arXiv subjects

Djuna Croon

Publications and source records attributed to Djuna Croon.

At least 19 recordsLinked to original sources

arXiv2026

Supernova microlensing as a probe of ultracompact minihalos and primordial cosmology

We propose using supernova microlensing to search for ultracompact minihalos and other extended dark structures seeded by enhanced primordial small-scale power. Unlike conventional microlensing searches, which are often optimized for primordial black holes, cosmological supernovae are sensitive to lenses with physical sizes comparable to their Einstein radii (up to $\sim {\rm pc}\left(M/10^4M_\odot\right)^{1/2}$ scale), opening a qualitatively different region of dark object parameter space. We compute the extended object detection efficiency relative to point lenses. We then show how projected limits on the abundance of ultracompact minihalos can be mapped, in benchmark formation scenarios, onto constraints on the primordial power spectrum. With the Vera C. Rubin Observatory expected to deliver an unprecedented LSST supernova sample, supernova microlensing is poised to become a timely and complementary probe of small-scale structure and the physics of the early Universe.

astro-ph.CO↗

arXiv2026

The location of the upper edge of the pair-instability supernovae black hole mass gap

Gravitational wave observations are beginning to probe the upper edge of the pair-instability supernova (PISN) black hole mass gap, a key prediction of stellar evolution. In this work, we quantify the sensitivity of this boundary to uncertainties in stellar evolution using a suite of simulations that vary inputs including nuclear reaction rates, mixing processes, and stellar winds. We find that the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ reaction rate is the dominant source of uncertainty, shifting the upper edge by $\Delta M\sim30\,{\rm M}_\odot$, with the triple-$\alpha$ rate producing a comparable shift of $\sim25\,{\rm M}_\odot$. Notably, $^{16}{\rm O}+^{16}{\rm O}$ reactions shift the upper edge by $\sim15\,{\rm M}_\odot$ while leaving the lower edge unchanged, implying they can widen or narrow the mass gap. Other processes affect the location at the $\lesssim10\,{\rm M}_\odot$ level. In contrast to the lower edge, we find that the upper edge is robust to variations in spatial and temporal resolution, indicating that it is reliably resolved in current simulations. Our results demonstrate that the upper edge carries substantial theoretical uncertainty and, while comparatively less affected by astrophysical contamination than the lower edge, provides a direct probe of the nuclear processes governing pair instability. We discuss the implications for interpreting high-mass black hole detections in gravitational wave data.

astro-ph.HE↗

arXiv2026

Searching for ultralight bosons with Josephson junction interferometry

Ultralight bosons sourced by macroscopic objects can generate long-range spin-independent and spin-dependent potentials that are accessible to precision interferometry. Such potentials induce phase shifts in Josephson junctions, detectable through precision current measurements. We propose three experimental scenarios to probe photophilic scalar interactions, Lorentz-violating scalar-mediated interactions, and axion-mediated monopole-dipole interactions, depending on the nature (unpolarized or polarized) of the source. The proposed setups provide sensitivities to novel mixed couplings that are largely unconstrained by existing bounds and enables the exploration of new forces at centimeter to micrometer length scales.

hep-ph↗

arXiv2025

Caustic crossings in giant arcs with extended dark matter objects

Caustic-crossing stars observed in giant arcs behind galaxy clusters provide a powerful probe of dark matter substructure. While previous work has focused on point-like lenses such as primordial black holes, we extend this framework to extended dark objects (EDOs), including ultracompact minihalos formed from the collapse of primordial overdensities. We develop an analytic model of microlensing by EDOs embedded in a macrolensing cluster potential and derive the resulting caustics and light curves. Depending on the EDO size relative to the effective Einstein radius, we show that they may generate additional narrow caustics, leading to novel features in the light curve. Applying our framework to the MACS J1149 LS1 ``Icarus'' event, we constrain EDOs with radii up to $10^7 R_\odot$. Our results demonstrate that caustic-crossing events complement galactic microlensing searches, as they can probe EDOs with larger physical size. We discuss the implications for current and future observations, which promise to deliver a statistical sample of caustic transients and correspondingly sharper constraints on dark objects.

