We study constraints on the quantum chromodynamics (QCD) axion in F-theory, a strongly coupled limit of string theory. We build models of QCD from compactifications to four dimensions on elliptic fibrations over toric threefolds B_3, characterized by an integer N=h^1,1(B_3). The QCD axion mass increases with N, and we find that models with sufficiently high N are inconsistent with observations of the neutrino burst from supernova 1987A, disfavoring large regions of the moduli space. Specifically, at least 95% of models with N ≥ 8,791 – a regime that arguably contains the vast majority of known F-theory topologies – have a QCD axion mass m_QCD>15 meV and are thus constrained. This limit is independent of cosmology. We only consider weakly-curved threefolds, where α' corrections are plausibly negligible.

One fundamental physics interpretation of Dark Energy Spectroscopic Instrument (DESI) results is that the observed accelerated expansion of the Universe is driven not by the cosmological constant, but by a slowly rolling scalar field, a natural model for which is an axion with a decay constant close to the Planck scale. In the “string axiverse’’ one expects not one, but many, axions and this may allow the ability to engineer large effective decay constants. We thus investigate dark energy dynamics in a toy model with two axions, and compute the prior probability for the dark energy equation of state parameters ( w 0 , w a ) implied by priors on the fundamental parameters of the model conditioned weakly on the resulting cosmology. We find various interesting behaviours, for example where both fields can be in an oscillating regime while maintaining w 0 < − 1 / 3 , and models with opposite sign for w a than in a single-field model. The prior probability in ( w 0 , w a ) gains support away from the”thawing quintessence’’ behaviour preferred by DESI, and when the axions have cross-interactions this effect becomes stronger. When the axions interact, one in general still requires large decay constants, but if the axions are non-interacting the largest decay constant can be reduced. The novel equations of state we illustrate may also be of relevance to models of early dark energy or inflation/reheating with mutliple axions. Further exploration of dynamics in models with N ≫ 1 axions is warranted.

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.

Axion dark matter (DM) is predicted to convert into radio waves in neutron star magnetospheres. We assess the detectability of this signal using a 5 m radio telescope to be installed at the Fan Mountain Observatory, operating in the UHF, L- and S-bands from 0.5 to 4 GHz. We demonstrate that such a telescope can search new parameter space for axion-like particles over a broad range from 2 μeV<m_a<17 μeV for axion-photon couplings g_aγγ≳ 2× 10^-12 GeV^-1 with a three year observing period assuming the standard halo model – improving neutron star observations by more than an order of magnitude. The search is broadband and is thus complementary to other techniques in the same frequency range. We describe in detail our neutron star population model, noise model, and proposed observing strategy. Most constraining power comes from neutron stars at the Galactic centre, where the smooth DM halo is densest. If a DM spike exists at the Galactic centre, the search is sensitive in the QCD axion model band. UHF and L-band observations (0.5 to 2 GHz) represent the pathfinder phase of a wider program we call “Axion Search with Telescope for Radio Astronomy” (ASTRA). Future higher mass searches aimed at discovery potential for the post-inflation axion require further hardware development to cover S, C, X and Ku bands (2 to 18 GHz).

We study the cosmological core-halo relation in vector dark matter using three-component Schrödinger-Poisson simulations. Starting from cosmological vector-field initial conditions, which due to the evolution of the vector field during inflation are enhanced on small scales, we find that nonlinear evolution begins almost immediately following matter-radiation equality and produces compact self-gravitating Proca-star condensates at the centers of halos. After confirming the central condensates through their radial density profiles, we find the empirical relation M_⋆∝M_ h^0.6403 between the Proca star mass and halo mass, although interestingly we find that we are only able to confirm Proca stars in 𝒪(10%) of halos. This serves as important input for future studies of the abundance and merger rates of Proca stars in models of vector dark matter. We also examine the vector-field structure of the objects through global longitudinal and transverse polarization fractions and local spin density inside halos, which increases over cosmic time.

