Speaker
Description
In this talk, we introduce Asymptotically Cubic Galileon gravity: a luminal Horndeski theory designed to remain close to the cubic Galileon over most of cosmic history, whilst allowing late-time departures that are be broadly consistent with current observations including the recent DESI results. These departures are generated by promoting the coefficients of the Galileon terms to functions of the scalar field. We show that, once this explicit field dependence is introduced, the closed theory space is fully characterised, modulo scalar-field redefinitions, by two invariant functions. Working in a convenient field parametrisation, we identify the functional behaviour these invariants must exhibit in order to lift the theory away from the cubic Galileon’s eternally phantom regime and allow a crossing of the phantom divide. We then use the ISW effect and the modified force in cosmic voids to identify phenomenologically viable regions of the theory space, before confronting these regions with DESI, Planck, and supernova data through an MCMC analysis. For the models studied, the data-favoured regions can overlap the phenomenologically viable sector, although the void modified-force condition remains the limiting constraint.