Axionlike scalar fields can induce temporary deviations from the standard expansion history of the Universe. The scalar field’s contribution to the energy density of the Universe grows while the field is held constant by Hubble friction, but when the scalar field starts to evolve, its energy density decreases faster than the radiation density for some potentials. We explore the observational signatures of such a scalar field that becomes dynamical between big bang nucleosynthesis and matter-radiation equality, which we call very early dark energy (vEDE). If vEDE momentarily dominates the energy density of the Universe, it generates a distinctive feature in the matter power spectrum that includes a bump on scales that enter the horizon just after the scalar field starts to evolve. For $k\gtrsim 10h{\mathrm{Mpc}}^{-1}$, the amplitude of this bump can exceed the amplitude of the standard matter spectrum. The power on scales on either side of this peak is suppressed relative to the standard power spectrum, but only scales that are within the horizon while the scalar field makes a significant contribution to the total energy density are affected. We determine how vEDE scenarios are constrained by observations of the cosmic microwave background, measurements of the primordial deuterium abundance, and probes of the late-time expansion history. We find that these observations are consistent with vEDE scenarios that enhance power on scales $k\gtrsim 30h{\mathrm{Mpc}}^{-1}$ and nearly double the amplitude of the matter power spectrum around $200h{\mathrm{Mpc}}^{-1}$. These scenarios also suppress power on scales between 0.3 and $30h{\mathrm{Mpc}}^{-1}$.

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• Cosmological parameters
• Evolution of the Universe
• Large scale structure of the Universe