arXiv subjects

Thomas W. Baumgarte

Publications and source records attributed to Thomas W. Baumgarte.

At least 19 recordsLinked to original sources

arXiv2026

Accretion onto a moving black hole for stiff equations of state

We revisit accretion of an asymptotically uniform fluid onto a moving Schwarzschild black hole, the relativistic analog of Bondi-Hoyle-Lyttleton accretion. Expanding on previous treatments, we focus on relativistic accretion of stiff fluids with adiabatic indices $Γ\geq 5/3$, which is particularly relevant for the treatment of primordial black holes captured by neutron stars. We perform numerical simulations to systematically explore the dependence of the steady-state accretion and drag rates on the asymptotic sound speed $a_\infty$ and relative speed $v_\infty$, and identify different regimes, both subsonic and supersonic, in which these rates either increase or decrease with respect to the spherical accretion rate for $v_\infty = 0$. We propose simple analytical approximations that capture the qualitative features observed in the dependence of the accretion and drag rates on $a_\infty$ and $v_\infty$.

gr-qc↗

arXiv2026

Critical collapse of vacuum spacetimes: Nakamura wave initial data

We report on numerical simulations of critical phenomena in the collapse of axisymmetric vacuum gravitational waves, adopting families of initial data that, to the best of our knowledge, have not been used in this context before. Like Teukolsky waves, the data are based on linear wave solutions to the Einstein equations. We follow Nakamura's construction and encode the wave content in the extrinsic curvature rather than the spatial curvature, which leads to several simplifications when the data are "dressed up" so that they satisfy the nonlinear constraint equations. We are able to fine-tune these data to the onset of black hole formation slightly better than in our previous simulations, allowing us to observe and examine one more echo in the approximately self-similar threshold solution. Our findings are consistent with earlier studies: while we find threshold solutions that are approximately discretely self-similar, the self-similarity is not exact, and we find no evidence for a unique critical solution. We discuss common features between the different threshold solutions, including the appearance of alternating maxima in the direction of the poles and the equator.

gr-qc↗

arXiv2026

Comparing twist-free axisymmetric gravitational waves near the black hole threshold

The threshold of black hole formation in axisymmetric vacuum gravity is proving to be more complicated than had been anticipated but, following recent advances, a consensus between independent codes and methods is emerging. Building on earlier work we provide further details of a comparison between three independent numerical codes (the bamps, prague, and sphGR codes), paying special attention to the relative strengths and weaknesses of each and examining various features of near-threshold collapse of vacuum gravitational waves for the first time. In particular, we observe quasi-universal strong-field features appearing in curvature scalars. Focusing on geometric features on the symmetry axis, we construct reference coordinates to aid the comparison of strong-field data. We evolve, for the first time, time-asymmetric wave initial data within the bamps code. To the extent possible with current methods we compare apparent horizons and attempt to determine what causes difficulties in the classification of these strong-field and highly dynamical spacetimes. In all cases the results from the three codes agree very well.

gr-qc↗

arXiv2026

Starting inflation in asymptotically flat spacetimes

A key question in early universe cosmology is whether inflation can be successfully seeded by a generic, localised fluctuation in the inflaton field that probes the inflationary part of the potential. Past simulations have mainly considered periodic spacetimes representing either a closed universe of a specific size or a typical patch of a larger one, and as a result have needed to impose restrictive conditions on the extrinsic and intrinsic curvatures, which are arguably not generic. In this work we consider initial fluctuations in asymptotically flat spacetimes, allowing more general profiles of the intrinsic and extrinsic curvature. Our findings confirm and generalise the result of Goldwirth and Piran that a fluctuation with proper size several times the inflationary scale $(Gρ_{\rm infl})^{-1/2}$ is required for successful inflation. We also discuss inherent restrictions on the initial data, and how imposing a periodic length close to the inflationary scale may bias results.

gr-qc↗

arXiv2026

Primordial Black Holes in a Radiation-Dominated Universe

Primordial fluctuations frozen out during inflation re-enter the cosmological horizon and can collapse, leading to the formation of primordial black holes. We perform simulations of the direct collapse of over-dense regions re-entering the horizon during a radiation-dominated epoch, using full 3+1 general relativistic simulations with the BSSN formalism. Building on previous studies, we impose periodic boundary conditions and allow the matter content of the Universe to self-consistently drive its dynamics. We analyze the evolution of over-densities in both the collapse and dispersal regimes and find a threshold, $0.77<δ_c<0.83$, above which over-densities collapse and form primordial black holes. Our findings are consistent with previous analytic predictions as well as numerical studies that use different formalisms and computational approaches, and hence provide independent validation of those results.

