Dark energy that switches sign (IMAGE)
Caption
Dark energy that switches sign. The effective dark energy density, in units of today's critical density, in four cosmic histories, each calibrated to the cosmic microwave background. In the standard model (grey) dark energy is constant, and in the commonly studied branch of the exponential f(T) model (orange) it departs from that only modestly. In the previously overlooked branch of the same model, examined in the new study (blue), dark energy is negative in the early Universe, crosses zero about 9.5 billion years ago (redshift ≈ 1.5) and has been positive ever since. Extended with a cosmological constant and tuned to the locally measured expansion rate (dashed blue), the model's dark energy peaks and then declines, so that cosmic acceleration halts about 1 billion years ago. Triangles mark where cosmic acceleration begins; times follow the standard model's clock.
Credit
Istanbul Technical University, adapted from Figs. 5 and 8 of Akarsu et al., Phys. Rev. D 112, 083532 (2025)
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