arXiv subjects

Zacharias Roupas

Publications and source records attributed to Zacharias Roupas.

At least 19 recordsLinked to original sources

arXiv2026

Evidence for an accretion-driven subpopulation of black holes in GWTC-5.0

We report Bayesian evidence for a subpopulation of black holes, grown via accretion during the gaseous birth-stage of star clusters, in the cumulative GWTC-5.0 released by the LIGO-Virgo-KAGRA collaboration. This accretion channel predicts a saturating spin--mass relation, with low spins at low masses, rising continuously with mass through all intermediate spin values, to high spins at high masses. We test this prediction directly against the component spins of the catalogue. We single out 10 events with positive support and many others with weaker support across the whole component-mass range of the catalogue. We identify a preferred mass scale $m^t=20.7^{+11.6}_{-1.2}\,M_\odot$ (90\% credibility) separating regimes with different accretion-mixture fractions, with $\ln B=5.5$. Above this inferred scale, the evidence for an accretion-driven subpopulation is strongly concentrated, reaching a natural log Bayes factor of $16.6$ against the LVK fiducial spin population.

astro-ph.HE↗

arXiv2026

Black hole spin-mass correlation and vector resonant relaxation in gaseous star clusters: the origin of GW231123?

During the formation of a star cluster a spin-mass correlation of stellar black holes is generated as they grow via accretion of the residual gas. Moreover, the black hole spin tends to be anti-aligned with its orbital angular momentum in the cluster. We show that GW231123, reported by the LIGO-Virgo-KAGRA (LVK) collaboration, lies on our predicted high-mass, high-spin plateau of the spin-mass correlation with positive Bayesian evidence over the LVK prior. Furthermore, vector resonant relaxation (VRR) equilibrium is favored over the isotropic LVK prior in reproducing the distribution of the relative spin tilt. The joint Bayes factor suggests strong evidence for the favored cluster models. GW231123 is thus consistent with our proposed channel that generates correlated black hole masses and spins, and drives the spins' orientations.

astro-ph.HE↗

arXiv2026

Accretion-disk formation around orbiting stellar black holes in gaseous star clusters

We consider low-mass black holes (BHs) moving in regular orbits in the cores of non-rotating gaseous star clusters, representative of proto-stellar clusters or the centers of protogalaxies. We argue that as the BH's sphere of influence -- the Bondi sphere -- is advected along the BH trajectory, the transverse velocity shear between the inner and outer hemispheres injects angular momentum, driving the formation of an accretion disk. Coriolis forces oppose angular momentum injection, delaying disk formation but not preventing it. The disk lies in the BH orbital plane and is counter-rotating with respect to the orbital BH motion. We verify this picture with 2D and 3D hydrodynamic simulations in the non-inertial frame of the orbiting BH. We find that the disk-formation timescale following a disruption event is of order the Bondi crossing timescale, $τ_{\rm d} \sim R_{\rm B}/V_{\bullet}$, and that the disk radius is of order $R_{\rm d} \sim ω_{\bullet}^2 R_{\rm B}^4/ G m_{\bullet} $, set by the circularization radius of gas captured in the Bondi sphere. For the case of a BH with $m_{\bullet} = 50\,{\rm M}_\odot$, inside the core of a typical compact proto-stellar cluster, these values read $τ_{\rm d} \sim 0.1 P_{\rm orbit}$ and $R_{\rm d} \sim 10^{-3} R_{\rm B}$.

astro-ph.GA↗

arXiv2026

Spin-up and spin distribution of stellar black holes grown by gas accretion in proto-stellar clusters

