Collisionless Cold Dark Matter Research Articles

Self-Interacting Dark Matter Solves the Final Parsec Problem of Supermassive Black Hole Mergers.

Evidence for a stochastic gravitational wave (GW) background, plausibly originating from the merger of supermassive black holes (SMBHs), is accumulating with observations from pulsar timing arrays. An outstanding question is how inspiraling SMBHs get past the "final parsec" of separation, where they have a tendency to stall before GW emission alone can make the binary coalesce. We argue that dynamical friction from the dark matter (DM) spike surrounding the black holes is sufficient to resolve this puzzle, if the DM has a self-interaction cross section of order cm^{2}/g. The same effect leads to a softening of the GW spectrum at low frequencies as suggested by the current data. For collisionless cold DM, the friction deposits so much energy that the spike is disrupted and cannot bridge the final parsec, while for self-interacting DM, the isothermal core of the halo can act as a reservoir for the energy liberated from the SMBH orbits. A realistic velocity dependence, such as generated by the exchange of a massive mediator like a dark photon, is favored to give a good fit to the GW spectrum while providing a large enough core. A similar velocity dependence has been advocated for solving the small-scale structure problems of cold DM.

Dianoga SIDM: Galaxy cluster self-interacting dark matter simulations

Self-interacting dark matter (SIDM) can tackle or alleviate small-scale issues within the cosmological standard model Lambda CDM, and diverse flavours of SIDM can produce unique astrophysical predictions, resulting in different possible signatures which can be used to test these models with dedicated observations of galaxy clusters This work aims to assess the impact of dark matter self-interactions on the properties of galaxy clusters. In particular, the goal is to study the angular dependence of the cross section by testing rare (large angle scattering) and frequent (small angle scattering) SIDM models with velocity-dependent cross sections. We re-simulated six galaxy cluster zoom-in initial conditions with a dark matter-only run and with full-physics set-up simulations that include a self-consistent treatment of baryon physics. We tested the dark matter-only setup and the full physics setup with the collisionless cold dark matter, rare self-interacting dark matter, and frequent self-interacting dark matter models. We then studied their matter density profiles as well as their subhalo population. Our dark matter-only SIDM simulations agree with theoretical models, and when baryons are included in simulations, our SIDM models substantially increase the central density of galaxy cluster cores compared to full-physics simulations using collisionless dark matter. SIDM subhalo suppression in full-physics simulations is milder compared to the one found in the dark matter-only simulations because of the cuspier baryonic potential that prevents subhalo disruption. Moreover, SIDM with small-angle scattering significantly suppresses a larger number of subhaloes compared to large-angle scattering SIDM models. Additionally, SIDM models generate a broader range of subhalo concentration values, including a tail of more diffuse subhaloes in the outskirts of galaxy clusters and a population of more compact subhaloes in the cluster cores.

Examining the self-interaction of dark matter through central cluster galaxy offsets

ABSTRACT While collisionless cold dark matter models have been largely successful in explaining a wide range of observational data, some tensions still exist, and it remains possible that dark matter possesses a non-negligible level of self-interactions. In this paper, we investigate a possible observable consequence of self-interacting dark matter: offsets between the central galaxy and the centre of mass of its parent halo. We examine 23 relaxed galaxy clusters in a redshift range of 0.1–0.3 drawn from clusters in the Dark Energy Survey and the Sloan Digital Sky Survey which have archival Chandra X-ray data of sufficient depth for centre and relaxation determination. We find that most clusters in our sample show non-zero offsets between the X-ray centre, taken to be the centroid within the cluster core, and the central galaxy position. All of the measured offsets are larger, typically by an order of magnitude, than the uncertainty in the X-ray position due to Poisson noise. In all but six clusters, the measured offsets are also larger than the estimated, combined astrometric uncertainties in the X-ray and optical positions. A more conservative cut on concentration to select relaxed clusters marginally reduces but does not eliminate the observed offset. With our more conservative sample, we find an estimated median X-ray to central galaxy offset of $\mu = 6.0 ^{+ 1.4}_{- 1.5}$ kpc. Comparing to recent simulations, this distribution of offsets is consistent with some level of dark matter self-interaction, though further simulation work is needed to place constraints.

