Cosmological constraints on Galileon dark energy with broken shift symmetry

Current cosmological data seem to show that dark energy is evolving in time and that it possibly crossed the phantom divide in the past. So far the only theories that lead to such a behavior involve a nontrivial coupling between dark energy, in the form of a scalar field, and the gravitational or matter sector. We show that there is another possibility involving both a nontrivial kinetic sector in a cubic Galileon theory and a scalar field potential that breaks the Galileon shift symmetry, which can lead to a similar phenomenology on large scales. We perform a full Bayesian analysis using the latest cosmological data, including DESI DR2 baryonic acoustic oscillation measurements, type Ia SNe measurements from DESY5, Union3, and Pantheon+, and cosmic microwave background data from Planck and ACT. We find that it is statistically strongly favored over a universe dominated by a cosmological constant (with a Bayes factor of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>log</a:mi> <a:mi>B</a:mi> <a:mo>≃</a:mo> <a:mn>6.5</a:mn> </a:math> ). Yet, as with other nonminimally coupled theories, it has severe ancillary gravitational effects. These can be mitigated to some extent, but as with other viable theories, the penalty is ever more elaborate scalar field models of dark energy.