Proto-stellar clusters, likely progenitors of globular clusters, are compact with typical mass $\sim 10^6\,{\rm M}_\odot$ and size $\sim 1\,{\rm pc}$, as revealed recently by JWST observations at $z\sim 10$. Sufficiently high compactness can provide a time window for early-formed stellar black holes (BHs) to accrete primordial gas. We develop a model to determine the final spin distribution of stellar BHs which grow in mass via gas accretion within compact gaseous proto-stellar clusters. The velocity shear within a BH's sphere of influence induces the formation of an accretion disk which is repeatedly disrupted by stochastic perturbations to the BH motion. We assume low initial BH spins $a_{*,{\rm ini}} = 0.01$, and restrict initial BH masses below the upper BH mass gap, $m_{\rm BH,ini} < 55\,{\rm M}_\odot$. Our analysis shows a strong BH spin-mass correlation, obtained within $\sim 10 \,{\rm Myr}$ when gas is depleted. Low-spin BHs, $a_{*} \leq 0.3$, are predominantly low-mass, $m_{\rm BH} \lesssim 25\,{\rm M}_\odot$, in contrast to high-spin black holes, $a_{*} \geq 0.7$, which are predominantly high-mass, $m_{\rm BH} \gtrsim 65\,{\rm M}_\odot$. Notably, there exist also low-spin, high-mass outliers with $\sim 1$ mass-gap BH per cluster expected to have $a_{*} \sim 0.1$. The general trend, however, expressed by the median spin as a function of final BH mass is well fit by a high-spin saturating exponential with transition mass $\sim 50\,{\rm M}_{\odot}$. For $m_{\rm BH} \geq 100\,{\rm M}_\odot$ the median spin is $\bar{a}_{*} \sim 0.90$ with the central $68\%$ of the distribution spanning $a_{*} \sim 0.70 - 0.96$, in striking agreement with the estimated spins of the gravitational-wave signal GW231123. These spin values persist up to the highest masses generated by our mechanism, $m_{\rm BH} \sim 10^3\,{\rm M}_\odot$.

astro-ph.GA↗

arXiv2026

Massive Black Hole formation in proto-stellar clusters via early gas accretion

We review our semi-analytic model of stellar black hole (BH) mass growth by gas accretion in gas-rich stellar clusters during their birthstage within the first $\sim 10\,{\rm Myr}$ after the first stellar formation event. Such proto-stellar clusters are massive and compact, with typical masses $\sim 10^6\,{\rm M}_\odot$ and sizes $\sim 1\,{\rm pc}$, suggested by recent James Webb Space Telescope (JWST) observations. We find that the BH masses are shifted by the end of gas depletion to values within and above the BH mass gap, well within the range of components of the recent gravitational-wave (GW) signal GW231123, and up to masses $\sim 10^3\,{\rm M}_\odot$.

astro-ph.GA↗

arXiv2025

Black hole mass function shift in proto-stellar-clusters driven by gas accretion

The James Webb Space Telescope (JWST) has observed compact, massive proto-stellar clusters of low metallicity in the Cosmic Gems arc galaxy at high redshift, which represent likely precursors to globular clusters. We model the mass growth of stellar black holes (BHs) during the first few Myr of the life of a massive, compact, gaseous stellar cluster before stellar feedback expels the primordial gas. At high redshift, in a lower metallicity environment stellar winds get weaker allowing for larger gas-depletion time-scales in the cluster despite of energetic pair-instability supernova (PISN) feedback for sufficiently compact clusters. Mass segregation drives the massive stellar progenitors of BHs in the center of the cluster where gas is most dense. We estimate the conditions for which the initial black hole mass function (BHMF), with a PISN-induced cut-off $<55{\rm M}_\odot$, gets shifted to values within the upper BH mass gap, $\sim 60-130{\rm M}_\odot$, or higher, as observed by Gravitational Wave (GW) experiments LIGO-Virgo-KAGRA. We find that the BHs are shifted by the end of gas depletion to values within and above the mass gap, well within the range of BH components of the recent GW-signal GW231123, depending on total mass, star formation efficiency, metallicity and compactness. The individual BH mass increase follows approximately a surprisingly steep power law with respect to initial BH mass with an exponent in the range $\approx 4-6$. This occurs in gaseous proto-stellar clusters that are sufficiently massive and compact, with typical values of total mass $\sim 10^6{\rm M}_\odot$ and size $\sim 1{\rm pc}$. Our analysis suggests that proto-stellar clusters at high redshift such as Cosmic Gems arc clusters have generated through early gas accretion, BHs as heavy as $\sim 10^2-10^3{\rm M}_\odot$.