Cluster halo shapes in CDM and SIDM models: unveiling the DM particle nature using a weak-lensing approach

ABSTRACT Self-interacting dark matter (SIDM) is an alternative to the standard collisionless cold dark matter model (CDM), allowing for interactions between the dark-matter particles through the introduction of a self-scattering cross-section. However, the observable effects between these two scenarios are hard to detect. In this work, we present a detailed analysis of an application of galaxy–galaxy lensing to measure with high precision the shapes of cluster haloes and how this approach can be used to obtain information regarding the nature of the dark-matter particle. Using two sets of simulated data, SIDM and CDM simulations, we compute stacked shear maps centred on several subsets of haloes with masses ≳1013.5 M⊙. From these maps, we obtain the quadrupole profiles related to the mean projected elongation of the particle distribution from which the shape parameters are derived. Accounting for a radial shape variation, this technique provides an enhancement of the observed differences between the simulated data sets. In particular, we obtain a higher slope of the power law for the shape-radial relation for the haloes identified in the SIDM simulation, which are rounder towards the centre. Also, as approaching to the mean virial radius, the projected semi-axis ratios converge to similar values than in the CDM simulation. Moreover, we account for the impact of the neighbouring mass, where more strongly elongated distributions are found for the haloes in the SIDM simulation, indicating that under dark matter self interaction, the large-scale structure imprints a more coherent accretion process.

Simulating Atomic Dark Matter in Milky Way Analogs

Dark sector theories naturally lead to multicomponent scenarios for dark matter where a subcomponent can dissipate energy through self-interactions, allowing it to efficiently cool inside galaxies. We present the first cosmological hydrodynamical simulations of Milky Way analogs where the majority of dark matter is collisionless cold dark matter (CDM) but a subcomponent (6%) is strongly dissipative minimal atomic dark matter (ADM). The simulations, implemented in GIZMO and utilizing FIRE-2 galaxy formation physics to model the standard baryonic sector, demonstrate that the addition of even a small fraction of dissipative dark matter can significantly impact galactic evolution despite being consistent with current cosmological constraints. We show that ADM gas with roughly standard model–like masses and couplings can cool to form a rotating “dark disk” with angular momentum closely aligned with the visible stellar disk. The morphology of the disk depends sensitively on the parameters of the ADM model, which affect the cooling rates in the dark sector. The majority of the ADM gas gravitationally collapses into dark “clumps” (regions of black hole or mirror star formation), which form a prominent bulge and a rotating thick disk in the central galaxy. These clumps form early and quickly sink to the inner ∼kiloparsec of the galaxy, affecting the galaxy’s star formation history and present-day baryonic and CDM distributions.

Constraints on Self-Interacting dark matter from relaxed galaxy groups

Self-interacting dark matter (SIDM) has been proposed as an alternative to the standard collisionless cold dark matter to explain the diversity of galactic rotation curves and core-cusp problems seen at small scales. Here, we estimate the constraints on SIDM for a sample of 11 relaxed galaxy groups with X-ray observations from Chandra and XMM-Newton. We fit the dark matter density distribution to the Einasto profile and use the estimated Einasto α parameter to constrain the SIDM cross-section, based on the empirical relation between the two, which was obtained in Eckert et al. (2022). We obtain a non-zero central estimate for the cross-section per unit mass (σ/m) for seven groups, with the most precise estimate obtained for NGC 5044, given by σ/m=0.165±0.025cm2/g, for dark matter velocity dispersion of about 300 km/s. For the remaining four groups, we obtain 95% c.l. upper limits on σ/m< 0.16-6.61 cm2/g with dark matter velocity dispersions between 200–500 km/s, with the most stringent limit for our sample obtained for the group MKW 4, given by σ/m<0.16cm2/g for dark matter velocity dispersion of about 350 km/s.

On particle scattering in Gross-Pitaevskii theory and implications for dark matter halos

Bose-Einstein-condensed dark matter (BEC-DM), also called scalar field dark matter (SFDM), has become a popular alternative to the standard, collisionless cold dark matter (CDM) model, due to its long-held potential to resolve the small-scale crisis of CDM. Halos made of BEC-DM have been modelled using the Gross-Pitaevskii (GP) equation coupled to the Poisson equation; the so-called GPP equations of motion. These equations are based on fundamental microphysical conditions that need to be fulfilled in order for the equations to be valid in the first place, related to the diluteness of the DM gas and the nature of the particle scattering model. We use these conditions in order to derive the implications for the BEC-DM parameters, the 2-particle self-interaction coupling strength g and the particle mass m. We compare the derived bounds with the constraint that results from the assumption of virial equilibrium of the central cores of halos, deriving a relationship that connects g and m. We find that the GPP conditions are greatly fulfilled, for BEC-DM particle masses of interest, if such models also obey the virial condition that turns out to be the strongest constraint. We also derive the implications for the elastic scattering cross section (per particle mass) in BEC-DM halos, based on the scattering model of GPP, and find a huge range of possible values, depending on the self-interaction regime. We put our results into context to recent literature which predicts sub-kpc core size in BEC-DM halos.

