Gravastars, a concept first proposed by scientists Mazur, and Mottola in their research study, are becoming a topic of interest for astrophysicists all over the world.1 Compact objects like black holes are formed when stars tend to collapse at the end of their life cycle. Gravastars are regarded as alternatives to conventional black holes, and scientists have recently made some ground-breaking discoveries.
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A Brief Introduction to Gravastars
A gravastar, just like a black hole, consists of an extremely dense core. It is made up of three regions. The core or inner region consists of densely packed dark energy. Ultra-relativistic stiff fluid discovered and introduced by Zeldovich, surrounds the dense dark energy core, to form the second layer.
This stiff fluid thin layer is also sometimes referred to as the shell of a gravastar. Beyond the shell, the exterior of the gravastar constitutes another layer, described using different models depending on the circumstances.2
Research studies have been carried out to study primordial black holes, the compact objects formed at the early stages of the universe. Around 4 years ago, the detection of gravitational waves by the LIGO Scientific, and Virgo Collaborations, has confirmed the existence of primordial gravastars, just like primordial black holes. Researchers have found that a gravitational vacuum star or gravastar expels gravitational waves, which contain waveform with primary signals identical to that of black holes.3
In short, a gravastar is essentially a de Sitter space bubble, primarily representing a space of negative black energy. This unique model, based on quantum energy fluctuations explaining the development of an extremely thin finite shell around the core, turns out to be accurate with the idea of a constantly expanding universe.
A Conceptual Comparison between Gravastars and Black Holes
With the applications of general relativity in astrophysics, black holes became the center of attention. However, many scientists considered the existence of an event horizon - a boundary not allowing even light to escape - and the presence of gravitational singularity very strange. In this regard, the gravastar configuration model is considered a more realistic model, as it is not characterized by the presence of an event horizon at the core.
Initially, it was believed that black holes and gravastars were the same, as both these high-density objects cast undetectable shadows. However, the confirmation of the absence of an event horizon is instrumental in differentiating them. Furthermore, studying oscillation modes of black holes and gravastars with quasi-normal frequency mode detection also reveals that even if the gravitational mass is identical, both these cosmological bodies emit different quasi-normal spectrums. Researchers have also found differences in the decay rate of gravastars, and black holes.
The literature also points out a major finding: if the mass of a star is above 3 solar masses (M☉), and less than 64 M☉, it collapses to form black holes. The gravastar cosmological model doesn’t follow this mass range restriction. Additionally, another major difference between the two compact objects is the proof of gravastars being much more thermodynamically stable than black holes.4
What are Nested Gravastars?
Although gravastars prove to be the solution to various concerns raised with general relativity and black holes, researchers have recently developed a refined model allowing the nesting of 2 gravastars into each other, opening up new avenues in astrophysics.
The nesting of gravastars leads to a much thicker shell region in comparison to the conventional thin-shell gravastar. This model is much more realistic, and in this model, the pressure considered for numerical computations was anisotropic, in comparison to the isotropic pressure model for conventional gravastars.
The researchers also mentioned the concept of a nestar, which is a multi-layered dense body made up of 2 nested gravastars. Conventional stars can’t be expected to follow this concept, as the hydrostatic equilibrium created during the nesting process has only been confirmed numerically for gravastars.
The equilibrium achieved by the layering of nested gravastars enables the development of models that consider the formation of nestars with an even greater number of nested gravitational stars, and the multi-layered nested gravastars could prove to be an accurate solution to unexplained black hole limitations.5
Modified Gravastar Model, and Implications for Cosmology
The gravitational vacuum stars (gravastars) model integrates the expanded Bose-Einstein condensation phenomena effects, highlighting the phase transitions taking place in the core (de Sitter core), balancing the forces to prevent the formation of an event horizon. Although this answers various questions, researchers are still trying to integrate modified theories instead of general relativity, particularly to explain cosmological processes such as the expansion of the universe.
The study of astrophysical processes points out the continuous expansion of the universe, with negative pressure, explained by quintessence dark energy. Furthermore, string theory hints at the existence of 1-D strings as the basic building block of nature. A modified gravastar model with the integration of clouds of string, and quintessence dark energy has recently been devised when researchers computed modified Einstein field equations to study the interplay between string theory, dark matter, gravastars, and the processes taking place in the universe.
Even the modified equation of states, and the study of modified energy states also reveal that no singularity is present at the core of compact objects. Furthermore, the calculations reveal that the energy density and pressure of the gravastar system didn’t change, which is in line with the properties of dark matter.
The clouds of strings enhanced the stability parameters of the gravastars, while the stability region decreased with an increase in the quintessence field parameter. Furthermore, the thickness of the shell also affects the total energy, as an increase in thickness leads to an increase in energy and in entropy. The modified model of gravastars is proof of the expanding universe and explains the processes occurring at the core of celestial bodies while incorporating the string theory, and dark matter principles.6
What Does the Future Hold?
In the future, we can expect greater advancements in the domain of gravastars. Although the concept of the nested gravastar model has been discussed recently, researchers still need to study the effects of disturbances on the nestar model and determine the quasi-normal mode spectrum. A detailed study involving the analysis of perturbations will be key in understanding the existence of gravastars, and provide a much more detailed platform to distinguish them more clearly from black holes.
Furthermore, observatory instruments such as the Event Horizon Telescope, and GRAVITY+ instruments are becoming more precise, and their resolution is increasing significantly, allowing for more accurate data collection and enabling the imaging of distant and far-off astronomical objects. This should help to validate the gravastar model and differentiate between different types of astronomical objects while studying astrophysical phenomena.
The nestars model and incorporation of string theory provide a dynamic and thorough framework for using the gravastars model as a tool for astronomical study.
Further Reading
- Mazur P. et. al. (2023). Gravitational Condensate Stars: An Alternative to Black Holes. Universe. 9(2). 88. Available at: https://doi.org/10.3390/universe9020088
- Ray, S. et. al. (2020). Gravastar: An alternative to black hole. International Journal of Modern Physics D, 29(05), 2030004. Available at: https://www.doi.org/10.1142/S0218271820300049
- Wang, Y. et. al. (2019). Primordial gravastar from inflation. Physics Letters B, 795, 314-318. Available at: https://doi.org/10.1016/j.physletb.2019.06.036
- Antoniou, I. (2022), Version 3. Black hole or Gravastar? The GW190521 case. arXiv preprint arXiv:2010.05354. Parana J. Sci. Educ., 8(8). 14-18. Available at: https://doi.org/10.48550/arXiv.2010.05354
- Jampolski, D., & Rezzolla, L. (2024). Nested solutions of gravitational condensate stars. Classical and Quantum Gravity, 41(6), 065014. Available at: https://www.doi.org/10.1088/1361-6382/ad2317
- Javed, F. et. al. (2024). Novel gravastar solutions: Investigating stability, energy, and entropy in the presence of cloud of strings and quintessence. Chinese Journal of Physics, 88, 786-798. Available at: https://doi.org/10.1016/j.cjph.2024.02.033
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