Dark radiation from particle decay: cosmological constraints and opportunities

Abstract

We study particle decay as the origin of dark radiation. After elaborating general properties and useful parametrisations we provide model-independent and easy-to-use constraints from nucleosynthesis, the cosmic microwave background and structure formation. Bounds on branching ratios and mass hierarchies depend in a unique way on the time of decay. We demonstrate their power to exclude well-motivated scenarios taking the example of the lightest ordinary sparticle decaying into the gravitino. We point out signatures and opportunities in cosmological observations and structure formation. For example, if there are two dark decay modes, dark radiation and the observed dark matter with adjustable free-streaming can originate from the same decaying particle, solving small-scale problems of structure formation. Hot dark matter mimicking a neutrino mass scale as deduced from cosmological observations can arise and possibly be distinguished after a discovery. Our results can be used as a guideline for model building.

41 pages + refs, including 14 figures; v2: references added, typos corrected, results and conclusions unchanged; contains minor improvements in presentation compared to published version

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• Is the CMB telling us that dark matter is weaker than weakly interacting?
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• Dark Photon as Fractional Cosmic Neutrino Masquerader
• Neutrino Self-Interactions
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• Reconciling Cosmological Tensions with Inelastic Dark Matter and Dark Radiation in a $\mathbf{U}(1{)}_{\mathbf{D}}$ Framework
• Shining Light on Cosmogenic Axions with Neutrino Experiments
• Forecasting Constraints on Non-Thermal Light Massive Relics from Future CMB Experiments (CMB-S4/Simons Observatory)
• Searching for dark radiation at the LHC