Late-Time Oscillating Quintessence in Light of DESI

Abstract

Recent DESI baryon acoustic oscillation measurements, especially when combined with Type Ia supernova and CMB data, sharpen the case for possible low-redshift dynamics in the dark energy sector. We study a simple and physically transparent realization of such dynamics: a quintessence field that is Hubble frozen for most of cosmic history and starts to oscillate around its minimum recently (at a redshift $z\approx 0.1$). This late onset of oscillations can occur in a broad class of models where the quintessence potentials have a shallow slope away from the minimum and steepen near it. This class of models can improve the fit relative to $\Lambda$CDM, with $\Delta\chi^2\simeq -9$, while remaining competitive with common phenomenological dark energy parameterizations with the same number of parameters. The preference is driven mainly by the background expansion history, and near the best-fit region the resonant growth of quintessence perturbations and the associated Integrated Sachs-Wolfe (ISW) contribution remain small. More precise low-redshift distance measurements, together with late-time probes such as the ISW effect and lensing, may help distinguish this oscillating quintessence scenario from other forms of late-time dark energy dynamics.