arXiv subjects

Janna Levin

Publications and source records attributed to Janna Levin.

At least 19 recordsLinked to original sources

arXiv2025

Klein Bottle Cosmology

We explore a higher-dimensional universe that is a product of Minkowski space and the nonorientable Klein bottle. The topology explicitly breaks important symmetries, such as translational invariance and (5+1)-dimensional CP invariance. Somewhat surprisingly, the (3+1)-dimensional cp of the Minkowski space can also be broken by the Klein bottle, both explicitly and in the presence of a brane. The topology enforces a background of fermion correlations that amounts to a condensate wall localized in the Klein bottle. The wall acts as an order parameter for the broken symmetries. If a brane passes through the wall, brane fermions that couple to the condensate are produced as quantified by the Bogoliubov coefficients for a time-dependent mass. The scenario meets the conditions, including cp violation, to potentially generate the matter-antimatter asymmetry of the universe.

hep-th↗

arXiv2025

Compactification Without Orientation, or a Topological Scenario for $CP$ Violation

In higher dimensional theories, we often assume that the extra dimensions form an orientable space, perhaps with singularities. However, many physical theories are well-defined on non-orientable spaces, and many spaces are not orientable, so it is reasonable to explore what happens if the assumption of orientability is relaxed. Here we consider the simplest example of free 6D theories compactified on a flat Klein bottle. We focus on a Dirac fermion in 6D, with boundary conditions that define ${\rm pin}^+$ and ${\rm pin}^-$ structures. Translation invariance is broken by the boundary conditions, which leads to sharp features localized near the parity walls (fixed points of the reflection used to construct the Klein bottle). For a scalar field, there is a position-dependent energy density, peaked near the parity walls. A Dirac fermion can lead to breaking of parity, charge conjugation and $CP$ in 3+1 dimensions. Order parameters for this breaking are provided by the vevs of certain fermion bilinears, again peaked near the parity walls. As one potential application, these results suggest mechanisms for $CP$ violation and baryogenesis, enabled by compactification on a Klein bottle.

hep-th↗

arXiv2023

Back to the Future: Causality on a Moving Braneworld

Brane observers executing appropriate motion through a partially compactified Lorentz invariant bulk spacetime, such as $M_4 \times S^1$, can send signals along the brane that are instantaneous or even travel backward in time. Nevertheless, causality in the braneworld remains intact. We establish these results, which follow from superluminal signal propagation reported in arXiv:2206.13590, through classical analysis and then extend our reasoning by examining quantum mechanical microcausality. One implication is the capacity for real time communication across arbitrarily large distances.

gr-qc↗

arXiv2022

Charging up Boosted Black Holes

Contrary to a prevailing assumption that black holes would swiftly discharge, we argue that black holes can charge preferentially when boosted through an ambient magnetic field. Though the details are very different, the preference for charge is related to the precipitation of the Wald charge on a spinning black hole in an ambient magnetic field. The gravito-electrodynamics upstage naive arguments about screening electric fields in determining the value of the charge accrued. Charged test particles, which build up the black hole charge, exhibit chaotic behavior as evidenced by fractal basin boundaries between dynamical regions. Charged, boosted black holes will generate their own electromagnetic fields and thereby their own luminous signatures, even if they are initially bare. We therefore add boosted black holes to the growing list of potentially observable black hole signatures, alongside black hole batteries and black hole pulsars. The implications should be relevant for supermassive black holes that are boosted relative to a galactic magnetic field as well as black holes merging with magnetized neutron stars.

gr-qc↗

arXiv2022

Superluminal Propagation on a Moving Braneworld

We consider a braneworld scenario in the simplest setting, $M_4 \times S^1$, with a 4D Minkowski metric induced on the brane, and establish the possibility of superluminal propagation. If the brane is at rest, the 4D Lorentz symmetry of the brane is exact, but if the brane is in motion, it is broken globally by the compactification. By measuring bulk fields, an observer on the brane sees a slice through a higher-dimensional field profile, which carries an imprint of the extra dimensions even when the brane is at rest. If the brane is in motion we find that bulk fields can propagate outside the brane lightcone by a parametrically large amount set by the brane velocity. We mention observational tests and possible applications to cosmology.

hep-th↗

arXiv2021

Multi-Messenger Constraints on Magnetic Fields in Merging Black Hole-Neutron Star Binaries