astro-ph.CO↗

arXiv2025

Dark matter silences Cepheids in the Galactic Center

Upcoming near-infrared facilities (e.g. JWST/NIRCam, ELT/MICADO) will dramatically increase the detectability of galactic center Cepheids despite extreme extinction at optical wavelengths. In this work, we study the impact of dark matter (DM) annihilation on Cepheid stars in the inner parsec of the Milky Way. We show that at captured densities $\rho\sim10^5\,$GeV cm$^{-3}$, blue loop evolution can be suppressed, preventing the formation of low-mass ($3$-$6~{\rm M}_\odot$) short-period ($1$-$6$ days) Cepheids. For even slightly higher DM densities, Cepheids are suppressed across their entire mass range. A dearth of such variables could provide indirect evidence for DM heating. Notably, this effect occurs at lower DM densities than required to impact main-sequence stars. Future surveys will thus offer a novel, complementary probe of DM properties in galactic nuclei.

hep-ph↗

arXiv2025

Can GW231123 have a stellar origin?

The gravitational wave event GW231123 detected by the LIGO interferometers during their fourth observing run features two black holes with source-frame masses of $137^{+23}_{-18} M_\odot$ and $101^{+22}_{-50} M_\odot$ -- in the range of the pair-instability black hole mass gap predicted by standard stellar evolution theory. Both black holes are also inferred to be rapidly spinning ($\chi_1 \simeq 0.9$, $\chi_2 \simeq 0.8$). The primary object in GW231123 is the heaviest stellar mass black hole detected to date, which, together with its extreme rotation, raises questions about its astrophysical origin. Accounting for the unusually large spin of $\sim 0.9$ with hierarchical mergers requires some degree of fine tuning. We investigate whether such a massive, highly spinning object could plausibly form from the collapse of a single rotating massive star. We simulate stars with an initial core mass of $160\,M_\odot$ -- sufficient to produce BH masses at the upper edge of the 90\% credible interval for $m_1$ in GW231123 -- across a range of rotation rates and $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ reaction rates. We allow for differential rotation to explore the high-spin regime. In this limit of weak angular momentum transport, we find that: (i) rotation shifts the pair-instability mass gap to higher masses, introducing an important correlation between masses and spins in gravitational wave predictions; and (ii) highly spinning BHs with masses $\gtrsim 150 \rm M_\odot$ can form above the mass gap. Our results suggest that the primary object of GW231123 may be the first directly observed black hole that formed via direct core collapse following the photodisintegration instability.

astro-ph.HE↗

arXiv2025

Interpreting the 95 GeV resonance in the Two Higgs Doublet Model: Implications for the Electroweak Phase Transition

We investigate if the recent mass resonance excesses seen around 95 GeV at the Large Hadron Collider (LHC) can be reconciled with a first-order electroweak phase transition. Performing the first large-scale parameter scan of the Type I Two Higgs Doublet Model (2HDM) using high-temperature dimensionally reduced effective field theory, we focus on regions of parameter space consistent with interpreting the excess as an additional pseudoscalar state. We find that, in contrast to the Standard Model, the electroweak transition pattern in the 2HDM is generically first-order, proceeding either in a single or in two steps. While transition strengths can reach up to $v_c/T_C \sim 1.3$, the viable, collider-constrained parameter space yields $v_c/T_C \lesssim 1$. Thus, the gravitational wave signals lie below the projected reach of future interferometer experiments and are likely insufficient to support successful electroweak baryogenesis.

hep-ph↗

arXiv2025

Dark Classification Matters: Searching for Primordial Black Holes with LSST

We present projected constraints on the abundance of primordial black holes (PBHs) as a constituent of dark matter, based on microlensing observations from the upcoming Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory. We use a catalogue of microlensing light curves simulated with Rubin Observatory's OpSims to demonstrate that competitive constraints crucially rely on minimising the false positive rate (FPR) of the classification algorithm. We propose the Bayesian information criterion and a Boosted Decision Tree as effective discriminators and compare their derived efficiency and FPR to a more standard $\chi^2$-test.

