Monodromic dark energy and DESI

Recent baryon acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) Collaboration have renewed interest in dynamical dark energy models, particularly those that cross the “phantom divide” ( w DE = − 1 ). We present leading observational constraints on , a physically motivated scalar field dark energy scenario capable of realizing rapid oscillations about the phantom divide. Using cosmic microwave background (CMB) information, DESI DR2 BAO measurements, and type Ia supernova observations, we constrain the amplitude, frequency, phase, and power-law index describing the monodromic k-essence scenario at the background level. We find that the monodromic dark energy scenario can fit these datasets with a χ 2 that is comparable to the phenomenological w 0 − w a parametrization. While the CMB and BAO data alone are consistent with the standard Λ CDM model, the inclusion of DESY5 supernovae shows a preference for a nonzero amplitude, A = 0.4 4 − 0.12 + 0.16 (fully marginalized 68% CL). Conversely, inclusion of the Pantheon-Plus supernovae provides no evidence for monodromic k-essence, with A < 0.43 (95% CL). We show that constraints on both monodromic dark energy and w 0 − w a models are sensitive to the DESI DR2 LRG2 BAO distance, especially in the absence of supernova data.