About

Stuart Louis Shapiro is a Professor Emeritus and Research Professor in the departments of Physics and Astronomy. His research expertise encompasses a broad range of topics in astrophysics and general relativity, with a particular focus on black hole physics, neutron stars, gravitational waves, and the equations of state governing dense matter. He has contributed extensively to the understanding of phenomena such as binary neutron star mergers, magnetic fields in astrophysical contexts, and the dynamics of massive stars and black holes. Shapiro's work also includes studies on the growth of intermediate-mass black holes, the formation of black holes in mass gaps, and the dynamical evolution of star clusters and dwarf galaxies using conduction fluid simulations. His research output is substantial, with numerous peer-reviewed articles and contributions to leading journals in physics and astrophysics. Recognized for his significant contributions to the field, Shapiro is a Fellow of the American Physical Society and a recipient of the Hans A. Bethe Prize, highlighting his impact in nuclear astrophysics and related areas.

Research topics

• Physics
• Astrophysics
• Quantum mechanics
• Theoretical physics
• Geometry
• Classical mechanics

Selected publications

• Gravitational Wave Memory from Binary Neutron Star Mergers

  Physical Review Letters · 2025-12-11 · 1 citations

  articleOpen access

  The gravitational wave signal produced by the merger of two compact objects includes both an oscillatory transient and a nonoscillatory part, the so-called memory effect. This produces a permanent displacement of test masses and has not yet been measured. We use general relativistic magnetohydrodynamic simulations, including neutrinos, with several representative viable equations of state, to quantify-for the first time-the effects of the neutron star magnetic field, neutrino emission, and the ejected mass on the linear and nonlinear displacement memory in binary neutron star mergers. We find that the additional contributions due to the emission of electromagnetic radiation, neutrinos and baryonic ejecta can be ∼15% of the total memory for moderate magnetic fields and up to ∼50% for extreme magnetic fields. The memory is most affected by changes in the equation of state, the binary mass, and the magnetic field. In particular, for moderate premerger field strengths, the dominant impact of the electromagnetic field is the change in the gravitational wave luminosity, and the associated gravitational wave null memory, due to the unstable growth of the magnetic field and the resulting redistribution of angular momentum it induces in the remnant. While the direct electromagnetic contribution to the null memory is additive, the change in the gravitational wave null memory can-in some cases-result in the total memory being smaller than that from the corresponding nonmagnetized binary. Furthermore, in contrast to binary black hole mergers, the growth of the memory in binary neutron star mergers is extended due to the long emission timescale of electromagnetic fields, neutrinos, and ejecta. These results necessitate the consideration of the magnetic field, as well as the equation of state, for accurate parameter estimation in future analyses of gravitational wave memory data.

  PublisherOA PDFDOI

• Can Premature Collapse Form Black Holes in the Upper and Lower Mass Gaps?

  Physical Review Letters · 2025-10-22 · 3 citations

  articleSenior author

  Observations of gravitational waves from binary black hole mergers, including the recent signals GW231123 and GW230529, have revealed multiple progenitor black holes in the so-called upper and lower mass gaps, respectively. It is generally assumed that massive stars cannot form black holes in the upper mass gap because pair instabilities in the late stage of stellar evolution disrupt the stars, whereas the lower mass gap refers to the gap between the maximum allowed neutron-star mass and the smallest black hole mass expected to form in supernova explosions. Here we explore a premature collapse scenario in which upper mass gap stars collapse and form black holes before they reach the late stage of stellar evolution. The mechanism for triggering a premature collapse is the capture of a smaller black hole, possibly primordial in nature. A similar capture scenario can occur to produce black holes in the lower mass gap. At least for massive stars, typical stellar rotation rates would likely result in rapidly rotating black holes in such a scenario, naturally explaining the rapid spins inferred from GW231123. Even though our estimates hinge on several parameters with rather large uncertainties, they suggest that, at least in galactic disks, the likelihood of such a capture is small for stars in the upper mass gap, but may lead to a significant population of black holes in the lower mass gap and, in fact, even below the lower mass gap.

  PublisherDOI

• Extracting the Temperature Analytically in Hydrodynamics Simulations with Gas and Radiation Pressure

  The Astrophysical Journal · 2025-03-26

  articleOpen accessSenior author

  Abstract Numerical hydrodynamics simulations of gases dominated by ideal, nondegenerate matter pressure and thermal radiation pressure in equilibrium entail finding the temperature as part of the evolution. Since the temperature is not typically a variable that is evolved independently, it must be extracted from the evolved variables (e.g., the rest-mass density and specific internal energy). This extraction requires solving a quartic equation, which, in many applications, is done numerically using an iterative root-finding method. Here we show instead how the equation can be solved analytically and provide explicit expressions for the solution. We also derive Taylor expansions in limiting regimes and discuss the respective advantages and disadvantages of the iterative versus analytic approaches to solving the quartic.

