Ultracompact anisotropic stars and gravastars in general relativity
Abstract
We study static, spherically symmetric ultracompact objects in General Relativity with anisotropic stress. Using the covariant anisotropic equation of state and assuming homogeneous density, we construct a mostly analytical model admitting configurations beyond the Buchdahl limit and with compactness arbitrarily close to the black hole value. The solutions exhibit two regimes separated by a critical curve in the parameter space of compactness and anisotropy, associated with the divergence of the central pressure: regular anisotropic configurations with positive central pressure, and singular configurations with negative central pressure for arbitrary anisotropy. In the latter case, we show that introducing a thick shell removes the pressure divergence, yielding regular ultracompact gravastar configurations. We further construct both anisotropic and isotropic gravastar models using this thick-shell construction.
Versions
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BibTeX
@misc{puttasiddappa2026ultracompact,
title = {{Ultracompact anisotropic stars and gravastars in general relativity}},
author = {Prajwal Hassan Puttasiddappa and Nora Bretón and Santiago Esteban Perez Bergliaffa},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.10991},
eprint = {2608.10991},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.10991},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Ultracompact anisotropic stars and gravastars in general relativity
AU - Prajwal Hassan Puttasiddappa
AU - Nora Bretón
AU - Santiago Esteban Perez Bergliaffa
PY - 2026
DA - 2026/10/02
PB - arXiv
DO - 10.48550/arxiv.2608.10991
UR - https://arxiv.org/abs/2608.10991
AB - We study static, spherically symmetric ultracompact objects in General Relativity with anisotropic stress. Using the covariant anisotropic equation of state and assuming homogeneous density, we construct a mostly analytical model admitting configurations beyond the Buchdahl limit and with compactness arbitrarily close to the black hole value. The solutions exhibit two regimes separated by a critical curve in the parameter space of compactness and anisotropy, associated with the divergence of the central pressure: regular anisotropic configurations with positive central pressure, and singular configurations with negative central pressure for arbitrary anisotropy. In the latter case, we show that introducing a thick shell removes the pressure divergence, yielding regular ultracompact gravastar configurations. We further construct both anisotropic and isotropic gravastar models using this thick-shell construction.
ER -