arXiv subjects

David I. Kaiser

Publications and source records attributed to David I. Kaiser.

At least 19 recordsLinked to original sources

arXiv2026

Electromagnetic Signatures From Primordial Black Holes in the Solar System

Primordial black holes (PBHs) in the asteroid-mass range, with typical masses $10^{17}\,{\rm g}\lesssim M \lesssim 10^{23}\,{\rm g}$, have drawn significant recent attention as a viable dark matter candidate. The peak frequencies of photons emitted via Hawking radiation from asteroid-mass PBHs range from infrared to $γ$-ray bands. We calculate expected local transit rates for extended PBH mass distributions which could comprise all the dark matter. We evaluate prospects for detecting Hawking-radiated photons from local PBH transits through the inner Solar System and from PBH explosions in the far outer edges of the Solar System. We consider several existing and proposed ground-based and space-based instruments sensitive to photons from the radio band to ultrahigh energy $γ$-rays. We find that proposed instruments, such as the AMEGO-X satellite, could reliably detect PBH transits within ${\cal O} (0.1 \, {\rm AU})$ of the Earth, while the HAWC and LHAASO observatories are both sensitive to PBH explosions out to ${\cal O}(0.1 \, {\rm pc})$ and ${\cal O}(0.5 \, {\rm pc})$ respectively. We conclude by specifically considering potential companion electromagnetic signatures in the case of a PBH explosion about $10^3\,{\rm AU}$ from Earth, which has been suggested as a potential source for the $\sim 220 \, {\rm PeV}$ ultrahigh-energy KM3-230213A neutrino event observed by the KM3NeT collaboration in 2023. Whereas we find that the recent KM3NeT event would not have yielded detectable electromagnetic signals -- due to its location on the sky, proposed distance from Earth, and the offline status of the HAWC observatory at that time -- we demonstrate that future PBH explosions at comparable distances could yield measurable electromagnetic signals at Earth, depending on alignment of the PBH burst with detector fields of view.

hep-ph↗

arXiv2026

Baryogenesis from Exploding Primordial Black Holes

Exploding primordial black holes can source baryon asymmetry soon after the electroweak phase transition, as high-energy Hawking radiation drives ultrarelativistic shocks in the surrounding plasma. The shocks and their trailing rarefaction waves delineate two bubble-like walls around a shell of superheated fluid, in which electroweak symmetry is restored. These moving interfaces source chiral charge, which is converted to baryon number. Upon adding a simple CP-violating operator at the TeV scale, this mechanism yields the observed baryon asymmetry with minimal dependence on PBH model parameters.

hep-ph↗

arXiv2026

Shocks from Exploding Primordial Black Holes in the Early Universe

We investigate how Hawking radiation from low-mass primordial black holes deposits energy into the early-universe plasma and show that the resulting phenomena are hydrodynamic rather than purely diffusive. Combining analytic arguments with relativistic hydrodynamic simulations, we find that the plasma first develops a quasi-steady outflow during the slow evaporation stage, while the final runaway phase of evaporation produces an expanding fireball that launches a shock wave into the surrounding medium. We characterize the thermalization scale of the Hawking products, the conditions under which shocks form, and the evolution and propagation of shocks. Additionally, we show that these shocks can locally restore electroweak symmetry, identifying exploding PBHs as a potentially important source of out-of-equilibrium dynamics in the early universe with profound phenomenological implications.

