Dark energy stars with a phantom field

Abstract

Dark energy is the constituent with an enormous abundance in the present Universe, responsible for the Universe's accelerated expansion. Therefore, it is plausible that dark energy may interact within any compact astrophysical object. The author in [S. S. Yazadjiev, Phys. Rev. D 83, 127501 (2011)] constructs an exact star solution consisting of ordinary matter and a phantom field from a constant density star (CDS) known as the Schwarzschild interior solution. The star denotes a dark energy star (DES). The author claims that the phantom field represents dark energy within the star. So far, the role of the phantom field as dark energy in a DES is not systematically studied yet. Related to this issue, we analyze the energy condition of a DES. We expect that DESs shall violate the strong energy condition (SEC) for a particular condition. We discover that a SEC is fully violated only when the compactness reaches the Buchdahl limit. Furthermore, we also investigate the causal conditions and stabilities due to the convective motion and gravitational cracking. We also find that those conditions are violated. These results indicate that DES is not physically stable. However, we may consider DES as an ultracompact object of which we can calculate the gravitational wave echo time and echo frequency and compare them to those of CDS. We find that the contribution of the phantom field delays the gravitational wave echoes. The effective potential of the perturbed DES is also studied. The potential also enjoys a potential well like CDS but with a deeper well. We also investigate the possibility that DES could form a gravastar when $C=1$. It is found that a gravastar produced from DES possesses no singularity with a dS-like phase as the interior. These results could open more opportunities for the observational study of dark energy in the near future, mostly from the compact astrophysical objects.