arXiv subjects

Michael F. Wondrak

Publications and source records attributed to Michael F. Wondrak.

12 recordsLinked to original sources

arXiv2026

Particle Creation in a Cosmological Background in Analogy to the Schwinger Effect

We consider a gravitational analogue of the Schwinger effect in a cosmological context. While the Schwinger effect is usually attributed to a static electric background, its derivation is actually based on a switching on/off of the electric field in the infinite past/future. Motivated by this, and our previous work on particle production in a gravitational background, we consider a long pulse of the gravitational field in an FLRW-spacetime, thus simulating a static background. We rigorously derive particle production by a novel application of the Heun equation. In fact, the recently obtained connecting formulas between its local solutions can be used to determine the Bogolyubov coefficients, and subsequently the particle production probabilities, in the limit of an infinitely long pulse. The particle production in the FLRW-model is found to have a lower threshold on the late time frequencies, which can be related to the duration of the time interval of the switching on/off the background field. For large frequencies and in the spatially flat case, we find a Planckian frequency spectrum whose temperature is inversely proportional to the duration of scale change. We compare our findings to Schwinger's result on particle production for a long pulse of an electromagnetic background field, for which we also include a detailed derivation.

gr-qc↗

arXiv2025

An upper limit to the lifetime of stellar remnants from gravitational pair production

Black holes are assumed to decay via Hawking radiation. Recently we found evidence that spacetime curvature alone without the need for an event horizon leads to black hole evaporation. Here we investigate the evaporation rate and decay time of a non-rotating star of constant density due to spacetime curvature-induced pair production and apply this to compact stellar remnants such as neutron stars and white dwarfs. We calculate the creation of virtual pairs of massless scalar particles in spherically symmetric asymptotically flat curved spacetimes. This calculation is based on covariant perturbation theory with the quantum field representing, e.g.,\ gravitons or photons. We find that in this picture the evaporation timescale, $τ$, of massive objects scales with the average mass density, $ρ$, as $τ\proptoρ^{-3/2}$. The maximum age of neutron stars, $τ\sim 10^{68}\,\text{yr}$, is comparable to that of low-mass stellar black holes. White dwarfs, supermassive black holes, and dark matter supercluster halos evaporate on longer, but also finite timescales. Neutron stars and white dwarfs decay similarly to black holes, ending in an explosive event when they become unstable. This sets a general upper limit for the lifetime of matter in the universe, which in general is much longer than the Hubble--Lemaître time, although primordial objects with densities above $ρ_\text{max} \approx 3\times 10^{53}\,\text{g/}\text{cm}^3$ should have dissolved by now. As a consequence, fossil stellar remnants from a previous universe could be present in our current universe only if the recurrence time of star forming universes is smaller than about $\sim 10^{68}\,\text{years}$.

gr-qc↗

arXiv2025

Reply to "Comment on 'Gravitational Pair Production and Black Hole Evaporation'"

In a recent letter, the authors presented a unified derivation of the electric Schwinger effect and a generalized Hawking effect with an additional radiation component. The approach discloses a radial profile of black hole pair production and traces the emission back to local tidal forces which are independent of the black hole event horizon. It uses an effective action valid to second order in curvature and arbitrary order in proper time. A comment on the letter supposed two inconsistencies when applying the central formula to the Schwinger effect in the presence of magnetic fields. The present letter points out that the partially flawed argumentation does not cast doubt on the results.

gr-qc↗

arXiv2023

Early time dynamics far from equilibrium via holography

We investigate the early time dynamics of heavy ion collisions studying the time evolution of the energy-momentum tensor as well as energy-momentum correlations within a uniformly thermalizing holographic QGP. From these quantities, we suggest a far-from equilibrium definition of shear viscosity, which is a crucial property of QCD matter as it significantly determines the generation of elliptic flow already at early times. During an exemplary initial heating phase of the holographic QGP the shear viscosity of entropy density ratio decreases down to 60%, followed by an overshoot to 110% of the near-equilibrium value, $η/s=1/(4π)$. Implications for the QCD QGP are discussed. Subsequently, we consider a holographic QGP which is Bjorken-expanding. Its energy-momentum tensor components have a known hydrodynamic attractor to which all time evolutions collapse independent of the initial conditions. Based on this, we propose a definition for a far from equilibrium speed of sound, and analytically compute its hydrodynamic attractor. Subjecting this Bjorken-expanding plasma to an external magnetic field and an axial chemical potential, we study the chiral magnetic effect far from equilibrium.

nucl-th↗

arXiv2023

Gravitational Pair Production and Black Hole Evaporation

We present a new avenue to black hole evaporation using a heat-kernel approach analogous as for the Schwinger effect. Applying this method to an uncharged massless scalar field in a Schwarzschild spacetime, we show that spacetime curvature takes a similar role as the electric field strength in the Schwinger effect. We interpret our results as local pair production in a gravitational field and derive a radial production profile. The resulting emission peaks near the unstable photon orbit. Comparing the particle number and energy flux to the Hawking case, we find both effects to be of similar order. However, our pair production mechanism itself does not explicitly make use of the presence of a black hole event horizon.