astro-ph.CO↗

arXiv2026

Gravitational Ionization by Schwarzschild Primordial Black Holes

Primordial black holes (PBHs) are theorized to form from the collapse of overdensities in the very early Universe. PBHs in the asteroid-mass range $10^{17} \, {\rm g}\lesssim M \lesssim 10^{23} \, {\rm g}$ could serve as all or most of the dark matter today, but are particularly difficult to detect due to their modest rates of Hawking emission and sub-micron Schwarzschild radii. We consider whether the steep gradients of a PBH's gravitational field could generate tidal forces strong enough to disrupt atoms and nuclei. Such phenomena may yield new observables that could uniquely distinguish a PBH from a macroscopic object of the same mass. We first consider the gravitational ionization of ambient neutral hydrogen and evaluate prospects for detecting photon radiation from the recombination of ionized atoms. During the present epoch, this effect would be swamped by Hawking radiation -- which would itself be difficult to detect for PBHs at the upper end of the asteroid-mass window. We then consider the gravitational ionization and heating of neutral hydrogen immediately following recombination at $z\simeq1090$, and identify a broad class of PBH distributions with typical mass $5\times10^{21}\,{\rm g}\lesssim M \lesssim 10^{23}\, {\rm g}$ within which gravitational interactions would have been the dominant form of energy deposition to the medium. We also identify conditions under which tidal forces from a transiting PBH could overcome the strong nuclear force, either by dissociating deuterons, which would be relevant during big bang nucleosynthesis (BBN), or by inducing fission of heavy nuclei. We find that gravitational dissociation of deuterons dominates photodissociation rates due to Hawking radiation for PBHs with masses $10^{14}\,{\rm g}\lesssim M \lesssim 10^{16}\,{\rm g}$. We additionally identify the phenomenon of gravitationally induced fission of heavy nuclei via tidal deformation.

hep-ph↗

arXiv2026

Hawking Radiation Signatures from Primordial Black Holes Transiting the Inner Solar System: Prospects for Detection

Primordial black holes (PBHs) arise from the collapse of density perturbations in the early universe and serve as a dark matter (DM) candidate and a probe of fundamental physics. There remains an unconstrained ``asteroid-mass'' window where PBHs of masses $10^{17} {\rm g} \lesssim M \lesssim 10^{23} {\rm g}$ could comprise up to $100\%$ of the dark matter. Current $e^{\pm}$ Hawking radiation constraints on the DM fraction of PBHs are set by comparing observed spatial- and time-integrated cosmic ray flux measurements with predicted Hawking emission fluxes from the galactic DM halo. These constraints depend on cosmic ray production and propagation models, the galactic DM density distribution, and the PBH mass function. We propose to mitigate these model dependencies by developing a new local, time-dependent Hawking radiation signature to detect low-mass PBHs transiting through the inner Solar System. We calculate transit rates for PBHs that form with initial masses $M \lesssim 5\times10^{17}\text{g}$. We then simulate time-dependent positron signals from individual PBH flybys as measured by the Alpha Magnetic Spectrometer (AMS) experiment in low-Earth orbit. We find that AMS is sensitive to PBHs with masses $M\lesssim 2\times10^{14} \, {\rm g}$ due to its lower energy threshold of $500 \, {\rm MeV}$. We demonstrate that a dataset of daily positron fluxes over the energy range $5-500 \, {\rm MeV}$, with similar levels of precision to the existing AMS data, would enable detection of PBHs drawn from present-day distributions that peak within the asteroid-mass window. Our simulations yield ${\cal O} (1)$ detectable PBH transits per year across wide regions of parameter space, which may be used to constrain PBH mass functions. This technique could be extended to detect $γ$-ray and X-ray Hawking emission to probe further into the asteroid-mass window.