  PublisherDOI

• Boosting the growth of intermediate-mass black holes: Collisions with massive stars

  Physical review. D/Physical review. D. · 2025-03-14 · 6 citations

  articleSenior author

  We perform fully relativistic simulations of the head-on collisions between intermediate-mass black holes and very massive stars. Such collisions are expected to occur in dense stellar clusters and may play an important role in growing the mass of the seed black hole. For the cases considered here, for which the masses of the black holes and stars are comparable, the vast majority of the stellar material is accreted onto the black hole within a stellar dynamical timescale, as expected from analytical estimates, and leads to a rapid growth of the black hole. A small amount of mass, which is shock-heated in the wake of the black hole, is ejected from the collision and will contribute to the interstellar material in the cluster.

  PublisherDOI

• Dynamical evolutions in globular clusters and dwarf galaxies: Conduction fluid simulations

  Physical review. D/Physical review. D. · 2025-06-27 · 2 citations

  articleOpen accessSenior author

  We present a new two-fluid conduction scheme to simulate the evolution of an isolated, self-gravitating, equilibrium cluster of stars and collisionless dark matter on secular (gravothermal) timescales. We integrate the equations in Lagrangian coordinates via a second-order, semi-implicit algorithm, which is unconditionally stable when the mass of the lighter species is much less than that of the heavier species. The method can be straightforwardly generalized to handle a multispecies system with a population of stars or components beyond collisionless dark matter and stars. We apply the method to simulate the dynamical evolution of stellar-dark matter systems, exploring the consequences of mass segregation and gravothermal core collapse, and assessing those effects for observed globular clusters and dwarf galaxies in the Local Volume. © 2025 American Physical Society.

  PublisherOA PDFDOI

• Impact of Magnetic Field Topology on Electromagnetic and Gravitational Waves from Binary Neutron Star Merger Remnants

  ePrints Soton (University of Southampton) · 2025-10-20

  preprintOpen accessSenior author

  We perform general relativistic magnetohydrodynamic (GRMHD) simulations of binary neutron star (BNS) mergers with four distinct magnetic field topologies: (i) a dipole pulsar-like configuration, (ii) a mixed linear superposition of poloidal and toroidal components inside the star, and (iii-iv) two topologies featuring a smooth transition from a confined mixed core to a pulsar-like structure at radii $0.95\,R_{\rm NS}$ and $0.5\,R_{\rm NS}$, with $R_{\rm NS}$ the radius of the star. The latter topologies are explored in BNS merger studies for the first time. We evolve systems with two equations of state (EoS), SLy and WFF1, with ADM masses 2.7 and 2.6, respectively, and include an additional lower-mass SLy binary to probe the behavior of long-lived remnants. We perform an extensive analysis of the emission properties of the systems, both electromagnetic and gravitational waves, and of the properties of the remnants, namely their frequency modes, density eigenfunctions, rotation, temperature, and convective stability. We report three key results: (1) for the first time, we assess the convective stability of magnetized remnants, extending previous unmagnetized analyses; (2) we identify a clear secondary peak in the gravitational-wave spectrum of pulsar-like configurations, consistent with the nonlinear coupling of the $m=0$ and $m=2$ modes, which is absent in other topologies; and (3) the magnetic field topology strongly influences the gravitational wave emission properties to the extent that nearby ($<50\,{\rm Mpc}$) events could allow one to observationally distinguish between different field structures with future gravitational-wave detectors. Across all models, we obtain luminosities compatible with short gamma-ray bursts (sGRBs), with purely poloidal configurations being the most efficient in driving possible relativistic jets.

  OA PDFDOI

• Extracting the Temperature Analytically In Hydrodynamics Simulations with Gas and Radiation Pressure

  ArXiv.org · 2025-01-30

  preprintOpen accessSenior author

  Numerical hydrodynamics simulations of gases dominated by ideal, nondegenerate matter pressure and thermal radiation pressure in equilibrium entail finding the temperature as part of the evolution. Since the temperature is not typically a variable that is evolved independently, it must be extracted from the the evolved variables (e.g. the rest-mass density and specific internal energy). This extraction requires solving a quartic equation, which, in many applications, is done numerically using an iterative root-finding method. Here we show instead how the equation can be solved analytically and provide explicit expressions for the solution. We also derive Taylor expansions in limiting regimes and discuss the respective advantages and disadvantages of the iterative versus analytic approaches to solving the quartic.