Asymmetric dark matter with a spontaneously broken U(1)′ : Self-interaction and gravitational waves

Motivated by the collisionless cold dark matter small scale structure problem, we propose an asymmetric dark matter model where dark matter particle interact with each other via a massive dark gauge boson. This model easily avoid the strong limits from cosmic microwave background (CMB) observation, and have a large parameter space to be consistent with small scale structure data. We focus on a special scenario where portals between dark sector and visible sector are too weak to be detected by traditional methods. We find that this scenario can increase the effective number of neutrinos ($N_{\text{eff}}$).In addition, the spontaneous $U(1)'$ symmetry breaking process, which makes dark gauge boson massive, can generate stochastic gravitational waves with peak frequency around $10^{-6} - 10^{-7} \text{ Hz}$.

Modelling self-interacting dark matter substructures – I. Calibration with N-body simulations of a Milky-Way-sized halo and its satellite

ABSTRACT We study evolution of single subhaloes with their masses of ∼109 M⊙ in a Milky Way-sized host halo for self-interacting dark matter (SIDM) models. We perform dark-matter-only N-body simulations of dynamical evolution of individual subhaloes orbiting its host by varying self-scattering cross-sections (including a velocity-dependent scenario), subhalo orbits, and internal properties of the subhalo. We calibrate a gravothermal fluid model to predict time evolution in spherical mass density profiles of isolated SIDM haloes with the simulations. We find that tidal effects of SIDM subhaloes can be described with a framework developed for the case of collision-less cold dark matter (CDM), but a shorter typical time-scale for the mass loss due to tidal stripping is required to explain our SIDM simulation results. As long as the cross-section is less than $\sim \! 10\, \mathrm{cm}^2\,\mathrm{g}^{ -1}$ and initial states of subhaloes are set within a 2σ-level scatter at redshifts of ∼2 predicted by the standard ΛCDM cosmology, our simulations do not exhibit a prominent feature of gravothermal collapse in the subhalo central density for 10 Gyr. We develop a semi-analytic model of SIDM subhaloes in a time-evolving density core of the host with tidal stripping and self-scattering ram pressure effects. Our semi-analytic approach provides a simple, efficient, and physically intuitive prediction of SIDM subhaloes, but further improvements are needed to account for baryonic effects in the host and the gravothermal instability accelerated by tidal stripping effects.

Shapes of Milky-Way-mass galaxies with self-interacting dark matter

ABSTRACT Self-interacting dark matter (SIDM) models offer one way to reconcile inconsistencies between observations and predictions from collisionless cold dark matter (CDM) models on dwarf-galaxy scales. In order to incorporate the effects of both baryonic and SIDM interactions, we study a suite of cosmological-baryonic simulations of Milky-Way (MW)-mass galaxies from the Feedback in Realistic Environments (FIRE-2) project where we vary the SIDM self-interaction cross-section σ/m. We compare the shape of the main dark matter (DM) halo at redshift z = 0 predicted by SIDM simulations (at σ/m = 0.1, 1, and 10 cm2 g−1) with CDM simulations using the same initial conditions. In the presence of baryonic feedback effects, we find that SIDM models do not produce the large differences in the inner structure of MW-mass galaxies predicted by SIDM-only models. However, we do find that the radius where the shape of the total mass distribution begins to differ from that of the stellar mass distribution is dependent on σ/m. This transition could potentially be used to set limits on the SIDM cross-section in the MW.

Cosmic filament spin from dark matter vortices

The recent observational evidence for cosmic filament spin on megaparsec scales Wang et al. (2021) [41] demands an explanation in the physics of dark matter. Conventional collisionless cold particle dark matter is conjectured to generate cosmic filament spin through tidal torquing, but this explanation requires extrapolating from the quasi-linear regime to the non-linear regime. Meanwhile no alternative explanation exists in the context of ultra-light (e.g., axion) dark matter, and indeed these models would naively predict zero spin for cosmic filaments. In this Letter we study cosmic filament spin in theories of ultra-light dark matter, such as ultra-light axions, and bosonic and fermionic condensates, such as superfluids and superconductors. These models are distinguished from conventional particle dark matter models by the possibility of dark matter vortices. We take a model agnostic approach, and demonstrate that a collection of dark vortices can explain the data reported in Wang et al. Modeling a collection of vortices with a simple two-parameter analytic model, corresponding to an averaging of the velocity field, we find an excellent fit to the data. We perform a Markov Chain Monte Carlo analysis and find constraints on the number of vortices, the dark matter mass, and the radius of the inner core region where the vortices are distributed, in order for ultra-light dark matter to explain spinning cosmic filaments.