astro-ph.CO↗

arXiv2025

Primordial Black Holes from Inflation with a Spectator Field

How is the production of primordial black holes (PBHs) in single-field models of inflation impacted by the presence of additional scalar fields? We consider the effect of a spectator field - a free scalar field with sub-Hubble mass, no direct coupling to the inflaton, and which makes a subdominant contribution to the total energy density - in the context of single-field models of inflation featuring a transient phase of ultra-slow roll (USR) evolution. Despite the modest title, a spectator field can have a dramatic impact: the slow-roll evolution of the spectator prevents the combined inflaton-and-spectator system from entering into USR, which naively might be expected to preclude the production of PBHs. However, we demonstrate that the growth of perturbations is maintained or enhanced by the spectator, through the rich interplay of curvature and isocurvature perturbations. We show in a model-independent way that the single-field phase of ultra-slow-roll is replaced by two turns in field space encompassing a phase of tachyonic instability for the isocurvature perturbations and a transfer of power from isocurvature to curvature modes. Furthermore, we highlight a degeneracy between the fine-tuning of the feature in the inflaton potential and the parameters of the spectator, leading to an overall resilience of model predictions to parameter variations. This makes it easier for the underlying PBH model to accommodate both high-precision CMB constraints and production of PBHs in the asteroid-mass range.

astro-ph.CO↗

arXiv2025

Gravitational wave signals from primordial black holes orbiting solar-type stars

Primordial black holes (PBHs) with masses between $10^{14}$ and $10^{20}$ kg are candidates to contribute a substantial fraction of the total dark matter abundance. When in orbit around the center of a star, which can possibly be a completely interior orbit, such objects would emit gravitational waves, as predicted by general relativity. In this work, we examine the gravitational wave signals emitted by such objects when they orbit typical stars, such as the Sun. We show that the magnitude of the waves that could eventually be detected on Earth from a possible PBH orbiting the Sun or a neighboring Sun-like star within our galaxy can be significantly stronger than those originating from a PBH orbiting a denser but more distant neutron star (NS). Such signals may be detectable by the LISA gravitational-wave detector. In addition, we estimate the contribution that a large collection of such PBH-star systems would make to the stochastic gravitational-wave background (SGWB) within a range of frequencies to which pulsar timing arrays are sensitive.

gr-qc↗

arXiv2025

Ultra-High-Energy Neutrinos from Primordial Black Holes

The KM3NeT Collaboration recently announced the detection of a neutrino with energy 220 PeV. One possible source of such ultra-high-energy particles is the rapid emission of energetic Hawking radiation from a primordial black hole (PBH) near the end of its evaporation lifetime. The mass distribution for PBHs features a power-law tail for small masses; a small subset of PBHs would be undergoing late-stage evaporation today. We find that recent high-energy neutrino events detected by the IceCube and KM3NeT Collaborations, with energies ${\cal O} (1 - 10^2) \, {\rm PeV}$, are consistent with event-rate expectations if a significant fraction of the dark matter consists of PBHs.

hep-ph↗

arXiv2025

Toward quantum tunneling from excited states: Recovering imaginary-time instantons from a real-time analysis

We revisit the path integral description of quantum tunneling and lay the groundwork for its generalization to excites states through real-time path integral techniques. For clarity, we focus on the simple toy model of a point particle in a double-well potential, for which we perform all steps explicitly. Instead of performing the familiar Wick rotation from physical to imaginary time -- which is inconsistent with the requisite boundary conditions when treating tunneling from states other than the false vacuum -- we regularize the path integral by adding an infinitesimal complex contribution to the Hamiltonian, while keeping time strictly real. We find that this gives rise to a complex stationary-phase solution, in agreement with recent insights from Picard-Lefshitz theory. We then show that there exists a class of analytic solutions for the corresponding equations of motion, which can be made to match the appropriate boundary conditions in the physically relevant limits of a vanishing regulator and an infinite physical time. We provide a detailed discussion of this non-trivial limit. We find that, for systems without an explicit time-dependence, our approach reproduces the picture of an instanton-like solution defined on a finite Euclidean-time interval. Lastly, we discuss the generalization of our approach to broader classes of systems, for which it serves as a reliable framework for high-precision calculations.

