Phantom crossing or dark interaction?

Abstract

Recent results from DESI BAO measurements, together with Planck CMB and Pantheon+ data, suggest that there may be a `phantom' phase ($w_{\mathrm{de}}<-1$) in the expansion of the Universe. This inference follows when the $w_0,w_a$ parametrization for the dark energy equation of state $w_{\mathrm{de}}$ is used to fit the data. Since phantom dark energy in general relativity is unphysical, we investigate the possibility that the phantom behaviour is not intrinsic, but effective -- due to a non-gravitational interaction between dark matter and non-phantom dark energy. To this end, we assume a physically motivated thawing quintessence-like form of the intrinsic dark energy equation of state $w_{\mathrm{de}}$. Then we use a $w_0,w_a$ model for the \emph{effective} equation of state of dark energy. We find that the data favours a phantom crossing for the effective dark energy, but only at low significance. The intrinsic equation of state of dark energy is non-phantom, without imposing any non-phantom priors. A nonzero interaction is favoured at more than $3\sigma$ at $z\sim 0.3$. The energy flows from dark matter to dark energy at early times and reverses at later times.

16 pages, 5 figures, 5 tables

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