Black hole (BH) superradiance can provide strong constraints on the properties of ultralight bosons (ULBs). While most of the previous work has focused on the theoretical predictions. Here, we investigate the most suitable statistical framework to constrain ULB masses and self-interactions using BH spin measurements. We argue that a Bayesian approach based on a simple time-scales analysis provides a clear statistical interpretation, deals with limitations regarding the reproducibility of existing BH analyses, incorporates the full information from BH data, and allows us to include additional nuisance parameters or to perform hierarchical modelling with BH populations in the future. We demonstrate the feasibility of our approach using mass and spin posterior samples for the X-ray binary BH M33 X-7 and, for the first time in this context, the supermassive BH IRAS 09149-6206. We explain the differences to existing ULB constraints in the literature and illustrate the effects of various assumptions about the superradiance process (equilibrium regime versus cloud collapse, higher occupation levels). As a result, our procedure yields the most statistically rigorous ULB constraints available in the literature, with important implications for the quantum chromodynamics (QCD) axion and axion-like particles. We encourage all groups analysing BH data to publish likelihood functions or posterior samples as supplementary material to facilitate this type of analysis, and for theory developments to compress their findings to effective time-scale modifications (https://github.com/sebhoof/bhsr).

Axions that couple to electromagnetism are produced in the early Universe by, among other channels, freeze-in via the Primakoff process. For sufficiently large axion masses, the same coupling causes the axions to decay into two photons, which subsequently ionize the intergalactic medium. If this decay occurs in the redshift range $20 \lesssim z \lesssim 1100$, then the contribution to the cosmic microwave background optical depth $τ_{\rm reio}$ can lead to a conflict with observations, excluding models with sufficiently strongly coupled, heavy axions and high reheating temperatures, $T_{\rm reh}$. Using large ensembles of explicit type IIB string theory models with up to $h^{1,1} = 100$ axions, we compute the full cosmic reionization history caused by the decays of multiple axions. We compare this to the posterior on the high-$z$ component of $τ_{\rm reio}$ derived from model-independent constraints on the ionization state of the Universe, obtained in a full \textit{Planck} analysis presented in a companion paper. For $h^{1,1} = 20, 50, 100$, we find that approximately 15\%, 15\%, and 10\% of the models in the ensemble prefer $T_{\rm reh} \lesssim 10^{10}\,\text{GeV}$ at 95\% CL. We provide a publicly available code at:~\href{https://github.com/ZiwenYin/Reionization-with-multi-axions-decay}{github.com/ZiwenYin/Reionization-with-multi-axions-decay}, which computes the reionization history for arbitrary ensembles of decaying axions. Our analysis opens the door for future large-scale work studying the preference for low-temperature reheating in models with multiple axions.

We study fuzzy axion dark matter in type IIB string theory, for axions descending from the Ramond-Ramond four-form in compactifications on orientifolds of Calabi-Yau hypersurfaces. Such models can be tested by cosmological measurements if a significant relic abundance of fuzzy dark matter arises, which we argue is most common in models with small numbers of axions. We construct a topologically exhaustive ensemble of more than 350,000 Calabi-Yau compactifications yielding up to seven axions, and in this setting we perform a systematic analysis of misalignment production of fuzzy dark matter. In typical regions of moduli space, the fuzzy axion, the QCD axion, and other axions have comparable decay constants of f_a ≈ 10^16 GeV. We find that overproduction of heavier axions is problematic, except at special loci in moduli space where decay constant hierarchies can occur: without a contrived reheating epoch, it is necessary to fine-tune initial displacements. The resulting dark matter is typically a mix of fuzzy axions and heavier axions, including the QCD axion. Dark photons are typically present as a consequence of the orientifold projection. We examine the signatures of these models by simulating halos with multiple fuzzy axions, and by computing new cosmological constraints on ultralight axions and dark radiation. We also give evidence that cosmic birefringence is possible in this setting. Our findings determine the phenomenological correlates of fuzzy axion dark matter in a corner of the landscape.

