Nested solitons in two-field fuzzy dark matter

Fuzzy dark matter (FDM), comprised of ultralight ($m \sim 10^{-22}\,{\rm eV}$) boson particles, has received significant attention as a viable alternative to cold dark matter (CDM), as it approximates CDM on large scales (${\gtrsim}1$ Mpc) while potentially resolving some of its small-scale problems via kiloparsec-scale quantum interference. However, the most basic FDM model, with one free parameter (the boson mass), is subject to a tension: small boson masses yield the desired cores of dwarf galaxies but underpredict structure in the Lyman-α forest, while large boson masses render FDM effectively identical to CDM. This Catch-22 problem may be alleviated by considering an axion-like particle with attractive particle self-interactions. We simulate an idealized FDM halo with self-interactions parametrized by an energy decay constant $f \sim 10^{15}~\rm {GeV}$ related to the axion symmetry-breaking conjectured to solve the strong-CP problem in particle physics. We observe solitons, a hallmark of FDM, condensing within a broader halo envelope, and find that the density profile and soliton mass depend on self-interaction strength. We propose generalized formulae to extend those from previous works to include self-interactions. We also investigate a critical mass threshold predicted for strong interactions at which the soliton collapses into a compact, unresolved state. We find that the collapse happens quickly, and its effects are initially contained to the central region of the halo.

The 21-cm signals from cosmic dawn and the epoch of reionization contain valuable information on cosmological structure formation dominated by dark matter. Measurements of the 21-cm power spectrum can thus probe certain dark matter candidates. Here we investigate the impacts of fuzzy dark matter (FDM) on the 21-cm signals, taking into account both the linear matter power spectrum and the halo mass function (HMF) in FDM cosmologies. The full FDM dynamics are implemented in reionization simulations, along with a new ansatz on modulation of the FDM HMF by the linear overdensity. Not only does the suppression of FDM halos on small scales give rise to delay of the signature epochs during cosmic reionization, but these epochs are also shortened relative to the cold dark matter cosmology. In addition, we find that while the FDM effects on the 21-cm power spectrum are dominated by its linear dynamics early in cosmic dawn, a correct FDM HMF resulting from nonlinear wave dynamics must be considered when x-ray heating begins. We forecast the constraints on the FDM model parameters from upcoming 21-cm power spectrum measurements by SKA1-Low (central area). In FDM cosmologies with ${m}_{\mathrm{FDM}}={10}^{\ensuremath{-}21}\text{ }\text{ }\mathrm{eV}$, SKA1-Low will be able to constrain the boson mass to within $\ensuremath{\sim}10%$ at $2\ensuremath{\sigma}$ confidence with a mock 1080-hour observation, if the ionizing efficiency is mass independent. However, our results show that realistic astrophysical processes are degenerate with the FDM effects, which shall severely loosen the constraints on the boson mass from 21-cm power spectrum data alone.

Fuzzy Dark Matter (FDM) represents an alternative and intriguing description of the standard Cold Dark Matter (CDM) fluid, able to explain the lack of direct detection of dark matter particles in the GeV sector and to alleviate small scales tensions in the cosmic large-scale structure formation. Cosmological simulations of FDM models in the literature were performed either with very expensive high-resolution grid-based simulations of individual haloes or through N-body simulations encompassing larger cosmic volumes but resorting on significant approximations in the FDM non-linear dynamics to reduce their computational cost. With the use of the new N-body cosmological hydrodynamical code AX-GADGET, we are now able not only to overcome such numerical problems, but also to combine a fully consistent treatment of FDM dynamics with the presence of gas particles and baryonic physical processes, in order to quantify the FDM impact on specific astrophysical observables. In particular, in this paper we perform and analyse several hydrodynamical simulations in order to constrain the FDM mass by quantifying the impact of FDM on Lyman-$\alpha$ forest observations, as obtained for the first time in the literature in a N-body setup without approximating the FDM dynamics. We also study the statistical properties of haloes, exploiting the large available sample, to extract information on how FDM affects the abundance, the shape, and density profiles of dark matter haloes.

