Bradley J. Kavanagh

Spikes are steep enhancements in the dark matter (DM) distribution around a heavy compact object. If the compact object is primordial, and the bulk of the DM is also composed of (lighter) primordial compact objects, for instance asteroid-mass primordial black holes (PBHs), the phenomenology of spike formation is highly non-trivial. In fact, lighter PBHs have negligible angular momentum at formation with respect to the massive central object and would therefore be captured unless enough torque is exerted from either small-scale or large-scale matter fluctuations. In this paper, we present the first comprehensive assessment of this scenario. We define the mechanisms and the initial conditions that allow light PBHs to avoid capture. We then quantify the different types of torque and follow the corresponding angular momentum evolution with a combination of analytical prescriptions and numerical simulations. We find that in the innermost region no mechanism studied here is capable of providing sufficient torque; the resulting inner core is expected to be significantly less dense than in particle scenarios, with potential consequences for the spike phenomenology and its observational signatures.

The meV mass range has emerged as a focal point in axion physics, where advances in theory, cosmology, astrophysics, and experimental techniques converge. Axions in this mass range are theoretically well motivated, can arise in ultraviolet-complete models, and can have significant cosmological impacts as dark matter or dark radiation. In parallel, their efficient production in stellar and supernova environments provides powerful astrophysical probes. Here, we provide a comprehensive overview of meV axions across these domains, highlighting both established results and open questions. We discuss the theoretical underpinnings of meV axions, their cosmological and astrophysical signatures, and the diverse experimental strategies — ranging from helioscopes and haloscopes to quasiparticle systems and large-volume Cherenkov detectors — that aim to explore this regime. The convergence of these approaches emphasizes the pivotal role of the meV mass range for axion discovery in the coming years, identifying meV axions as a key probe for testing beyond-Standard-Model physics. This review document is the direct outcome of the discussions at the dedicated workshop “The meV Mass Axion Frontier: Challenges and Opportunities”, held at Laboratori Nazionali di Frascati (IT) on 27–28 October 2025, and organized by the EU funded COST Action “Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments” (CA21106, https://www.cost.eu/actions/CA21106). Its aim is to provide an overview of current efforts in meV axion research, their motivations, and the research goals that animate the community involved in this search.

Recent work has highlighted the importance of a fully relativistic treatment of the dephasing of gravitational waves induced by dark-matter overdensities in extreme mass-ratio inspirals (EMRIs). However, a general-relativistic description of the dark matter phase-space distribution is currently available only for the case of a dark matter “spike” arising from adiabatic black hole growth. Here we develop a fully general-relativistic formalism for the more realistic scenario in which a supermassive stellar progenitor collapses to a black hole and produces a shallower dark matter overdensity, or “mound.” We follow self-consistently the evolution of the supermassive star, its collapse, and the subsequent growth of the resulting black hole, together with the collisionless dark matter orbits. We find that in the regime where the collapse becomes nonadiabatic, the dark matter distribution function is significantly reshaped, with a clear depletion in the low-binding-energy region of phase space. Our results provide a more realistic prediction for the dark matter phase-space distribution around supermassive black holes, which is an essential step in using future EMRI observations to extract information about their formation history and the nature of dark matter.

Caustic-crossing stars observed in giant arcs behind galaxy clusters provide a powerful probe of dark matter substructure. While previous work has focused on pointlike lenses such as primordial black holes, we extend this framework to (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 ⊙ . 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.