astro-ph.GA↗

arXiv2024

Self-gravitating isothermal sphere in an expanding background

Spatially homogeneous thermal equilibria of self-gravitating gas, being impossible otherwise, are nevertheless allowed in an expanding background accounting for Universe's expansion. Furthermore, a fixed density at the boundary of a perturbation is a natural boundary condition keeping the mass finite inside without the need to invoke any unphysical walls. These facts allow us to develop a consistent gravitational thermodynamics of isothermal spheres inside an expanding Universe. In the canonical and grand canonical ensembles we identify an instability for both homogeneous and inhomogeneous equilibria. We discuss a potential astrophysical application. If such an instability is triggered on baryonic gas at high redshift $z > 137$ when the primary baryonic component, namely atomic hydrogen, was still thermally locked to the Cosmic Microwave Background radiation, then the corresponding destabilized gaseous clouds have baryonic mass $\geq 0.8\cdot 10^5 {\rm M}_\odot$ and radius $\geq 15{\rm pc}$.

astro-ph.GA↗

arXiv2023

Thermodynamic instability of dark energy equation of state

We derive the dark energy fluid equation of state $P = -ε= {\rm const.}$ as an extremum of entropy, subject to the Hamiltonian constraint of General Relativity. However, we identify perturbations that can render this extremum an entropy minimum designating a thermodynamic instability and specify the mathematical condition for this to occur.

gr-qc↗

arXiv2022

The Cosmological Black Hole

We briefly review the recent novel solution of General Relativity, we call the cosmological black hole, firstly discovered in [Roupas, Z. Eur. Phys. J. C 82, 255 (2022)]. A dark energy universe and a Schwartzschild black hole are matched on a common dual event horizon which is finitely thick due to quantum indeterminacy. The system gets stabilized by a finite tangential pressure applied on the dual horizon. The fluid entropy of the system at a Tolman temperature identified with the cosmological horizon temperature is calculated to be equal with the Bekenstein-Hawking entropy.

gr-qc↗

arXiv2022

Detectable universes inside regular black holes

While spacetime in the vicinity outside astrophysical black holes is believed to be well understood, the event horizon and the interior remain elusive. Here, we discover a degenerate infinite spectrum of novel general relativity solutions with the same mass-energy and entropy that describe a dark energy universe inside an astrophysical black hole. This regular cosmological black hole is stabilized by a finite tangential pressure applied on the dual cosmological-black hole event horizon, localized up to a quantum indeterminacy. We recover the Bekenstein-Hawking entropy formula from the classical fluid entropy, calculated at a Tolman temperature equal to the cosmological horizon temperature. We further calculate its gravitational quasi-normal modes. We find that cosmological black holes are detectable by gravitational-wave experiments operating within the $μ{\rm Hz}-{\rm Hz}$ range, like LISA space-interferometer.

gr-qc↗

arXiv2021

Thermodynamic origin of quantum time-energy uncertainty relation

The problem of time is a notable obstacle towards the recognition of quantum theory as the ultimate fundamental description of nature. Quantum theory may not be complete if founded upon classical notions. Louis de Broglie, seeming to be more or less convinced about the ontology of his proposed matter waves, tried to develop a theory of sub-quantum degrees of freedom relying on statistical thermodynamics. He realized a quantum particle as a fluctuating dense corpuscle formed via non-linear effects from a sub-quantum medium. A wave on the medium guides the vibrating corpuscle. He argued that an intrinsic clock of a quantum particle is related to its Brownian motion at the sub-quantum level. This led him to conjecture a relation between the de Broglie clock frequency $m c^2/h$ and its implicit temperature, which equals that of the surrounding sub-quantum medium. About the same time, Mandelbrot was the first to derive in a classical setting a thermodynamic uncertainty relation between energy and temperature, that was, coincidentally or not, anticipated by Bohr and Heisenberg in the first years of development of quantum theory. We show here that, when the de Broglie temperature-time conjecture is assumed, the thermodynamic temperature-energy uncertainty relation leads to the quantum time-energy uncertainty relation.