astro-ph.CO↗

arXiv2026

Subsolar mass black holes from stellar collapse induced by primordial black holes

While no gravitational-wave detection of subsolar mass black holes has been confirmed to date, a number of candidate detections invite us to speculate on the origin of such black holes should a detection be confirmed. It is generally assumed that the observation of a black hole with subsolar mass $M_{\rm obs}$ would provide strong evidence for primordial black holes (PBHs). The mass $M_{\rm PBH}$ of the PBH, however, does not necessarily have to be equal to $M_{\rm obs}$, as it would in what we term a ``direct PBH scenario". Instead, a black hole of mass $M_{\rm obs}$ may form in a capture of a much smaller primordial black hole, $M_{\rm PBH} \ll M_{\rm obs}$, by a dwarf star of mass $M_* \simeq M_{\rm obs}$, followed by the total consumption of the star by the PBH. We provide some rough estimates and demonstrate that such an ``indirect PBH scenario" may also lead to significant populations of black holes with mass $M_{\rm obs}$, especially in dwarf galaxies, and may be able to explain rare subsolar mass events.

astro-ph.HE↗

arXiv2025

Restrictions on Initial Conditions in Cosmological Scenarios and Implications for Simulations of Primordial Black Holes and Inflation

Numerical relativity simulations provide a means by which to study the evolution and end point of strong over-densities in cosmological spacetimes. Specific applications include studies of primordial black hole formation and the robustness of inflation. Here we adopt a toy model previously used in asymptotically flat spacetimes to show that, for given values of the over-density and the mean curvature, solutions to the Hamiltonian constraint need not exist, and if they do exist they are not unique. Specifically, pairs of solutions exist on two branches, corresponding to strong-field and weak-field solutions, that join at a maximum beyond which solutions cease to exist. As a result, there is a limit to the extent to which an over-density can be balanced by intrinsic rather than extrinsic curvature on the initial slice. Even below this limit, iterative methods to construct initial data may converge to solutions on either one of the two branches, depending on the starting guess, leading to potentially inconsistent physical results in the evolution.

gr-qc↗

arXiv2025

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

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.

astro-ph.HE↗

arXiv2025

Can "premature collapse" form black holes in the upper and lower mass gaps?

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.

astro-ph.HE↗

arXiv2025

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

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.

astro-ph.HE↗

arXiv2024

Constraining the nuclear equation of state from orbits of primordial black holes inside neutron stars

Lacking terrestrial experimental data, our best constraints on the behavior of matter at high densities up to and above nuclear density arise from observations of neutron stars. Current constraints include those based on measurements of stellar masses, radii, and tidal deformabilities. Here we explore how orbits of primordial black holes - should they exist - inside neutron stars could provide complementary constraints on the nuclear equation of state (EOS). Specifically, we consider a sample of candidate EOSs, construct neutron star models for these EOSs, and compute orbits of primordial black holes inside these stars. We discuss how the pericenter advance of eccentric orbits, i.e. orbital precession, results in beat phenomena in the emitted gravitational wave signal. Observing this beat frequency could constrain the nuclear EOS and break possible degeneracies arising from other constraints, as well as provide information about the host star.

astro-ph.HE↗

arXiv2024

Primordial black hole capture, gravitational wave beats, and the nuclear equation of state

Primordial black holes (PBHs), if captured by neutron stars (NSs), would emit a characteristic gravitational wave (GW) signal as they orbit inside the host star. We identify a specific and qualitatively new feature of these signals, namely quasi-periodic beats caused by the precession of noncircular PBH orbits. We demonstrate numerically and analytically that the beat frequency depends rather sensitively on the NS structure, so that hypothetical future observations with next-generation GW detectors would provide valuable constraints on the nuclear equation of state.

gr-qc↗

arXiv2024

Primordial black holes captured by neutron stars: relativistic point-mass treatment

Primordial black holes (PBHs), if they exist, may collide with and be captured by neutron stars. We adopt a relativistic point-mass approximation to study this capture, the subsequent confinement of the PBH of mass $m$ inside the neutron star of mass $M_* \gg m$, and the PBH's growth by accretion of stellar material. Building on earlier treatments we systematically study the capture, confinement, and accretion process, characterize the emitted quasiperiodic continuous gravitational-wave signal, track the evolution of the PBH's orbital parameters, and compare the effects of different choices for the prescription of the dissipative forces. Our point-mass treatment here is applicable in the limit of small PBH masses, for which its effects on the neutron star can be ignored.