We investigate non-linear structure formation in the context of the fuzzy dark matter (FDM) model and compare it to the cold dark matter (CDM) model from a weak lensing perspective. Employing Eulerian perturbation theory (PT) up to fourth order, we calculate the tree-level matter trispectra and the one-loop matter spectra and bispectra from consistently chosen initial conditions. Furthermore, we conduct N-body simulations with CDM and FDM initial conditions to predict the non-linear matter power spectra. Subsequently, we derive the respective lensing spectra, bispectra, and trispectra for CDM and FDM within the framework of a Euclid-like weak lensing survey. In our analysis, we compute attainable cumulative signal-to-noise ratios and estimate χ2-functionals, aimed at distinguishing FDM from CDM at particle masses of m = 10−21 eV, m = 10−22 eV, and m = 10−23 eV. Our results indicate that PT predictions are insufficient for distinguishing between the CDM and FDM models within the context of our simulated weak lensing survey for the considered particle masses. Assuming that N-body simulations overestimate late-time small-scale power in the FDM model, future weak lensing surveys may provide the means to discriminate between FDM and CDM up to a mass of m = 10−23 eV. However, for stronger constraints on the FDM mass, observations of the local high-z universe may be more suitable.

In hierarchical models of structure formation, the first galaxies form in low-mass dark matter potential wells, probing the behavior of dark matter on kiloparsec scales. Even though these objects are below the detection threshold of current telescopes, future missions will open an observational window into this emergent world. In this Letter, we investigate how the first galaxies are assembled in a "fuzzy" dark matter (FDM) cosmology where dark matter is an ultralight ∼10^{-22} eV boson and the primordial stars are expected to form along dense dark matter filaments. Using a first-of-its-kind cosmological hydrodynamical simulation, we explore the interplay between baryonic physics and unique wavelike features inherent to FDM. In our simulation, the dark matter filaments show coherent interference patterns on the boson de Broglie scale and develop cylindrical solitonlike cores, which are unstable under gravity and collapse into kiloparsec-scale spherical solitons. Features of the dark matter distribution are largely unaffected by the baryonic feedback. On the contrary, the distributions of gas and stars, which do form along the entire filament, exhibit central cores imprinted by dark matter-a smoking gun signature of FDM.

A plausible dark matter candidate is an ultralight bosonic particle referred to as fuzzy dark matter. The equivalent mass-energy of the fuzzy dark matter boson is ∼ 10-22 eV and has a corresponding de Broglie wavelength of kiloparsec scale, thus exhibiting wave behaviour in scales comparable to a galactic core, which could not appear in conventional cold dark matter models. The presence of fuzzy dark matter in galactic clusters will impact the motion of their members through dynamical friction. In this work, we present simulations of the dynamical friction on satellites traversing an initially uniform fuzzy dark matter halo. We focus on the satellites whose shapes are beyond spherical symmetry described by ellipsoidal and logarithmic potentials. We find that the wakes created on the fuzzy dark matter halo due to the passage of such satellites are qualitatively different from those generated by spherically symmetric ones. Furthermore, we quantify the dynamical friction coefficient for such systems, finding that the same satellite may experience a drag differing by a factor of 5 depending on its ellipticity and the direction of motion. Finally, we find that the dynamical friction time-scale is close to Hubble time, assuming a satellite of 1011 M ⊙ traversing at 103 km/s a FDM halo whose mean density is ∼ 106 M ⊙ kpc-3.

We review a testable dark matter (DM) model outside of the standard WIMP paradigm. The model is unique in a sense that the observed ratio Ωdark ≃ Ωvisible for visible and dark matter densities finds its natural explanation as a result of their common QCD origin when both types of matter (DM and visible) are formed during the QCD phase transition and both are proportional to single dimensional parameter of the system, Λqcd. We argue that the charge separation effect also inevitably occurs during the same QCD phase transition in the presence of the CP odd axion field a(x). It leads to preferential formation of one species of nuggets on the scales of the visible Universe where the axion field a(x) is coherent. A natural outcome of this preferential evolution is that only one type of the visible baryons (not anti- baryons) remain in the system after the nuggets complete their formation. Unlike conventional WIMP dark matter candidates, the nuggets and anti-nuggets are strongly interacting but macroscopically large objects. The rare events of annihilation of the anti-nuggets with visible matter lead to a number of observable effects. We argue that the relative intensities for a number of measured excesses of emission from the centre of galaxy (covering more than 11 orders of magnitude) are determined by standard and well established physics. At the same time the absolute intensity of emission is determined by a single new fundamental parameter of the theory, the axion mass, 10−6eV ≲ ma ≲ 10−3eV. Finally, we comment on implications of these studies for the axion search experiments, including microwave cavity and the Orpheus experiments.