Spin transfer from dark matter to gas during halo formation

ABSTRACT In the protogalactic density field, diffuse gas and collision-less cold dark matter (DM) are often assumed sufficiently mixed that both components experience identical tidal torques. However, haloes in cosmological simulations consistently end up with a higher specific angular momentum (sAM) in gas, even in simulations without radiative cooling and galaxy formation physics. We refine this result by analysing the spin distributions of gas and DM in ∼50 000 well-resolved haloes in a non-radiative cosmological simulation from the SURFS suite. The sAM of the halo gas on average ends up ∼40 per cent above that of the DM. This can be pinned down to an excess AM in the inner halo (&amp;lt;50 per cent virial radius), paralleled by a more coherent rotation pattern in the gas. We uncover the leading driver for this AM difference through a series of control simulations of a collapsing ellipsoidal top-hat, where gas and DM are initially well mixed. These runs reveal that the pressurized inner gas shells collapse more slowly, causing the DM ellipsoid to spin ahead of the gas ellipsoid. The arising torque generally transfers AM from the DM to the gas. The amount of AM transferred via this mode depends on the initial spin, the initial axes ratios, and the collapse factor. These quantities can be combined in a single dimensionless parameter, which robustly predicts the AM transfer of the ellipsoidal collapse. This simplistic model can quantitatively explain the average AM excess of the gas found in the more complex non-radiative cosmological simulation.

Cointeracting dark matter and conformally coupled light scalars

The increasing observational pressure on the standard cosmological model motivates analyses going beyond the paradigm of the collisionless cold dark matter (DM). Since the only clear evidence for the existence of DM is based on gravitational interactions, it seems particularly fitting to study them in this sector where extensions to the standard model can be naturally introduced. A promising avenue can be obtained using modifications of the space-time metric coupled to DM and induced by the presence of a new ultralight scalar field $\ensuremath{\phi}$. The $\ensuremath{\phi}$ field can contribute to the DM density and can couple all the matter species universally, including additional heavy DM particles. We present a simple two-component DM model employing derivative conformal interactions between the two DM species. This can simultaneously (1) guarantee the necessary symmetries to stabilize the dark species, (2) predict a subdominant thermal relic density of the heavy DM component, and (3) alleviate small-scale structure tensions of the cold DM scenario due to the possible cointeractions in the dark sector. The scenario is highly predictive with future observational prospects ranging from the Large Hadron Collider to gravitational-wave searches and can be generalized to more rich and realistic dark sector models.

Effective Self-interaction of Dark Matter from Gravitational Scattering

I show that gravitational scattering of dark matter objects of mass ∼104 M ⊙ and speeds of ∼10 km s−1, provides the cross section per unit mass required in self-interacting dark matter models that alleviate the small-scale structure challenges to the collisionless cold dark matter model. For primordial objects of mass 104 M 4 M ⊙, moving at the velocity dispersion characteristic of dwarf galaxies, 10v 1 km s−1, the cross section per unit mass for gravitational scattering is . The steep decline in interaction with increasing velocity explains why self-interaction is not evident in data on massive galaxies and clusters of galaxies.

Cosmological structure formation in scalar field dark matter with repulsive self-interaction: the incredible shrinking Jeans mass