This study investigates the characteristic polarization formation and evolution of vector dark matter (VDM) in the outer halo of galaxies. By employing numerical simulations, we analyze the behavior of VDM under different initial conditions-homogeneous, isotropic, and partially polarized. The simulations solve the Schr & ouml;dinger-Poisson equations, examining the spin density distribution and its evolution during gravitational collapse and halo formation. Our results reveal that VDM forms halos and central Proca stars from homogeneous and isotropic conditions, with the polarization density fluctuation amplitude mirroring VDM matter density. In scenarios with no initial polarization, spin density remains stable in the halo core but fluctuates in outer regions. Partially polarized initial conditions lead to a conservation of total polarization, with increased core polarization resulting in opposite polarization in the periphery. We examine the novel consequences of the partially polarized state for direct detection of dark photons, i.e., VDM kinetically mixed with ordinary photons.

Models of scalar field dark matter where the scalar is a dilaton have a special behavior, since nontrivial couplings, d, to matter result in a contribution to the potential for the field that is proportional to the trace of the stress-energy tensor. We look in more detail at the dilaton mass, m_{ϕ}, and initial conditions required to yield the correct relic abundance for couplings that are not already excluded by terrestrial experiments. In minimal models with only couplings accessible to terrestrial searches, we find that dilaton dark matter with m_{ϕ}≳10^{-10}  eV requires couplings suppressed compared to constraints from equivalence principle tests and fifth force searches in order to not produce too much dark matter, improving on the strongest current experimental constraints by up to ∼O(10), with consequences for the proposed mechanical resonator dilaton dark matter searches. In nonminimal or universally coupled models, the unconstrained couplings of the dilaton to, e.g., the top quark can strongly influence the relic abundance at all masses. In particular, this implies that atom interferometry searches at masses m_{ϕ}≈10^{-19}  eV are unable to constrain the early Universe behavior or UV physics of the dilaton. We also find that dilatonic couplings allow for compatibility of m_{ϕ}≳10^{-7}  eV with an observably large tensor-to-scalar ratio in the cosmic microwave background, which is not possible for a decoupled scalar of the same mass.

The axion is a hypothetical fundamental particle that is conjectured to correspond to the coherent oscillation of the θ field in quantum chromodynamics1,2. Its existence would solve multiple fundamental questions, including the strong CP problem of quantum chromodynamics and dark matter, but the axion has never been detected. Electrodynamics of condensed-matter systems can also give rise to a similar θ, so far studied as a static, quantized value to characterize the topology of materials3-5. Coherent oscillation of θ in condensed matter has been proposed to lead to physics directly analogous to the high-energy axion particle-the dynamical axion quasiparticle (DAQ)6-23. Here we report the observation of the DAQ in MnBi2Te4. By combining a two-dimensional electronic device with ultrafast pump-probe optics, we observe a coherent oscillation of θ at about 44 gigahertz, which is uniquely induced by its out-of-phase antiferromagnetic magnon. This represents direct evidence for the presence of the DAQ, which in two-dimensional MnBi2Te4 is found to arise from the magnon-induced coherent modulation of the Berry curvature. The DAQ also has implications in light-matter interaction and coherent antiferromagnetic spintronics24, as it might lead to axion polaritons and electric control of ultrafast spin polarization6,15-20. Finally, the DAQ could be used to detect axion particles21-23. We estimate the detection frequency range and sensitivity in the millielectronvolt regime, which has so far been poorly explored.

It is widely established that a lower bound on the dark matter particle mass, $m$, can be obtained by demanding that the de Broglie wavelength in a given galaxy must be smaller than the virial radius of the galaxy, leading to $m\gtrsim 10^{-22}\text{ eV}$ when applied to typical dwarf galaxies. This lower limit has never been derived precisely or rigorously. We use stellar kinematical data for the Milky Way satellite galaxy Leo II to self-consistently reconstruct a statistical ensemble of dark matter wavefunctions and corresponding density profiles. By comparison to a data-driven, model-independent reconstruction, and using a variant of the maximum mean discrepancy as a statistical measure, we determine that a self-consistent description of dark matter in the local Universe requires $m>2.2 \times 10^{-21}\,\mathrm{eV}\;\mathrm{(CL>95\%)}$. This lower limit is free of any assumptions pertaining to cosmology, microphysics (including spin), or dynamics of dark matter, and only assumes that it is predominantly composed of a single bosonic particle species.