quant-ph↗

arXiv2021

QCD color superconductivity in compact stars: color-flavor locked quark star candidate for the gravitational-wave signal GW190814

At sufficiently high densities and low temperatures matter is expected to behave as a degenerate Fermi gas of quarks forming Cooper pairs, namely a color superconductor, as was originally suggested by Alford, Rajagopal and Wilczek [Nuclear Physics B 537, 443 (1999)]. The ground state is a superfluid, an electromagnetic insulator that breaks chiral symmetry, called the color-flavor locked phase. If such a phase occurs in the cores of compact stars, the maximum mass may exceed that of hadronic matter. The gravitational-wave signal GW190814 involves a compact object with mass $2.6{\rm M}_\odot$, within the so-called low mass gap. Since it is too heavy to be a neutron star and too light to be a black hole, its nature has not been identified with certainty yet. Here, we show not only that a color-flavor locked quark star with this mass is viable, but also we calculate the range of the model-parameters, namely the superconducting gap $Δ$ and the bag constant $B$, that satisfies the strict LIGO constraints on the equation of state. We find that a color-flavor locked quark star with mass $2.6{\rm M}_\odot$ satisfies the observational constraints on the equation of state if $Δ\geq 200{\rm MeV}$ and $B\geq 83{ \rm MeV}/{\rm fm^3}$ for a strange quark mass $m_s=95~{\rm MeV}/c^2$, and attains a radius $(12.7-13.6) {\rm km}$ and central density $(7.5-9.8) 10^{14}{\rm g}/{\rm cm}^3$.

astro-ph.HE↗

arXiv2020

Relativistic Gravitational Collapse by Thermal Mass

Gravity and thermal energy are universal phenomena which compete over the stabilization of astrophysical systems. The former induces an inward pressure driving collapse and the latter a stabilizing outward pressure generated by random motion and energy dispersion. Since a contracting self-gravitating system is heated up one may wonder why is gravitational collapse not halted in all cases at a sufficient high temperature establishing either a gravo-thermal equilibrium or explosion. Here, based on the equivalence between mass and energy, we show that there always exists a temperature threshold beyond which the gravitation of thermal energy overcomes its stabilizing pressure and the system collapses under the weight of its own heat.

gr-qc↗

arXiv2020

Gravitational Brownian motion as inhomogeneous diffusion: black hole populations in globular clusters

Recent theoretical and numerical developments supported by observational evidence strongly suggest that many globular clusters host a black hole (BH) population in their centers. This stands in contrast to the prior long-standing belief that a BH subcluster would evaporate after undergoing core collapse and decoupling from the cluster. In this work, we propose that the inhomogeneous Brownian motion generated by fluctuations of the stellar gravitational field may act as a mechanism adding a stabilizing pressure to a BH population. We argue that the diffusion equation for Brownian motion in an inhomogeneous medium with spatially varying diffusion coefficient and temperature, which was first discovered by Van Kampen, also applies to self-gravitating systems. Applying the stationary phase space probability distribution to a single BH immersed in a Plummer globular cluster, we infer that it may wander as far as $\sim 0.05,\,0.1,\,0.5{\rm pc}$ for a mass of $m_{\rm b} \sim 10^3,\,10^2,\,10{\rm M}_\odot$, respectively. Furthermore, we find that the fluctuations of a fixed stellar mean gravitational field are sufficient to stabilize a BH population above the Spitzer instability threshold. Nevertheless, we identify an instability whose onset depends on the Spitzer parameter, $S = (M_{\rm b}/M_\star) (m_{\rm b}/m_\star)^{3/2} ,$ and parameter $B = ρ_{\rm b}(0) (4πr_c^3/M_b)(m_\star/m_{\rm b})^{3/2} $, where $ρ_{\rm b}(0)$ is the Brownian population central density. For a Plummer sphere, the instability occurs at $(B,S) = (140,0.25)$. For $B > 140,$ we get very cuspy BH subcluster profiles that are unstable with regard to the support of fluctuations alone. For $S > 0.25,$ there is no evidence of any stationary states for the BH population based on the inhomogeneous diffusion equation.