astro-ph.CO↗

arXiv2025

Updated constraints on the primordial power spectrum at sub-Mpc scales

The primordial power spectrum of matter density perturbations contains highly valuable information about new fundamental physics, in particular cosmological inflation, but is only very weakly constrained observationally for small cosmological scales $k\gtrsim 3\,{\rm Mpc}^{-1}$. We derive novel constraints, $\mathcal{P}_\mathcal{R}(k)\lesssim 5\cdot10^{-6}$ over a large range of such scales, from the formation of ultracompact minihalos in the early universe. Unlike most existing constraints of this type, our results do not rest on the assumption that dark matter can annihilate into ordinary matter.

astro-ph.CO↗

arXiv2025

Anomaly Detection to identify Transients in LSST Time Series Data

We introduce a novel approach to detecting microlensing events and other transients in light curves, utilising the isolation forest (iForest) algorithm for anomaly detection. Focusing on the Legacy Survey of Space and Time by the Vera C. Rubin Observatory, we show that an iForest trained on signal-less light curves can efficiently identify microlensing events by different types of dark objects and binaries, as well as variable stars. We further show that the iForest has real-time applicability through a drip-feed analysis, demonstrating its potential as a valuable tool for LSST alert brokers to efficiently prioritise and classify transient candidates for follow-up observations.

astro-ph.SR↗

arXiv2024

Gravitational Waves from Phase Transitions

We summarise the physics of first-order phase transitions in the early universe, and the possible ways in which they might come about. We then focus on gravitational waves, emphasising general qualitative features of stochastic backgrounds produced by early universe phase transitions and the cosmology of their present-day appearance. Finally, we conclude by discussing some of the ways in which a stochastic background might be detected.

hep-ph↗

arXiv2024

Hot Leptogenesis

We investigate a class of leptogenesis scenarios in which the sector containing the lightest right-handed neutrino establishes kinetic equilibrium at a temperature $T_{N_1} > T_\text{SM}$, where $T_\text{SM}$ is the temperature of the Standard Model sector. We study the reheating processes which realise this "hot leptogenesis" and the conditions under which kinetic and chemical equilibrium can be maintained. We derive and solve two sets of evolution equations, depending on the presence of chemical equilibrium within the hot sector, and numerically solve these for benchmark scenarios. We compare the viable parameter space of this model with standard leptogenesis scenarios with a thermal initial condition and find that hot leptogenesis resolves the neutrino and Higgs mass fine-tuning problems present in the standard scenario.

hep-ph↗

arXiv2024

Cosmology and Astrophysics of CP-Violating Axions

We study the cosmology and astrophysics of axion-like particles (ALPs) with CP-violating Yukawa couplings to nucleons. At finite nucleon density, the ALP's dynamics is governed by an effective potential which is the sum of the bare periodic potential and a linear potential whose strength depends on the nucleon density. We identify a critical nucleon density $\rho_c$ controlling the dynamics. At densities smaller than $\rho_c$ the effective potential is a tilted sinusoidal curve and the field is displaced from its zero-density minimum. At densities larger than $\rho_c$ the minima (and maxima) are absent, and the ALP is destabilized. Astrophysically, this implies that neutron stars can source a radial ALP field, providing a complementary probe to equivalence principle tests. Cosmologically, the ALP may have been destabilized in the early Universe and could have made large field excursions. We discuss model-building applications of our results for such early universe scenarios.

hep-ph↗

arXiv2024

Dark Dwarfs: Dark Matter-Powered Sub-Stellar Objects Awaiting Discovery at the Galactic Center

We investigate the effects of dark matter annihilation on objects with masses close to the sub-stellar limit, finding that the minimum mass for stable hydrogen burning is larger than the $\sim0.075 M_\odot $ value predicted in the Standard Model. Below this limit, cooling brown dwarfs evolve into stable dark matter-powered objects that we name dark dwarfs. The timescale of this transition depends on the ambient dark matter density $\rho_{\rm DM}$ and circular velocity $v_{\rm DM}$ but is independent of the dark matter mass. We predict a population of dark dwarfs close to the galactic center, where the dark matter density is expected to be $\rho_{\rm DM}\gtrsim 10^{3}$ GeV/cm$^3$. At larger galactic radii the dark matter density is too low for these objects to have yet formed within the age of the universe. Dark dwarfs retain their initial lithium-7 in mass ranges where brown/red dwarfs would destroy it, providing a method for detecting them.