hep-th↗

arXiv2025

Precision Unitarity Calculations in Inflationary Models

We revisit perturbative unitarity in scalar field inflation with a nonminimal coupling, with Higgs inflation serving as the most prominent example. Although such models are phenomenologically successful, it is critical to examine whether or not unitarity violations spoil their theoretical self-consistency. The analysis of these issues has so far typically relied on order-of-magnitude estimates of scattering amplitudes, which are appropriate for generic parameters. It is not evident that these methods apply to scenarios relying on a near-critical inflationary potential, for which an interplay of both small scalar self-couplings and nonminimal couplings could partially alleviate the unitarity issues. To allow for an exploration of this possibility, we consider the full $S$-matrix for the relevant scattering processes, taking into account important phase space volume factors, leading to a precise evaluation of the cut-off scale. In the single-field case, we demonstrate that near-criticality raises the cut-off scale considerably, compared to previous estimates. In the multifield case, momentum-dependent self-interactions in the kinetic sector lower the cut-off compared to the single-field case to a value comparable to but slightly larger than previous estimates. We carefully study both the single-field and multifield cases in metric and metric-affine (Palatini) formulations of gravity, as well as introduce a new phenomenologically viable model with a canonical kinetic term and a significantly raised cut-off, and discuss the importance of background field effects.

hep-ph↗

arXiv2025

Finite-Temperature Instantons from First Principles

We derive the finite-temperature quantum-tunneling rate from first principles. The rate depends on both real- and imaginary-time; we demonstrate that the relevant instantons should therefore be defined on a Schwinger-Keldysh contour, and how the familiar Euclidean-time result arises from it in the limit of large physical times. We generalize previous results for general initial states, and identify distinct behavior in the high- and low-temperature limits, incorporating effects from background fields. We construct a consistent perturbative scheme that incorporates large finite-temperature effects.

hep-th↗

arXiv2025

Light Scalar Fields Foster Production of Primordial Black Holes

Scalar fields are ubiquitous in theories of high-energy physics. In the context of cosmic inflation, this suggests the existence of spectator fields, which provide a subdominant source of energy density. We show that spectator fields boost the inflationary production of primordial black holes, with single-field ultra-slow roll evolution supplanted by a phase of evolution along the spectator direction, and primordial perturbations amplified by the resulting multifield dynamics. This generic mechanism is largely free from the severe fine-tuning that afflicts single-field inflationary PBH models.

astro-ph.CO↗

arXiv2025

Tracking the nonlinear formation of an interfacial wave spectral cascade from one to few to many

A hallmark of far-from-equilibrium systems is the emergence of a spectral cascade, where energy is transferred across length-scales following a simple power law. The universal nature of this phenomenon has led to advances in a range of disciplines, including climate forecasting, foreign exchange trading, and the modelling of neurological activity. For many diverse far-from-equilibrium scenarios, the scaling laws of steady states have been successfully predicted by the statistical theory of weak wave turbulence, originally developed by considering the leading order interactions between waves on a fluid surface. However, the predictive power of weak wave turbulence breaks down in the presence of large amplitudes, high dissipation, and finite-size effects. We offer new insight into these regimes by experimentally tracking the formation of a spectral cascade under these conditions in an externally driven fluid-fluid interface. We resolve individual wave modes and observe their time evolution from one to few to many, a process culminating in a steady state with a spectral density characterised by a power-law scaling. Our findings confirm that interfacial dynamics can be effectively modelled by a weakly nonlinear Lagrangian theory, a predictive framework encompassing both underlying wave interaction and emergent behaviours of the system. Such nonlinear interactions are experimentally quantified through statistical correlations, revealing a hierarchy in wave-mixing order that confirms a key assumption of weak wave turbulence. The Lagrangian formulation further aids our time-evolution analysis; specific interactions are tracked through time, and we predict the timescale until a cascade emerges. Our findings are transferable to other far-from-equilibrium systems, which we demonstrate by providing a mapping to reheating scenarios following cosmic inflation in the early Universe.

gr-qc↗

arXiv2024

Close encounters of the primordial kind: a new observable for primordial black holes as dark matter