astro-ph.GA↗

arXiv2020

Anisotropic Neutron Stars Modelling: Constraints in Krori-Barua Spacetime

Dense nuclear matter is expected to be anisotropic due to effects such as solidification, superfluidity, strong magnetic fields, hyperons, pion-condesation. Therefore an anisotropic neutron star core seems more realistic than an ideally isotropic one. We model anisotropic neutron stars working in the Krori-Barua (KB) ansatz without preassuming an equation of state. We show that the physics of general KB solutions is encapsulated in the compactness. Imposing physical and stability requirements yields a maximum allowed compactness $2GM/Rc^2 < 0.71$ for a KB-spacetime. We further input observational data from numerous pulsars and calculate the boundary density. We focus especially on data from the LIGO/Virgo collaboration as well as recent independent measurements of mass and radius of miilisecond pulsars with white dwarf companions by the Neutron Star Interior Composition Explorer (NICER). For these data the KB-spacetime gives the same boundary density which surprisingly equals the nuclear saturation density within the data precision. Since this value designates the boundary of a neutron core, the KB-spacetime applies naturally to neutron stars. For this boundary condition we calculate a maximum mass of 4.1 solar masses.

gr-qc↗

arXiv2020

Ejection of supermassive black holes and implications for merger rates in fuzzy dark matter haloes

Fuzzy dark matter (FDM) consisting of ultra-light axions has been invoked to alleviate galactic-scale problems in the cold dark matter scenario. FDM fluctuations, created via the superposition of waves, can impact the motion of a central supermassive black hole (SMBH) immersed in an FDM halo. The SMBH will undergo a random walk, induced by FDM fluctuations, that can result in its ejection from the central region. This effect is strongest in dwarf galaxies, accounting for wandering SMBHs and the low detection rate of AGN in dwarf spheroidal galaxies. In addition, a lower bound on the allowed axion masses is inferred both for Sagittarius $A^*$ and heavier SMBH; to avoid ejection from the galactic centres, axion masses of the order of $10^{-22}{\rm eV}$ or lighter are excluded. Stronger limits are inferred for merging galaxies. We find that the event rate of SMBH mergers in FDM haloes and the associated SMBH growth rates can be reduced by at least an order of magnitude.

astro-ph.GA↗

arXiv2020

Secondary component of gravitational-wave signal GW190814 as an anisotropic neutron star

The gravitational-wave signal GW190814 involves a compact object with mass $(2.50-2.67){\rm M}_\odot$ within the so-called low mass gap. As yet, a general consensus on its nature, being a black hole, a neutron star or an exotic star, has not been achieved. We investigate the possibility this compact object to be an anisotropic neutron star. Anisotropies in a neutron star core arise naturally by effects such as superfluidity, hyperons, strong magnetic fields and allow the maximum mass to exceed that of the ideally isotropic stars. We consider the Krori-Barua ansatz to model an anisotropic core and constrain the equation of state with LIGO/Virgo observations GW170817 and GW190814. We find that the GW190814 secondary component can be an anisotropic neutron star compatible with LIGO/Virgo constraints if the radius attains a value in the range $(13.2-14.0)\,{\rm km}$ with the anisotropic core's boundary density in the range $(3.5-4.0)\cdot 10^{14}{\rm g}/{\rm cm}^3$.

gr-qc↗

arXiv2020

Gravitational potential from maximum entropy principle

It is shown here in the framework of standard General Relativity that the gravitational potential in static spacetime, equivalently the redshift factor, inside any kind of matter, can be derived from maximum entropy principle. It is used only the Hamiltonian constraint, without further invoking Einstein's equations or any new principle. The Newtonian potential arises from the same procedure.

gr-qc↗