Laboratory experiments have the capacity to detect the QCD axion in the next decade, and precisely measure its mass, if it composes the majority of the dark matter. In type IIB string theory on Calabi-Yau threefolds in the geometric regime, the QCD axion mass, ma, is strongly correlated with the topological Hodge number h1,1. We compute ma in a scan of 185965 compactifications of type IIB string theory on toric hypersurface Calabi-Yau threefolds. We compute the range of h1,1 probed by different experiments under the condition that the QCD axion can provide the observed dark matter density with minimal fine-tuning. Taking the experiments DMRadio, ADMX, MADMAX, and BREAD as indicative on different mass ranges, the h1,1 distributions peak near h1,1=24.9, 65.4, 196.8, and 310.9, respectively. We furthermore conclude that, without severe fine-tuning, detection of the QCD axion as dark matter disfavors 80% of models with h1,1=491, which is thought to have the most known Calabi-Yau threefolds. Measurement of the solar axion mass with IAXO is the dominant probe of all models with h1,1≳250. This Letter demonstrates the immense importance of axion detection in experimentally constraining the string landscape. Published by the American Physical Society 2025

We develop tools of Bayesian inference on the moduli space of Calabi–Yau (CY) manifolds. We sample from the invariant Weil–Petersson (WP) measure using Markov Chain Monte Carlo and normalising flows on moduli space with dimension up to h^1,1=30, and present results on the spectrum of the CY volume and properties of divisors when the measure is restricted in physically meaningful ways. We furthermore present a theory-informed prior on axion masses and decay constants (m_a,f_a) marginalised over the WP measure for all inequivalent CYs constructable from the Kreuzer–Skarke database with h^1,1≤ 5. We then impose likelihoods based on axion physics. We demonstrate how detection of a relatively heavy QCD axion at small h^1,1, e.g. by ADMX, provides detailed information about CY geometry and topology. Finally, we compute a full forward model incorporating likelihoods from the cosmic microwave background and Lyman-alpha forest and find the maximum posterior probability region on the moduli space of a given CY favoured by a resolution of the tension in these data by an ultralight axion composing 𝒪(1%) of the dark matter. This demonstration serves as a blueprint for future statistical analyses within string phenomenology.

The large-angle polarization anisotropies in the Cosmic Microwave Background (CMB) arise from Thomson scattering of CMB photons off free electrons in the post-recombination Universe. In the standard $Λ$ cold dark matter cosmological model, the free electron density increases at redshifts $z \lesssim 10$ as the first stars form, reionizing the intergalactic medium. We use \emph{Gaussian processes} to perform a model-independent reconstruction of the cosmic reionization history constrained by \textit{Planck} CMB data. Our approach recovers the standard reionization at $z \lesssim 10$ and places stringent limits on any additional high-$z$ reionization. From this reconstruction, we define a new derived parameter, the high-redshift contribution to the CMB optical depth, $τ_{\mathrm{highz}}$, whose posterior distribution provides robust constraints on exotic energy injection scenarios. We demonstrate this for decaying dark matter with particle masses in the range $\mathcal{O}(1\,\text{MeV})$. A companion paper applies this framework to multi-axion models. All data and code are publicly available at: \href{https://github.com/Cheng-Hanyu/CLASS_reio_gpr}{github.com/Cheng-Hanyu/CLASS\_reio\_gpr}.

It is widely established that a lower bound on the dark matter particle mass, m, can be obtained by demanding that the de Broglie wavelength in a given galaxy must be smaller than the virial radius of the galaxy, leading to m≳10^{-22}  eV when applied to typical dwarf galaxies. This lower limit has never been derived precisely or rigorously. We use stellar kinematical data for the Milky Way satellite galaxy Leo II to self-consistently reconstruct a statistical ensemble of dark matter wave functions and corresponding density profiles. By comparison to a data-driven, model-independent reconstruction, and using a variant of the maximum mean discrepancy as a statistical measure, we determine that a self-consistent description of dark matter in the local Universe requires m>2.2×10^{-21}  eV  (CL>95%). This lower limit is free of any assumptions pertaining to cosmology, microphysics (including spin), or dynamics of dark matter, and only assumes that it is predominantly composed of a single bosonic particle species.

