As a potential candidate for the late-time accelerating expansion of the
Universe, the Chaplygin gas and its generalized models have significant
implications to modern cosmology. In this work we investigate the effects of
dark energy on the internal structure of a neutron star composed of two phases,
which leads us to wonder: Do stable neutron stars have a dark-energy core? To
address this question, we focus on the radial stability of stellar
configurations composed by a dark-energy core — described by a Chaplygin-type
equation of state (EoS) — and an ordinary-matter external layer which is
described by a polytropic EoS. We examine the impact of the rate of energy
densities at the phase-splitting surface, defined as $alpha= rho_{rm
dis}^-/rho_{rm dis}^+$, on the radius, total gravitational mass and
oscillation spectrum. The resulting mass-radius diagrams are notably different
from dark energy stars without a common-matter crust. Specifically, it is found
that both the mass and the radius of the maximum-mass configuration decrease as
$alpha$ becomes smaller. Furthermore, our theoretical predictions for
mass-radius relations consistently describe the observational measurements of
different massive millisecond pulsars as well as the central compact object
within the supernova remnant HESS J1731-347. The analysis of the normal
oscillation modes reveals that there are two regions of instability on the
$M(rho_c)$ curve when $alpha$ is small enough indicating that the usual
stability criterion $dM/drho_c>0$ still holds for rapid phase transitions.
However, this is no longer true for the case of slow transitions.
Based on the analysis conducted in this work, the conclusion is reached that stable neutron stars can indeed have a dark-energy core. The study focuses on the stability of neutron star configurations composed of a dark-energy core described by a Chaplygin-type equation of state and an ordinary-matter external layer described by a polytropic equation of state.
The study establishes that the rate of energy densities at the phase-splitting surface, represented by the parameter $alpha$, has a significant impact on the properties of these neutron stars. Specifically, it is observed that as $alpha$ becomes smaller, both the mass and radius of the maximum-mass configuration decrease.
The theoretical predictions obtained in this study are found to be consistent with observational measurements of different massive millisecond pulsars and the central compact object within the supernova remnant HESS J1731-347.
Furthermore, the analysis of normal oscillation modes reveals that there are two regions of instability on the mass-density curve when $alpha$ is small enough, indicating that the usual stability criterion holds for rapid phase transitions. However, this criterion no longer holds for slow transitions.
Roadmap for the Future
Challenges
- Further refinement and validation of the theoretical framework used to describe neutron stars with a dark-energy core.
- Investigation of additional properties and effects of dark energy on neutron star internal structures.
- Gaining a deeper understanding of the physical processes that govern phase transitions between dark-energy and ordinary-matter phases.
- Exploring the implications of dark-energy cores in neutron stars on other astrophysical phenomena and observations.
Opportunities
- Expanding the study to consider different equations of state for both dark energy and ordinary matter.
- Investigating the possibility of observable signatures of dark-energy cores in neutron star mergers and gravitational wave detections.
- Collaborating with observational astronomers to compare theoretical predictions with new and existing data.
- Exploring the potential connection between dark-energy cores in neutron stars and the accelerating expansion of the Universe.
In conclusion, the study presented here provides valuable insights into the existence and properties of neutron stars with dark-energy cores. The road ahead involves addressing the challenges outlined above and capitalizing on the opportunities to further advance our understanding of these unique cosmic objects.