The LIGO-Virgo-KAGRA Collaboration recently detected gravitational waves (GWs) from the merger of black-hole-neutron-star (BHNS) binary systems GW200105 and GW200115. No coincident electromagnetic (EM) counterparts were detected. While the mass ratio and BH spin in both systems were not sufficient to tidally disrupt the NS outside of the BH event horizon, other, magnetospheric mechanisms for EM emission exist in this regime and depend sensitively on the NS magnetic field strength. Combining GW measurements with EM flux upper limits, we place upper limits on the NS surface magnetic field strength above which magnetospheric emission models would have generated an observable EM counterpart. We consider fireball models powered by the black-hole battery mechanism, where energy is output in gamma-rays over $\lesssim1$~second. Consistency with no detection by Fermi-GBM or INTEGRAL SPI-ACS constrains the NS surface magnetic field to $\lesssim10^{15}$~G. Hence, joint GW detection and EM upper limits rule out the theoretical possibility that the NSs in GW200105 and GW200115, and the putative NS in GW190814, retain $\gtrsim10^{15}$~G dipolar magnetic fields until merger. They also rule out formation scenarios where strongly magnetized magnetars quickly merge with BHs. We alternatively rule out operation of the BH-battery powered fireball mechanism in these systems. This is the first multi-messenger constraint on NS magnetic fields in BHNS systems and a novel approach to probe fields at this point in NS evolution. This demonstrates the constraining power that multi-messenger analyses of BHNS mergers have on BHNS formation scenarios, the magnetic-field evolution in NSs, and the physics of BHNS magnetospheric interactions.

astro-ph.HE↗

arXiv2021

Penrose process for a charged black hole in a uniform magnetic field

Spinning black holes create electromagnetic storms when immersed in ambient magnetic fields, illuminating the otherwise epically dark terrain. In an electromagnetic extension of the Penrose process, tremendous energy can be extracted, boosting the energy of radiating particles far more efficiently than the mechanical Penrose process. We locate the regions from which energy can be mined and demonstrate explicitly that they are no longer restricted to the ergosphere. We also show that there can be toroidal regions that trap negative energy particles in orbit around the black hole. We find that the effective charge coupling between the black hole and the super-radiant particles decreases as energy is extracted, much like the spin of a black hole decreases in the mechanical analogue. While the effective coupling decreases, the actual charge of the black hole increases in magnitude reaching the energetically-favored Wald value, at which point energy extraction is impeded. We demonstrate the array of orbits for products from the electromagnetic Penrose process.

gr-qc↗

arXiv2019

Black Hole Pulsar

In anticipation of a LIGO detection of a black hole/neutron star merger, we expand on the intriguing possibility of an electromagnetic counterpart. Black hole/Neutron star mergers could be disappointingly dark since most black holes will be large enough to swallow a neutron star whole, without tidal disruption and without the subsequent fireworks. Encouragingly, we previously found a promising source of luminosity since the black hole and the highly-magnetized neutron star establish an electronic circuit -- a black hole battery. In this paper, arguing against common lore, we consider the electric charge of the black hole as an overlooked source of electromagnetic radiation. Relying on the well known Wald mechanism by which a spinning black hole immersed in an external magnetic field acquires a stable net charge, we show that a strongly-magnetized neutron star in such a binary system will give rise to a large enough charge in the black hole to allow for potentially observable effects. Although the maximum charge is stable, we show there is a continuous flux of charges contributing to the luminosity. Most interestingly, the spinning charged black hole then creates its own magnetic dipole to power a black hole pulsar.

astro-ph.HE↗

arXiv2016

Bright transients from strongly-magnetized neutron star-black hole mergers

Direct detection of black hole-neutron star pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet black hole-neutron star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the x ray to $γ$ rays for neutron star field strengths ranging from $10^{12}$G to $10^{16}$G respectively and a $10M_{\odot} $ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the neutron star magnetic-field strength, and may be observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per year for neutron star field strengths $\gtrsim 10^{14}$G. We also discuss a possible decaying post-merger event which could accompany this signal. As an electromagnetic counterpart to these otherwise dark pairs, the black-hole battery should be of great value to the development of multi-messenger astronomy in the era of aLIGO.