We study the implications of relaxing the requirement for ultralight axions to account for all dark matter in the Universe by examining mixed dark matter (MDM) cosmologies with axion fractions f <= 0.3 within the fuzzy dark matter window 10(-25) eV less than or similar to m less than or similar to 10(-23) eV. Our simulations, using a new MDM gravity solver implemented in AxiREPO, capture wave dynamics across various scales with high accuracy down to redshifts z approximate to 1. We identify haloes with Rockstar using the cold dark matter component and find good agreement of inferred halo mass functions and concentration-mass relations with theoretical models across redshifts z=1-10. This justifies our halo finder approach a posteriori as well as the assumptions underlying the MDM halo model AxionHMcode. Using the inferred axion halo mass-cold halo mass relation Ma(Mc) and calibrating a generalized smoothing parameter alpha to our MDM simulations, we present a new version of AxionHMcode. The code exhibits excellent agreement with simulations on scales k<20hcMpc(-1) at redshifts z=1-3.5 for f <= 0.1 around the fiducial axion mass m=10(-24.5)eV=3.16x10(-25)eV, with maximum deviations remaining below 10 per cent. For axion fractions f <= 0.3, the model maintains accuracy with deviations under 20 per cent at redshifts z approximate to 1 and scales k<10hcMpc(-1), though deviations can reach up to 30 per cent for higher redshifts when f=0.3. Reducing the run-time for a single evaluation of AxionHMcode to below 1 min, these results highlight the potential of AxionHMcode to provide a robust framework for parameter sampling across MDM cosmologies in Bayesian constraint and forecast analyses.

Fuzzy (wave) dark matter (FDM), the dynamical model underlying an ultralight bosonic dark matter species, produces a rich set of nongravitational signatures that distinguishes it markedly from the phenomenologically related warm (particle) dark matter (WDM) scenario. The emergence of extended interference fringes hosted by cosmic filaments is one such phenomenon reported by cosmological simulations, and a detailed understanding of such may strengthen existing limits on the boson mass but also break the degeneracy with WDM, and provide a unique fingerprint of interference in cosmology. In this paper, we provide initial steps toward this goal. In particular, we show in a bottom-up approach, how the presence of interference in an idealized filament population can lead to a non-suppressive feature in the matter power spectrum --- an observation supported by cosmological FDM simulations. To this end, we build on a theoretically motivated and numerically observed steady-state approximation for filaments and express the equilibrium dynamics of such in an expansion of FDM eigenstates. We optimize the size of the expansion by incorporating classical phase-space information. Ellipsoidal collapse considerations were used to construct a fuzzy filament mass function which, together with the reconstructed FDM wave function, allowed us to efficiently compute the one-filament power spectrum. We showcase our non-perturbative interference model for a selection of boson masses and confirm our approach is able to produce the matter power boost observed in fully cosmological FDM simulations. More precisely, we find an excess in correlation between the spatial scale associated with the FDM ground state and the quantum pressure scale. We speculate about applications of this effect in data analysis.

The QCD axion and axionlike particles are compelling candidates for galactic dark matter. Theoretically, axions can convert into photons in the presence of a strong external magnetic field, which means it is possible to search for them experimentally. One approach is to use radio telescopes with high-resolution spectrometers to look for axion-photon conversion in the magnetospheres of neutron stars. In this paper, we describe the results obtained using a novel approach where we used the Green Bank Telescope (GBT) to search for radio transients produced by collisions between neutron stars and dark matter clumps known as axion miniclusters. We used the versatile GBT astronomical spectrometer and the X-band receiver (8-10 GHz) to observe the core of Andromeda. Our measurements are sensitive to axions with masses between 33 and 42 mu eV with Delta ma 1/4 3.8 x 10-4 mu eV. This paper gives a description of the search method we developed, including observation and analysis strategies. Given our analysis algorithm choices and the instrument sensitivity (similar to 2 mJy in each spectral channel), we did not find any candidate signals greater than 5 sigma. We are currently implementing this search method in other spectral bands.