Constraining Ultralight Dark Matter in Dynamical Dark Energy Cosmologies

Abstract

We investigate the cosmological implications of scalar-field dark matter (SFDM) in a dynamical dark energy framework, extending previous analyses that considered SFDM within the standard $Λ$CDM cosmological model. We consistently implement the scenario at both the background and linear perturbation levels. We perform Bayesian parameter inference using Planck CMB measurements, DESI DR2 baryon acoustic oscillation (BAO) data, and Pantheon+ Type Ia supernovae to constrain the model parameter space. Our theoretical predictions show that lighter scalar-field masses produce increasingly strong scale-dependent suppression of matter fluctuations and more pronounced modifications to the CMB temperature power spectrum, whereas heavier masses progressively approach the cold dark matter limit. For the SFDM mass, we obtain a one-sided 95\% lower limit of $\log_{10}(m_ϕ/{\rm eV})>-24.04$ from our main joint analysis. We find that replacing the standard cold dark matter fluid with SFDM does not qualitatively alter the constraints on dynamical dark energy or the evidence for its evolution, including a phantom crossing at low redshifts.