astro-ph.HE↗

arXiv2015

Fast Radio Bursts and Radio Transients from Black Hole Batteries

Most black holes (BHs) will absorb a neutron star (NS) companion fully intact, without tidal disruption, suggesting the pair will remain dark to telescopes. Even without tidal disruption, electromagnetic luminosity is generated from the battery phase of the binary when the BH interacts with the NS magnetic field. Originally the luminosity was expected in high-energy X-rays or gamma-rays, however we conjecture that some of the battery power is emitted in the radio bandwidth. While the luminosity and timescale are suggestive of fast radio bursts (FRBs; millisecond-scale radio transients) NS--BH coalescence rates are too low to make these a primary FRB source. Instead, we propose the transients form a FRB sub-population, distinguishable by a double peak with a precursor. The rapid ramp-up in luminosity manifests as a precursor to the burst which is $20\%-80\%$ as luminous, given 0.5~ms timing resolution. The main burst is from the peak luminosity before merger. The post-merger burst follows from the NS magnetic field migration to the BH, causing a shock. NS--BH pairs are especially desirable for ground-based gravitational wave (GW) observatories since the pair might not otherwise be detected, with electromagnetic counterparts greatly augmenting the scientific leverage beyond the GW signal. Valuably, the electromagnetic signal can break degeneracies in the parameters encoded in the GW as well as probe the NS magnetic field strength, yielding insights into open problems in NS magnetic field decay.

astro-ph.HE↗

arXiv2013

Big Black Hole, Little Neutron Star: Magnetic Dipole Fields in the Rindler Spacetime

As a black hole and neutron star approach during inspiral, the field lines of a magnetized neutron star eventually thread the black hole event horizon and a short-lived electromagnetic circuit is established. The black hole acts as a battery that provides power to the circuit, thereby lighting up the pair just before merger. Although originally suggested as a promising electromagnetic counterpart to gravitational-wave detection, the luminous signals are promising more generally as potentially detectable phenomena, such as short gamma-ray bursts. To aid in the theoretical understanding, we present analytic solutions for the electromagnetic fields of a magnetic dipole in the presence of an event horizon. In the limit that the neutron star is very close to a Schwarzschild horizon, the Rindler limit, we can solve Maxwell's equations exactly for a magnetic dipole on an arbitrary worldline. We present these solutions here and investigate a proxy for a small segment of the neutron star orbit around a big black hole. We find that the voltage the black hole battery can provide is in the range ~10^16 statvolts with a projected luminosity of 10^42 ergs/s for an M=10M_sun black hole, a neutron star with a B-field of 10^12 G, and an orbital velocity ~0.5c at a distance of 3M from the horizon. Larger black holes provide less power for binary separations at a fixed number of gravitational radii. The black hole/neutron star system therefore has a significant power supply to light up various elements in the circuit possibly powering jets, beamed radiation, or even a hot spot on the neutron star crust.

astro-ph.HE↗

arXiv2013

Kaluza-Klein Towers on General Manifolds

A higher-dimensional universe with compactified extra dimensions admits a four-dimensional description consisting of an infinite Kaluza-Klein tower of fields. We revisit the problem of describing the free part of the complete Kaluza-Klein tower of gauge fields, p-forms, gravity, and flux compactifications. In contrast to previous studies, we work with a generic internal manifold of any dimension, completely at the level of the action, in a gauge invariant formulation, and without resorting to the equations of motion or analysis of propagators. We demonstrate that the physical fields and Stuckelberg fields are naturally described by ingredients of the Hodge decomposition and its analog for symmetric tensors. The spectrum of states and stability conditions, in terms of the eigenvalues of various Laplacians on the internal manifold, is easily read from the action.

hep-th↗

arXiv2013

The Transient Gravitational-Wave Sky

Interferometric detectors will very soon give us an unprecedented view of the gravitational-wave sky, and in particular of the explosive and transient Universe. Now is the time to challenge our theoretical understanding of short-duration gravitational-wave signatures from cataclysmic events, their connection to more traditional electromagnetic and particle astrophysics, and the data analysis techniques that will make the observations a reality. This paper summarizes the state of the art, future science opportunities, and current challenges in understanding gravitational-wave transients.