Fuzzy dark matter (FDM) is a promising candidate for dark matter (DM), characterized by its ultra-light mass, which gives rise to wave effects at astrophysical scales. These effects offer potential solutions to the small-scale issues encountered within the standard cold dark matter (CDM) paradigm. In this paper, we investigate the large-scale structure of the cosmic web using FDM simulations, comparing them to CDM-only simulations and a simulation incorporating baryonic effects. Our study employs the nearest neighbour (NN) analysis as a new statistical tool for examining the structure and statistics of the cosmic web in an FDM universe. This analysis could capture the information absent in the two-point correlation functions. In particular, we analyse data related to the spherical contact, nearest neighbour distances (NND), and the angle between the first and second nearest neighbours of haloes (NNA). Specifically, we utilize probability distribution functions, statistical moments, and fitting parameters, as well as G(x), F(x), and J(x) functions to analyse the above data. Remarkably, the results from the FDM simulations differ significantly from the others across these analyses, while no noticeable distinction is observed between the baryonic and CDM-only simulations. Moreover, the lower FDM mass leads to more significant deviations from the CDM simulations. These compelling results highlight the efficiency of the NN analysis – mainly through the use of the J(x) function, $s_3$, $l_{3}$, and $a_4$ parameters – as a prominent new tool for investigating FDM on large scales and making observational predictions.

Using high-resolution cosmological radiation-hydrodynamic (RHD) simulations (thesan-hr), we explore the impact of alternative dark matter (altDM) models on galaxies during the Epoch of Reionization. The simulations adopt the IllustrisTNG galaxy formation model. We focus on altDM models that exhibit small-scale suppression of the matter power spectrum, namely warm dark matter (WDM), fuzzy dark matter (FDM), and interacting dark matter (IDM) with strong dark acoustic oscillations (sDAO). In altDM scenarios, both the halo mass functions and the ultraviolet luminosity functions at z ≳ 6 are suppressed at the low-mass/faint end, leading to delayed global star formation and reionization histories. However, strong non-linear effects enable altDM models to ‘catch up’ with cold dark matter (CDM) in terms of star formation and reionization. The specific star formation rates are enhanced in halos below the half-power mass in altDM models. This enhancement coincides with increased gas abundance, reduced gas depletion times, more compact galaxy sizes, and steeper metallicity gradients at the outskirts of the galaxies. These changes in galaxy properties can help disentangle altDM signatures from a range of astrophysical uncertainties. Meanwhile, it is the first time that altDM models have been studied in RHD simulations of galaxy formation. We uncover significant systematic uncertainties in reionization assumptions on the faint-end luminosity function. This underscores the necessity of accurately modeling the small-scale morphology of reionization in making predictions for the low-mass galaxy population. Upcoming James Webb Space Telescope imaging surveys of deep lensed fields hold potential for uncovering the faint low-mass galaxy population, which could provide constraints on altDM models.

(Abridged) Despite the success of Cold Dark Matter (CDM) in explaining a wide range of observations, the microscopic nature of dark matter is still unknown. Attempts to detect WIMPs, the most commonly studied form, have not yet succeeded. Some predictions are even in apparent conflict with observations (e.g. cuspy-cored halos and `missing satellite problem'). Therefore, it is important to consider different forms of CDM. We focus on dark matter comprised of ultralight bosons that form a Bose-Einstein Condensate (BEC), described by a complex scalar field, and solve the Klein-Gordon and Einstein field equations to evolve the Friedmann-Robertson-Walker universe. We find, in addition to phases of radiation-, matter- and \Lambda-domination, an earlier phase of scalar-field-domination. Moreover, while WIMP CDM is non-relativistic at all times after it decouples, BEC scalar field dark matter (SFDM) is relativistic at early times, evolving from stiff to radiation-like, before it becomes non-relativistic (CDM-like) at late times. The timing of transitions between these phases yields fundamental constraints on SFDM model parameters, particle mass m and self-interaction coupling strength \lambda. We derive the range of particle parameters required to match observations of the evolving background universe, including the CMB and abundances of light elements produced by BBN, characterized by N_eff, the effective number of neutrino species, and the epoch of matter-radiation equality z_eq. This yields m >= 2.4*10^{-21} eV/c^2 and 9.5*10^{-19} eV^{-1}cm^3 <= \lambda/(mc^2)^2 <= 4*10^{-17} eV^{-1}cm^3. Our model accommodates current observations in which N_eff at BBN is higher than at z_eq, as probed by the CMB, otherwise unexplained by WIMP CDM. SFDM without self-interaction (`Fuzzy Dark Matter') cannot satisfy current BBN constraints within 68% confidence and is therefore disfavored.

