Interacting Dark Energy and Dark Matter in O(3) No-Scale Gravity
Dark Energy (DE) and Dark Matter (DM) are among the greatest mysteries in particle physics and cosmology, since their origins remain unclear. It is intriguing to consider whether both may originate from a purely gravitational sector. We propose that they arise from degrees of freedom associated with the partners of the Brans-Dicke boson in No-Scale Gravity, a fundamental scale-invariant gravitational theory in which the Planck scale is illusional. We consider the Brans-Dicke boson, together with two scalar fields, to form a vector multiplet under an $O(3)$ symmetry, whose angular directions in the Einstein frame are identified with DM and DE. Small explicit breaking of $O(3)$ generates their masses and interaction, and we show that the resulting model reproduces the required background cosmological evolution and present-day abundances, with the lighter field providing a dynamical DE component at late times. Although the DE field remains canonical, the interacting cosmology exhibits an effective phantom-like evolution at late times, of the type suggested by DESI in combination with other cosmological probes, without introducing a fundamental phantom degree of freedom. The model provides a first-principles realization of interacting DE, in which DM, DE, and their coupling emerge from the symmetry structure of the underlying gravitational theory.