hep-ph↗

arXiv2024

Repository for extended dark matter object constraints

Extended dark matter objects (EDOs) are popular dark matter candidates that interact gravitationally with the Standard Model. These gravitational interactions can be used to constrain their allowed parameter space. However, EDOs can have different formation mechanisms, sizes, and shapes, requiring a case-by-case analysis when studying their impact on different areas of cosmology. We thus present a repository of all available bounds for these objects, with a code that allows plotting user-defined combinations of all up-to-date bounds for a given shape and different radii. We propose a standard for the EDOs' mass profiles so that different sets of bounds are consistent with each other, and provide instructions on using the code and contributing to the repository.

astro-ph.CO↗

arXiv2024

Multi-Generational Black Hole Population Analysis with an Astrophysically Informed Mass Function

We analyze the population statistics of black holes in the LIGO/Virgo/KAGRA GWTC-3 catalog using a parametric mass function derived from simulations of massive stars experiencing pulsational pair-instability supernovae (PPISN). Our formalism enables us to separate the black hole mass function into sub-populations corresponding to mergers between objects formed via different astrophysical pathways, allowing us to infer the properties of black holes formed from stellar collapse and black holes formed via prior mergers separately. Applying this formalism, we find that this model fits the data better than the powerlaw+peak model with Bayes factor $ 9.7\pm0.1$. We measure the location of the lower edge of the upper black hole mass gap to be $M_{\rm BHMG}=84.05_{-12.88}^{+17.19}{\rm M}_{\odot}$, providing evidence that the $35{\rm M}_{\odot}$ Gaussian peak detected in the data using other models is not associated with the PPISN pile-up predicted to precede this gap. Incorporating spin, we find that the normalized spins of stellar remnant black holes are close to zero while those of higher generation black holes tend to larger values. All of these results are in accordance with the predictions of stellar structure theory and black hole merger scenarios. Finally, we combine our mass function with the spectral siren method for measuring the Hubble constant to find $H_0=36.19_{-10.91}^{17.50}$ km/s/Mpc and discuss potential explanations of this low value. Our results demonstrate how astrophysically-informed mass functions can facilitate the interpretation of gravitational wave catalog data to provide information about black hole formation and cosmology. Future data releases will improve the precision of our measurements.

astro-ph.HE↗

arXiv2024

Cosmic microwave background constraints on extended dark matter objects

Primordially formed extended dark objects would accrete baryonic matter and impact the ionisation history of the Universe. Insisting on consistency with the anisotropies of the cosmic microwave background, we derive constraints on the dark matter fraction for various classes of objects, of different sizes. We introduce a novel scaling technique to speed up numerical calculations and release our calculation framework in the form of a Mathematica notebook. Conservatively, we focus on spherical accretion and collisional ionisation. We find strong constraints limiting the dark matter fraction to subpercent level for objects of up to $10^4$ AU in size.

astro-ph.CO↗

arXiv2024

Microlensing signatures of extended dark objects using machine learning

This paper presents a machine learning-based method for the detection of the unique gravitational microlensing signatures of extended dark objects, such as boson stars, axion miniclusters and subhalos. We adapt MicroLIA, a machine learning-based package tailored to handle the challenges posed by low-cadence data in microlensing surveys. Using realistic observational timestamps, our models are trained on simulated light curves to distinguish between microlensing by point-like and extended lenses, as well as from other object classes which give a variable magnitude. We show that boson stars, examples of objects with a relatively flat mass distribution, can be confidently identified for $0.8 \lesssim r/r_E\lesssim 3$. Intriguingly, we also find that more sharply peaked structures, such as NFW-subhalos, can be distinctly recognized from point-lenses under regular observation cadence. Our findings significantly advance the potential of microlensing data in uncovering the elusive nature of extended dark objects. The code and dataset used are also provided.

astro-ph.CO↗