In this talk, I showcase models for fuzzy axion dark matter within the framework of type IIB string theory, focusing on axions originating from the Ramond-Ramond four-form in compactifications on Calabi-Yau orientifold hypersurfaces. These models are amenable to cosmological tests if a substantial relic abundance of fuzzy dark matter is produced. I present a topologically exhaustive ensemble of more than 350,000 Calabi-Yau compactifications with up to seven axions together with a systematic analysis of the misalignment production of fuzzy dark matter. The resulting dark matter composition is generally a mixture of fuzzy axions and heavier axions, including the QCD axion. Dark photons frequently emerge due to the orientifold projection. I will also comment on applications of optimisation strategies based on automatic differentiation for exploring the string axiverse. This talk is partially based on https://arxiv.org/abs/2412.12012.

We present a new module of the parallel N-Body code P-GADGET3 for cosmological simulations of light bosonic non-thermal dark matter, often referred as Fuzzy Dark Matter (FDM). The dynamics of the FDM features a highly non-linear Quantum Potential (QP) that suppresses the growth of structures at small scales. Most of the previous attempts of FDM simulations either evolved suppressed initial conditions, completely neglecting the dynamical effects of QP throughout cosmic evolution, or resorted to numerically challenging full-wave solvers. The code provides an interesting alternative, following the FDM evolution without impairing the overall performance. This is done by computing the QP acceleration through the Smoothed Particle Hydrodynamics (SPH) routines, with improved schemes to ensure precise and stable derivatives. As an extension of the P-GADGET3 code, it inherits all the additional physics modules implemented up to date, opening a wide range of possibilities to constrain FDM models and explore its degeneracies with other physical phenomena. Simulations are compared with analytical predictions and results of other codes, validating the QP as a crucial player in structure formation at small scales.

The axions produced during the QCD phase transition by vacuum realignment, string decay and domain wall decay thermalize as a result of their gravitational self-interactions when the photon temperature is approximately 500 eV. They then form a Bose-Einstein condensate (BEC). Because the axion BEC rethermalizes on time scales shorter than the age of the universe, it has properties that distinguish it from other forms of cold dark matter. The observational evidence for caustic rings of dark matter in galactic halos is explained if the dark matter is axions, at least in part, but not if the dark matter is entirely WIMPs or sterile neutrinos.

Fuzzy dark matter (FDM) is a dark matter candidate consisting of ultralight scalar particles with masses around $10^{-22}\, \mathrm{eV}/c^2$, a regime where cold bosonic matter behaves as a collective wave rather than individual particles. Although constraints on FDM are accumulating in many different contexts, very few have been verified by self-consistent numerical simulations. We present new large numerical simulations of cosmic structure formation with FDM, solving the full Schrödinger–Poisson (SP) equations using the AxiREPO code, which implements a pseudo-spectral numerical method. Combined with our previous simulations, they allow us to draw a four-way comparison of matter clustering, contrasting results (such as power spectra) for each combination of initial conditions (ICs; FDM versus cold dark matter, CDM) and dynamics (SP versus N-body). By disentangling the impact of ICs and non-linear dynamics in this manner, we can gauge the validity of approximate methods used in previous works, such as ordinary N-body simulations with an FDM initial power spectrum. Due to the comparatively large volume achieved in our FDM simulations, we are able to measure the FDM halo mass function from full wave simulations for the first time, and compare to previous results obtained using analytic or approximate approaches. We also investigate the density profiles of these filaments and compare to their ΛCDM counterparts.

Recent observations have uncovered a remarkably flat and extremely diffuse stellar distribution within the almost dark dwarf galaxy Nube, posing a challenge to the standard cold dark matter scenario. In this study, we employ numerical simulations to explore the possibility that this anomalous stellar distribution can be attributed to the dynamical heating effect of fuzzy dark matter (FDM). The relatively isolated location and low baryon fraction of Nube make it an ideal system for investigating this effect. Our findings indicate that by adopting a halo profile consistent with the dynamical mass estimation of Nube and an FDM particle mass on the order of 10−23 eV, the final 2D stellar distribution derived from simulation closely matches observational data. These results suggest that FDM could provide an explanation for the extremely diffuse stellar distribution of Nube.