  PublisherOA PDFDOI

• Boosting the growth of intermediate-mass black holes: collisions with massive stars

  ArXiv.org · 2025-02-20

  preprintOpen accessSenior author

  We perform fully relativistic simulations of the head-on collisions between intermediate-mass black holes and very massive stars. Such collisions are expected to occur in dense stellar clusters and may play an important role in growing the mass of the seed black hole. For the cases considered here, for which the masses of the black holes and stars are comparable, the vast majority of the stellar material is accreted onto the black hole within a stellar dynamical timescale, as expected from analytical estimates, and leads to a rapid growth of the black hole. A small amount of mass, which is shock-heated in the wake of the black hole, is ejected from the collision and will contribute to the interstellar material in the cluster.

  PublisherOA PDFDOI

• Postmerger multimessenger analysis of binary neutron stars: Effect of the magnetic field strength and topology

  Physical review. D/Physical review. D. · 2025-02-12 · 10 citations

  articleSenior author

  The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz postmerger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasiuniversal relations for the key oscillation frequencies, but most of them omit the effects of a magnetic field. We conduct full general-relativistic magnetohydrodynamics simulations of NSNS mergers with two different masses and two different EOSs (SLy and ALF2) with three different initial magnetic field topologies (poloidal and toroidal only, confined to the interior, and ``pulsarlike'': dipolar poloidal extending from the interior to the exterior), with four different magnetic field strengths with maximum values ranging from $5.5\ifmmode\times\else\texttimes\fi{}{10}^{15}$ to $2.2\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{G}$ at the time of insertion. We find that magnetic braking and magnetic effective turbulent viscosity drives the merger remnants towards uniform rotation and increases their overall angular momentum loss. As a result, the ${f}_{2}$ frequency of the dominant postmerger GW mode shifts upwards over time. The overall shift is up to $\ensuremath{\sim}200\text{ }\text{ }\mathrm{Hz}$ for the strongest magnetic field we consider and $\ensuremath{\sim}50\text{ }\text{ }\mathrm{Hz}$ for the median case and is therefore detectable in principle by future GW observatories, which should include the magnetic field in their analyses. We also explore the impact of the magnetic field on the postmerger electromagnetic emission, and demonstrate that an extremely large magnetic field, or alternatively a significant shear viscosity mechanism, can cause a supramassive NS remnant to collapse to a black hole in less than 100 ms and lead to jet formation, although we do not expect the conditions for such an outcome to be realistic.

  PublisherDOI

• Masking the Equation-of-State Effects in Binary Neutron Star Mergers

  Physical Review Letters · 2025-03-27 · 8 citations

  articleSenior author

  Recent nonmagnetized studies of binary neutron star mergers have indicated the possibility of identifying equation-of-state features, such as a phase transition or a quark-hadron crossover, based on the frequency shift of the main peak in the postmerger gravitational wave spectrum. By performing a series of general relativistic, magnetohydrodynamic simulations we show that similar frequency shifts can be obtained due to the effect of the magnetic field. The existing degeneracy can either mask or nullify a shift due to a specific equation-of-state feature, and therefore the interpretation of observational data is more complicated than previously thought, requiring a more complete treatment that would necessarily include the neutron star's magnetic field.

  PublisherDOI

Recent grants

• Theoretical Studies in Gravitation and Astrophysics

  NSF · $441k · 2004–2008

• Theoretical Studies in Gravitation and Astrophysics

  NSF · $423k · 2017–2021

• Theoretical Studies in Gravitation and Astrophysics

  NSF · $600k · 2013–2017

• Theoretical Studies in Gravitation and Astrophysics

  NSF · $510k · 2020–2025

• Theoretical Studies in Gravitation and Astrophysics

  NSF · $585k · 2007–2011

Frequent coauthors

• Thomas W. Baumgarte

  149 shared

• Saul A. Teukolsky

  63 shared

• Milton Ruiz

  61 shared

• Vasileios Paschalidis

  University of Arizona

  54 shared

• Z. B. Etienne

  West Virginia University

  43 shared

• Masaru Shibata

  42 shared

• Yuk Tung Liu

  University of Illinois Urbana-Champaign

  42 shared

• Antonios Tsokaros

  Academy of Athens

  41 shared

Labs

• Compact ObjectsPI