gr-qc↗

arXiv2013

Observing complete gravitational wave signals from dynamical capture binaries

We assess the detectability of the gravitational wave signals from highly eccentric compact binaries. We use a simple model for the inspiral, merger, and ringdown of these systems. The model is based on mapping the binary to an effective single black hole system described by a Kerr metric, thereby including certain relativistic effects such as zoom-whirl-type behavior. The resultant geodesics source quadrupolar radiation and, in turn, are evolved under its dissipative effects. At the light ring, we attach a merger model that was previously developed for quasicircular mergers but also performs well for eccentric mergers with little modification. We apply this model to determine the detectability of these sources for initial, Enhanced, and Advanced LIGO across the parameter space of nonspinning close capture compact binaries. We conclude that, should these systems exist in nature, the vast majority will be missed by conventional burst searches or by quasicircular waveform templates in the advanced detector era. Other methods, such as eccentric templates or, more practically, a stacked excess power search, must be developed to avoid losing these sources. These systems would also have been missed frequently in the initial LIGO data analysis. Thus, previous null coincidence results with detected gamma-ray bursts cannot exclude the possibility of coincident gravitational wave signals from eccentric binaries.

gr-qc↗

arXiv2012

Repeated Bursts from Relativistic Scattering of Compact Objects in Galactic Nuclei

Galactic nuclei are densely populated by stellar mass compact objects such as black holes and neutron stars. Bound, highly eccentric binaries form as a result of gravitational wave (GW) losses during close flybys between these objects. We study the evolution of these systems using 2.5 and 3.5 order post-Newtonian equations of motion. The GW signal consists of many thousand repeated bursts (RB) for minutes to days (depending on the impact parameter and masses), followed by a powerful GW chirp and an eccentric merger. We show that a significant signal to noise ratio (SNR) accumulates already in the RB phase, corresponding to a detection limit around 200-300 Mpc and 300--600 Mpc for Advanced LIGO for an average orientation BH/NS or BH/BH binary, respectively. The theoretical errors introduced by the inaccuracy of the PN templates are typically much less severe for the RB phase than in the following eccentric merger. The GW signal in the RB phase is broadband; we show that encounters involving intermediate mass black holes are detectable in multiple frequency bands coincidentally using LIGO and LISA.

astro-ph.CO↗

arXiv2011

Faster computation of adiabatic EMRIs using resonances

Motivated by the prohibitive computational cost of producing adiabatic extreme mass ratio inspirals, we explain how a judicious use of resonant orbits can dramatically expedite both that calculation and the generation of snapshot gravitational waves from geodesic sources. In the course of our argument, we clarify the resolution of a lingering debate on the appropriate adiabatic averaging prescription in favor of torus averaging over time averaging.

gr-qc↗

arXiv2011

Inspiral of Generic Black Hole Binaries: Spin, Precession, and Eccentricity

Given the absence of observations of black hole binaries, it is critical that the full range of accessible parameter space be explored in anticipation of future observation with gravitational wave detectors. To this end, we compile the Hamiltonian equations of motion describing the conservative dynamics of the most general black hole binaries, as computed by Will and collaborators, and incorporate an effective treatment of dissipation through gravitational radiation. We evolve these equations for systems with orbital eccentricity and precessing spins. We find that, while spin-spin coupling corrections can destroy constant radius orbits in principle, the effect is so small that orbits will reliably tend to quasi-spherical orbits as angular momentum and energy are lost to gravitational radiation. Still, binaries that are initially highly eccentric may retain eccentricity as they pass into the detectable bandwidth of ground-based gravitational wave detectors. We also show that a useful set of natural frequencies for an orbit demonstrating both spin precession and periastron precession is comprised of (1) the frequency of angular motion in the orbital plane, (2) the frequency of the plane precession, and (3) the frequency of radial oscillations. These three natural harmonics shape the observed waveform.

gr-qc↗

arXiv2011

The harmonic structure of generic Kerr orbits

Generic Kerr orbits exhibit intricate three-dimensional motion. We offer a classification scheme for these intricate orbits in terms of periodic orbits. The crucial insight is that for a given effective angular momentum $L$ and angle of inclination $ι$, there exists a discrete set of orbits that are geometrically $n$-leaf clovers in a precessing {\it orbital plane}. When viewed in the full three dimensions, these orbits are periodic in $r-θ$. Each $n$-leaf clover is associated with a rational number, $1+q_{rθ}=ω_θ/ω_r$, that measures the degree of perihelion precession in the precessing orbital plane. The rational number $q_{rθ}$ varies monotonically with the orbital energy and with the orbital eccentricity. Since any bound orbit can be approximated as near one of these periodic $n$-leaf clovers, this special set offers a skeleton that illuminates the structure of all bound Kerr orbits, in or out of the equatorial plane.

gr-qc↗