Primordial black holes (PBHs) remain a viable dark matter candidate in the asteroid-mass range. We point out that in this scenario, the PBH abundance would be large enough for at least one object to cross through the inner Solar System per decade. Since Solar System ephemerides are modeled and measured to extremely high precision, such close encounters could produce detectable perturbations to orbital trajectories with characteristic features. We evaluate this possibility with a suite of simple Solar System simulations, and we argue that the abundance of asteroid-mass PBHs can plausibly be probed by existing and near-future data.

astro-ph.CO↗

arXiv2024

Nonminimal Couplings and the Forgotten Field of Axion Inflation

We study the multifield dynamics of axion models nonminimally coupled to gravity. As usual, we consider a canonical $U(1)$ symmetry-breaking model in which the axion is the phase of a complex scalar field. If the complex scalar field has a nonminimal coupling to gravity, then the (oft-forgotten) radial component can drive a phase of inflation prior to an inflationary phase driven by the axion field. In this setup, the mass of the axion field is dependent on the radial field because of the nonminimal coupling, and the axion remains extremely light during the phase of radial inflation. As the radial field approaches the minimum of its potential, there is a transition to a second phase of inflation during which the axion field contributes a substantial fraction of the system's total energy density, yielding a phase of "natural inflation." This system exhibits ultra-light isocurvature perturbations, which are converted to adiabatic perturbations by a turning field-space trajectory. For models wherein the CMB pivot scale exited the horizon well before the turn, this acts to suppress the tensor-to-scalar ratio $r$, without generating CMB non-Gaussianity or observable isocurvature perturbations. Finally, we note that for certain parameters the interaction strength between axion and gauge fields can be suppressed during the first phase of inflation relative to its value during the second phase by several orders of magnitude. This decouples the constraints on the inflationary production of gauge fields (e.g., from primordial black holes) from the constraints on their production during (p)reheating.

hep-th↗

arXiv2024

Natural Inflation with Exponentially Small Tensor-To-Scalar Ratio

We demonstrate that "natural inflation", also known as "axion inflation", can be compatible with Planck 2018 measurements of the cosmic microwave background, while predicting an exponentially small tensor-to-scalar ratio, e.g., $r\sim 10^{-15}$. The strong suppression of $r$ arises from dynamics of the radial component of the complex scalar field, whose phase is the axion. Such tiny values of $r$ remain well below the threshold for detection by CMB-S4 or Simons Observatory B-mode searches. The model is testable with the running $α_s$ of the spectral index, which is within reach of next-generation CMB and large-scale structure experiments, motivating the running as a primary science goal for future experiments.

astro-ph.CO↗

arXiv2024

Orbital motion of primordial black holes crossing Solar-type stars

Primordial black holes (PBHs) are hypothetical objects that could have originated from density fluctuations in a very early phase of our Universe. Recent observations restrict the masses that such PBHs could have, if they are to constitute all of dark matter today: $10^{17} \, {\rm g} \leq m \leq 10^{23} \, {\rm g}$. With such low masses, general relativity predicts that the corresponding radii for the PBHs would be atomic or subatomic in size. When captured by a star, such a tiny PBH could exhibit an orbit completely or partially inside the body of the star, without significantly changing its mass for quite a long time. Here we examine the possible trajectories of a PBH that is captured by a Sun-like star. When in motion in the interior of the star, the amount of stellar mass that effectively interacts with the PBH will be a function of its distance to the center of the star. As a consequence, a strong effect on the shape of the orbits emerges, leading to PBH trajectories that could be open or closed, and exhibiting a rich variety of patterns.

astro-ph.CO↗

arXiv2024

Primordial Black Holes with QCD Color Charge

We describe a realistic mechanism whereby black holes with significant QCD color charge could have formed during the early universe. Primordial black holes (PBHs) could make up a significant fraction of the dark matter if they formed well before the QCD confinement transition. Such PBHs would form by absorbing unconfined quarks and gluons, and hence could acquire a net color charge. We estimate the number of PBHs per Hubble volume with near-extremal color charge for various scenarios, and discuss possible phenomenological implications.

hep-ph↗