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The potential energy from a time-dependent scalar field provides a possibleexplanation for the observed cosmic acceleration. In this paper, we investigatehow the redshift vs brightness data from the recent Pantheon+ survey of type Iasupernovae constrain the possible evolution of a single scalar field for theperiod of time (roughly half the age of the universe) over which supernova dataare available. Taking a linear approximation to the potential, we find thatmodels providing a good fit to the data typically have a decreasing potentialenergy at present (accounting for over 99% of the allowed parameter space) witha significant variation in scalar potential ($\langle {\rm Range}(V)/V_0\rangle \approx 0.97$) over the period of time corresponding to the availabledata ($z < 2.3$). Including quadratic terms in the potential, the data can befit well for a wide range of possible potentials including those with positiveor negative $V_2$ of large magnitude, and models where the universe has alreadystopped accelerating. We describe a few degeneracies and approximatedegeneracies in the model that help explain the somewhat surprising range ofallowed potentials.

Possible hints of decreasing dark energy from supernova data