Scalar Field Dark Energy in Light of DESI
Will Wolf
Graduate Student
University of Oxford
I present an assessment of the current observational status of single scalar field models of dark energy. Motivated by recent cosmological measurements -- including baryon acoustic oscillations, Type Ia supernovae, and CMB data -- I examine both the evidence from this data that appears to be hinting at dynamical evolution in the dark energy equation of state and whether a scalar field offers a viable explanation for the accelerated expansion of the Universe. Working within an effective field theory (EFT) framework, I argue that cosmological observations are fundamentally limited and can at most constrain a small number of parameters that govern scalar field dynamics. I argue that quintessence remains only marginally distinguishable from a cosmological constant, Λ, and that more general EFT extensions exhibit modest statistical preference, though such evidence is sensitive to data set selection and prior assumptions. These extended models generically predict fifth forces and modifications to the growth of structure, raising challenges from astrophysical constraints. I compare their predictions with current growth rate measurements, Integrated Sachs-Wolfe (ISW) effect and Solar System constraints. I argue that current ISW and growth data remain largely in agreement and do not yet offer strong discriminatory power between most scalar field constructs, while screening mechanisms remain theoretically non-trivial and difficult to assess observationally. Looking ahead to Stage IV surveys we forecast improvements in constraints on the dark energy behaviour; although there will be some tightening of bounds, I argue that the problem of underdetermination will persist. I conclude that while single scalar field dark energy remains a natural and flexible framework, its ultimate viability will hinge on improved low-redshift growth measurements and a clearer understanding of gravitational screening.