Three black-hole low-mass x-ray binaries (LMXBs) in the Milky Way show rates of period decay which cannot be easily explained by standard mechanisms. Recently, it has been claimed that the anomalous period decays in two of these systems may be explained by dynamical friction due to very high dark matter (DM) densities around the black holes. We critically assess these claims by performing N -body simulations of binaries embedded in dense DM “spikes.” We simulate the previously studied systems XTE J 1118 + 480 and A0620–00, as well as studying the third binary Nova Muscae 1991 for the first time in this context. These simulations show that feedback on the DM distribution plays a crucial role and we rule out previously claimed shallow DM spikes. We set lower limits on the steepness γ of DM density profiles required to explain the period decay in these LMXBs, requiring γ ≳ 2.15 – 2.20 in XTE J 1118 + 480 and A0620–00 and γ ≳ 2.3 in Nova Muscae 1991. Improved modeling of the long-term evolution of binaries embedded in DM spikes may allow us to exclude even larger densities in future.

Primordial Black Holes (PBHs) have not been experimentally detected so far, but their existence would provide important insights about the early Universe and serve as one of the possible candidates of dark matter (DM). In this work, we explore the accretion of radiation and matter by PBHs, with relevance for the growth of PBH seeds to form early Supermassive Black Holes; the emission from accreting PBHs; and constraints from gravitational wave observations, among others. We study the growth of PBH masses in the early Universe due to the accretion of radiation, highlighting uncertainties which arise from estimates of the PBH formation time. For baryonic accretion, we review the traditional Bondi-Hoyle-Lyttleton (BHL) and its refined version known as the Park-Ricotti (PR) model, which also includes radiative feedback. We find that in the BHL model, PBHs heavier than ∼ 100 M_⊙ can grow in mass by several orders of magnitude by z ≲ 10, though only when surrounded by DM halos and only when the accretion efficiency is large. By contrast, the inclusion of radiation feedback in the PR model can drastically suppress the baryonic accretion rate of PBHs, leading to a negligible change in PBH mass over cosmic time. Furthermore our calculations show that the accretion rate depends sensitively on the modelling of various parameters such as the speed of sound in the baryonic gas and the velocity of PBHs. These findings highlight the uncertainties associated with accretion onto PBHs, and we find that a large increase in the PBH mass due to accretion is by no means guaranteed.

Highly magnified stars (μ>100) are now routinely identified as transient events at cosmological distances thanks to microlensing by intra-cluster stars near the critical curves of galaxy clusters. Using the James Webb Space Telescope (JWST) in combination with the Hubble Space Telescope, we outline here an analytical framework that is applied to the Warhol arc (at z = 0.94) in the MACS 0416 galaxy cluster (at z = 0.396), where over a dozen microlensed stars have been detected to date. This method is general and can be applied to other lensed arcs. Within this lensed galaxy, we fit the spatially resolved spectral energy distribution spanned by eight JWST-NIRCam filters combined with three ACS filters, for accurate lensed star predictions in 2D. With this tool we can generate 2D maps of microlensed stars for well-resolved arcs in general, incorporating wavelength dependence and limiting apparent magnitude. These maps can be directly compared with planned cadenced campaigns from JWST and Hubble, offering a means to constrain the initial mass function and the level of dark matter substructure.

Recent observations have indicated a Cosmic Microwave Background (CMB) temperature decrement in the direction of local galaxies within the 2MASS Redshift Survey. We investigate this detection by analyzing its frequency dependence and sensitivity to component separation methods, suggesting that Galactic foregrounds are unlikely to be the cause. Contrary to previous studies, we find that the decrement is independent of galaxy type, indicating a possible correlation between the CMB and the overall matter density field. To test this hypothesis, we employ three analytical approaches: cross-correlation analysis, template fitting, and Bayes Factor calculation. Our analyses consistently reveal a moderate correlation (with significance levels in the 1% – 5% range) between the CMB and the 2MASS Redshift Survey projected matter density at distances below 50 Mpc/h. Importantly, we do not detect this signal beyond 50 Mpc/h, which constrains potential physical interpretations and suggests caution regarding possible look-elsewhere effects in our analysis. We discuss that the physical origin of this correlation could potentially be linked to the dark matter distribution in the halos of galaxies. Further investigation is required to confirm and understand this intriguing connection between the CMB and local matter distribution.