gr-qc↗

arXiv2022

Probing Quadratic Gravity with the Event Horizon Telescope

Quadratic gravity constitutes a prototypical example of a perturbatively renormalizable quantum theory of the gravitational interactions. In this work, we construct the associated phase space of static, spherically symmetric, and asymptotically flat spacetimes. It is found that the Schwarzschild geometry is embedded in a rich solution space comprising horizonless, naked singularities and wormhole solutions. Characteristically, the deformed solutions follow the Schwarzschild solution up outside of the photon sphere while they differ substantially close to the center of gravity. We then carry out an analytic analysis of observable signatures accessible to the Event Horizon Telescope, comprising the size of the black hole shadow as well as the radiation emitted by infalling matter. On this basis, we argue that it is the brightness within the shadow region which constrains the phase space of solutions. Our work constitutes the first step towards bounding the phase space of black hole type solutions with a clear quantum gravity interpretation based on observational data.

gr-qc↗

arXiv2020

Early-Stage Shear Viscosity far from Equilibrium via Holography

Shear viscosity is a crucial property of QCD matter which determines the collective behavior of the the quark-gluon plasma (QGP) in ultrarelativistic heavy-ion collisions. Extending the near-equilibrium, high-precision investigations in theory and experiment, we take into account the fact that, in a collision, the QGP is generated far from equilibrium. We use the AdS/CFT correspondence to study a strongly coupled plasma and find a significant impact on the ratio of shear viscosity to entropy density, $η/s$. In particular, we investigate the initial heating phase and find a decrease reaching down to below 60% followed by an overshoot to 110% of the near-equilibrium value. This finding might be highly relevant for the extraction of transport coefficients from anisotropic flow measurements at RHIC and LHC.

hep-ph↗

arXiv2020

Shear transport far from equilibrium via holography

In heavy-ion collisions, the quark-gluon plasma is produced far from equilibrium. This regime is currently inaccessible by quantum chromodynamics (QCD) computations. We calculate shear transport and entropy far from equilibrium in a holographic model, defining a time-dependent ratio of shear viscosity to entropy density, $η/s$. Large deviations of up to 60% from its near-equilibrium value, $1/4π$, are found for realistic situations at the Large Hadron Collider. We predict the far-from-equilibrium time-dependence of $η/s$ to substantially affect the evolution of the QCD plasma and to impact the extraction of QCD properties from flow coefficients in heavy-ion collision data.

hep-ph↗

arXiv2019

Constraints on the String T-Duality Propagator from the Hydrogen Atom

We investigate the implications of a string-theory modified propagator in the high-precision regime of quantum mechanics. In particular, we examine the situation in which string theory is compactified at the T-duality self-dual radius. The corresponding propagator is closely related to the one derived from the path integral duality. Our focus is on the hydrogen ground state energy and the $1\text{S}_{1/2}-2\text{S}_{1/2}$ transition frequency as they are the most precisely explored properties of the hydrogen atom. In our analysis, the T-duality propagator affects the photon field leading to a modified Coulomb potential. Thus, our study is complementary to investigations where the electron evolution is modified as in studies of a minimal length in the context of the generalized uncertainty principle. The first manifestation of the T-duality propagator arises at fourth order in the fine-structure constant, including a logarithmic term. The constraints on the underlying parameter, the zero-point length, reach down to $3.9 \times 10^{-19}\, \text{m}$ and are in full agreement with previous studies on black holes.

gr-qc↗

arXiv2019

Quantum Corrected Black Holes from String T-Duality

In this paper we present some stringy corrections to black hole spacetimes emerging from string T-duality. As a first step, we derive the static Newtonian potential by exploiting the relation between the T-duality and the path integral duality. We show that the intrinsic non-perturbative nature of stringy corrections introduce an ultraviolet cutoff known as zero-point length in the path integral duality literature. As a result, the static potential is found to be regular. We use this result to derive a consistent black hole metric for the spherically symmetric, electrically neutral case. It turns out that the new spacetime is regular and is formally equivalent to the Bardeen metric, apart from a different ultraviolet regulator. On the thermodynamics side, the Hawking temperature admits a maximum before a cooling down phase towards a thermodynamically stable end of the black hole evaporation process. The findings support the idea of universality of quantum black holes.

gr-qc↗

arXiv2018

Superradiance in Modified Gravity (MOG)

We consider the case of rotating black holes in a dark-matter-emulating theory of gravity called MOG. The latter introduces a gravitational vector field with an associated gravitational charge proportional to the black hole mass and a scalar field in place of the gravitational constant. The resulting black hole metrics resemble the Kerr-Newman geometry and enjoy superradiant scattering. MOG, however, presents important new features. By studying the scattering of a scalar field, we show that there is a marked reduction of the critical frequency of mode amplification. This corresponds to saying that the superradiance peak frequency is red shifted. Analyses of the reflected energy flux also show that MOG black holes are fainter with respect to the standard ones. The proposed results pave the way for testing MOG against astronomical observations.

gr-qc↗

arXiv2016

Unparticle contribution to the hydrogen atom ground state energy

In the present work we study the effect of unparticle modified static potentials on the energy levels of the hydrogen atom. By using Rayleigh-Schrödinger perturbation theory, we obtain the energy shift of the ground state and we compare it with experimental data. Bounds on the unparticle energy scale $Λ_\mathcal{U}$ as a function of the scaling dimension $d_\mathcal{U}$ and the coupling constant $λ$ are derived. We show that there exists a parameter region where bounds on $Λ_\mathcal{U}$ are stringent, signalling that unparticles could be tested in atomic physics experiments.

hep-ph↗