Signatures of Alternate Expansion Histories Between BBN and Recombination*

Popular extensions to the standard ΛCDM cosmology often introduce a new energy component to the early universe in an attempt to alleviate the Hubble tension or alter structure growth. One such example is the introduction of free-streaming dark radiation. We derive constraints on the injection of dark radiation after big bang nucleosynthesis (BBN) by considering the decay of a massive hidden sector particle into dark radiation. Pre-recombination decays are primarily restricted by observations of the cosmic microwave background (CMB) via their impact on the effective number of relativistic species. Meanwhile, long-lived decay scenarios in which the massive particle lifetime extends past recombination tend to decrease the late-time matter density inferred from the CMB and are thus subject to constraints from observations of baryon acoustic oscillations and type Ia supernovae. With limits on these decays being so stringent, neither short-lived nor long-lived scenarios are successful at substantially mitigating the Hubble tension. Another popular extension of ΛCDM that alters the pre-recombination expansion history is early dark energy. We demonstrate that the inclusion of a scalar field after BBN introduces features in the matter power spectrum that can enhance structure growth on certain scales while simultaneously suppressing growth on other scales.