gr-qc↗

arXiv2024

Simulations of gravitational collapse in null coordinates: I. Formulation and weak-field tests in generalised Bondi gauges

We present a code for numerical simulations of the collapse of regular initial data to a black hole in null coordinates. We restrict to twist-free axisymmetry with scalar field matter. Our coordinates are $(u,x,y,φ)$, where the retarded time $u$ labels outgoing null cones emerging from a regular central worldline, the angles $(θ,φ)$ label the null generators of each null cone, and the radial coordinate $x$ labels points along these generators. We focus on a class of generalised Bondi radial coordinates $x$ with the twin properties that $x=0$ is the central world line and that the numerical domain $(u\ge0$, $0\le x\le x_\text{max})$ is a subset of the domain of dependence of the initial data on $(u=0$, $0\le x\le x_\text{max}$). In critical collapse, an appropriate choice of these coordinates can be made to zoom in on the accumulation point of scale echos of the critical solution, without the need for explicit mesh refinement. We introduce a novel numerical scheme that in effect reduces the angular resolution at small radius, such that the time step $Δu$ for an explicit numerical scheme is limited by the radial resolution $Δx$, rather than $Δx(Δθ)^2$. We present convergence tests in the weak-field regime, where we have exact solutions to the linearised scalar and gravitational-wave equations.

gr-qc↗

arXiv2024

Simulations of gravitational collapse in null coordinates: II. Critical collapse of an axisymmetric scalar field

We present the first numerical simulations in null coordinates of the collapse of nonspherical regular initial data to a black hole. We restrict to twist-free axisymmetry, and re-investigate the critical collapse of a non-spherical massless scalar field. We find that the Choptuik solution governing scalar field critical collapse in spherical symmetry persists when fine-tuning moderately non-spherical initial data to the threshold of black hole formation. The non-sphericity evolves as an almost-linear perturbation until the end of the self-similar phase, and becomes dominant only in the final collapse to a black hole. We compare with numerical results of Choptuik et al, Baumgarte, and Marouda et al, and conclude that they have been able to evolve somewhat more non-spherical solutions. Future work with larger deviations from spherical symmetry, and in particular vacuum collapse, will require a different choice of radial coordinate that allows the null generators to reconverge locally.

gr-qc↗

arXiv2024

Primordial black holes captured by neutron stars: simulations in general relativity

We present self-consistent numerical simulations in general relativity of putative primordial black holes inside neutron stars. Complementing a companion paper in which we assumed the black hole mass $m$ to be much smaller than the mass $M_*$ of the neutron star, thereby justifying a point-mass treatment, we here consider black holes with masses large enough so that their effect on the neutron star cannot be neglected. We develop and employ several new numerical techniques, including initial data describing boosted black holes in neutron-star spacetimes, a relativistic determination of the escape speed, and a gauge condition that keeps the black hole hole at a fixed coordinate location. We then perform numerical simulations that highlight different aspects of the capture of primordial black holes by neutron stars. In particular, we simulate the initial passage of the black hole through the star, demonstrating that the neutron star remains dynamically stable provided the black-hole mass is sufficiently small, $m \lesssim 0.05 M_*$. We also model the late evolution of a black hole oscillating about the center of an initially stable neutron star while accreting stellar mass and ultimately triggering gravitational collapse.

gr-qc↗

arXiv2024

Could long-period transients be powered by primordial black hole capture?

Long-period radio transients have unusual properties that challenge their interpretation as pulsars or magnetars. We examine whether they might instead be powered by primordial black holes (PBHs) making repeated passages through a host star, thereby providing a signature of elusive dark-matter candidates. We demonstrate that constraints derived from the transients' period and period derivative alone already rule out this scenario for most potential host stars. While white dwarfs may satisfy these constraints, they are unlikely to capture PBHs in the required mass range.

astro-ph.HE↗

arXiv2023

Critical phenomena in the collapse of gravitational waves

Fine-tuning generic but smooth spherically-symmetric initial data for general relativity to the threshold of dynamical black hole formation creates arbitrarily large curvatures, mediated by a universal self-similar solution that acts as an intermediate attractor. For vacuum gravitational waves, however, these critical phenomena have been elusive. We present, for the first time, excellent agreement among three independent numerical simulations of this collapse. Surprisingly, we find no universality, and observe approximate self-similarity for some families of initial data but not for others.

gr-qc↗