Recent studies have revealed that two-flavor u-d quark matter may serve as the true ground state of quantum chromodynamics (QCD). Meanwhile, a growing number of supermassive compact stars have been detected observationally, which brings severe challenges to explaining the internal matter composition of these compact objects. In this work, we systematically explore the parameter stability window of u-d quark matter within the enhanced perturbative QCD (EpQCD) model, and further investigate the structural properties of u-d quark stars under the framework of gravity’s rainbow theory.
By constructing a well-defined renormalization subtraction point, the EpQCD model successfully eliminates the thermodynamic inconsistency plaguing conventional perturbative QCD, and partially incorporates the global effect of quark confinement into the theoretical framework. As a quantum gravity candidate, gravity’s rainbow theory can effectively modify gravitational interactions inside compact stars via tuning rainbow function parameters, which provides a feasible theoretical scheme to account for extremely massive compact stars.
The left panel illustrates the stability window of u-d quark matter, while the right panel displays the mass-radius relation of u-d quark stars derived from gravity’s rainbow. The stability diagram indicates that the green stable parameter region is far narrower than the forbidden and phase-transition zones, yet absolute stable u-d quark matter is still theoretically achievable. Selecting representative parameters from the stable region, we compute the equilibrium structure of u-d quark stars and find that the maximum stellar mass rises monotonically with the rainbow function parameter Σ. Under the limit of standard general relativity (Ξ = Σ = 1), the maximum mass of a u-d quark star reaches 2.34 M_⊙. Notably, when Σ = 1.2, the peak mass climbs to approximately 2.8 M_⊙, consistent with the mass range of the compact companion in the GW190814 gravitational-wave event. Our results provide a novel perspective to interpret the intrinsic nature of ultra-massive compact stars, and advance our physical insights into the low-energy regime of quantum gravity.