article · The Astrophysical Journal
Theoretical astrophysics research explores how alternative gravitational frameworks describe extremely dense celestial bodies. By applying Rastall gravity, which features a nonminimal coupling between matter and geometry, researchers modelled the structure of massive compact stars under a conformal limit on the speed of sound. This upper bound restricts sound speed to the speed of light divided by the square root of three in environments where core densities exceed nuclear saturation levels. Applying this condition alongside Krori-Barua potentials and an anisotropic fluid model allowed the estimation of the radius of PSR J0952-0607, the most massive observed pulsar. For an observed mass of 2.35 solar masses, the calculated radius is approximately 14.09 kilometres. Linking this framework to the MIT bag model yielded mass-radius relationships matching observational pulsar data within a 68 percent confidence level.
Understanding how matter behaves under extreme gravitational conditions helps physicists test the boundaries of fundamental physical laws. Pulsars provide natural laboratories containing matter at densities far beyond anything reproducible in terrestrial laboratories. Showing that modified gravity models successfully reproduce observational measurements helps clarify the behaviour of ultra-dense matter and refines general models of the universe.
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Abstract It has been shown that the nonminimal coupling between geometry and matter can provide models for massive compact stars that are consistent with the conformal bound on the sound speed, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0</mml:mn> <mml:mo>≤</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>≤</mml:mo> <mml:msup> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>3</mml:mn> </mml:math> , where the core density approaches a few times the nuclear saturation density. We impose the conformal upper bound on the sound speed on Rastall’s field equations of gravity, with Krori–Barua potentials in the presence of an anisotropic fluid as a matter source, to estimate the radius of the most massive pulsar ever observed, PSR J0952–0607. For its measured mass M = 2.35 ± 0.17 M ⊙ , we obtain a radius R = 14.087 ± 1.0186 km as inferred by the model. We investigate a possible connection between Rastall gravity and the MIT bag model with an equation of state, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>r</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:mi>ρ</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≈</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mfenced close=")" open="("> <mml:mrow> <mml:mi>ρ</mml:mi> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mfenced> </mml:math> , in the radial direction, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mi>c</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msqrt> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msqrt> </mml:math> and a surface density ρ s slightly above the nuclear saturation density ρ nuc = 2.7 × 10 14 g cm –3 . The corresponding mass–radius diagram is in agreement with our estimated value of the radius and with astrophysical observations of other pulsars at 68% confidence level.
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DOI: 10.3847/1538-4357/acd93c
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