curvature inhomogeneities. These fluctuations deviate from the expected Einstein-deSitter value and lead to a non-zero covariance between expansion rates at causally disconnected points. This covariance manifests as a super-horizon covariance, indicating that isocurvature modes can have effects on scales larger than the horizon. The authors also discuss the power spectrum resulting from uncertainties in baryonic physics and note that precise limits from small-scale clustering and large-scale cosmic microwave background observations go beyond what can be achieved with dark matter freeze-in models. It should be noted that some aspects of this work overlap with an independent study conducted by Holst Hu & Jenks (2023). In summary, this paper provides insights into how isocurvature inhomogeneities on super-horizon scales are generated from correlated curvature inhomogeneities and emphasizes the importance of understanding these isocurvature modes and their implications for cosmology, particularly regarding dark matter freeze-in models.