ABSTRACT Scalar field dark matter (SFDM) comprised of ultralight (≳ 10−22 eV) bosons is an alternative to standard, collisionless cold dark matter (CDM) that is CDM-like on large scales but inhibits small-scale structure formation. As a Bose–Einstein condensate, its free-field (‘fuzzy’) limit (FDM) suppresses structure below the de Broglie wavelength, λdeB, creating virialized haloes with central cores of radius ∼ λdeB, surrounded by CDM-like envelopes, and a halo mass function (HMF) with a sharp cut-off on small scales. With a strong enough repulsive self-interaction (SI), structure is inhibited, instead, below the Thomas–Fermi (TF) radius, RTF (the size of an SI-pressure-supported (n = 1)-polytrope), when RTF &amp;gt; λdeB. Previously, we developed tools to describe SFDM dynamics on scales above λdeB and showed that SFDM-TF haloes formed by Jeans-unstable collapse from non-cosmological initial conditions have RTF-sized cores, surrounded by CDM-like envelopes. Revisiting SFDM-TF in the cosmological context, we simulate halo formation by cosmological infall and collapse, and derive its transfer function from linear perturbation theory to produce cosmological initial conditions and predict statistical measures of structure formation, such as the HMF. Since FDM and SFDM-TF transfer functions both have small-scale cut-offs, we can align them to let observational constraints on FDM proxy for SFDM-TF, finding FDM with particle masses 1 ≲ m/(10−22 eV/c2) ≲ 30 corresponds to SFDM-TF with 10 ≳ RTF/(1 pc) ≳ 1, favouring subgalactic (sub-kpc) core size. The SFDM-TF HMF cuts off gradually, however, leaving more small-mass haloes: Its Jeans mass shrinks so fast that scales filtered early can still recover and grow!

Strongly-interacting ultralight millicharged particles

We consider the implications of an ultra-light fermionic dark matter candidate that carries baryon number. This naturally arises if dark matter has a small charge under standard model baryon number whilst having an asymmetry equal and opposite to that in the visible universe. A prototypical model is a theory of dark baryons of a non-Abelian gauge group, i.e., a dark Quantum Chromo-Dynamics (QCD). For sub-eV dark baryon masses, the inner region of dark matter halos is naturally at ‘nuclear density’, allowing for the formation of exotic states of matter, akin to neutron stars. The Tremaine-Gunn lower bound on the mass of fermionic dark matter, i.e., the dark baryons, is violated by the strong short-range self-interactions, cooling via emission of light dark pions, and the Cooper pairing of dark quarks that occurs at densities that are high relative to the (ultra-low) dark QCD scale. We develop the astrophysics of these STrongly-interacting Ultra-light Millicharged Particles (STUMPs) utilizing the equation of state of dense quark matter, and find halo cores consistent with observations of dwarf galaxies. These cores are prevented from core-collapse by pressure of the ‘neutron star’, which suggests ultra-light dark QCD as a resolution to core-cusp problem of collisionless cold dark matter. The model is distinguished from ultra-light bosonic dark matter through direct detection and collider signatures, as well as by phenomena associated with superconductivity, such as Andreev reflection and superconducting vortices.

Core-envelope haloes in scalar field dark matter with repulsive self-interaction: fluid dynamics beyond the de Broglie wavelength

ABSTRACT Scalar field dark matter (SFDM) comprised of ultralight bosons has attracted great interest as an alternative to standard, collisionless cold dark matter (CDM) because of its novel structure-formation dynamics, described by the coupled Schrödinger–Poisson equations. In the free-field (‘fuzzy’) limit of SFDM (FDM), structure is inhibited below the de Broglie wavelength, but resembles CDM on larger scales. Virialized haloes have ‘solitonic’ cores of radius ∼λdeB, surrounded by CDM-like envelopes. When a strong enough repulsive self-interaction (SI) is also present, structure can be inhibited below a second length-scale, λSI, with λSI &amp;gt; λdeB – called the Thomas–Fermi (TF) regime. FDM dynamics differ from CDM because of quantum pressure, and SFDM-TF differs further by adding SI pressure. In the small-λdeB limit, however, we can model all three by fluid conservation equations for a compressible, γ = 5/3 ideal gas, with ideal gas pressure sourced by internal velocity dispersion and, for the TF regime, an added SI pressure, PSI ∝ ρ2. We use these fluid equations to simulate halo formation from gravitational collapse in 1D, spherical symmetry, demonstrating for the first time that SFDM-TF haloes form with cores the size of RTF, the radius of an SI-pressure-supported (n = 1)-polytrope, surrounded by CDM-like envelopes. In comparison with rotation curves of dwarf galaxies in the local Universe, SFDM-TF haloes pass the [‘too-big-to-fail’ + ‘cusp–core’]-test if RTF ≳ 1 kpc.

To Observe, or Not to Observe, Quantum-Coherent Dark Matter in the Milky Way, That is a Question

In recent years, Bose-Einstein-condensed dark matter (BEC-DM) has become a popular alternative to standard, collisionless cold dark matter (CDM). This BEC-DM -also called scalar field dark matter (SFDM)- can suppress structure formation and thereby resolve the small-scale crisis of CDM for a range of boson masses. However, these same boson masses also entail implications for BEC-DM substructure within galaxies, especially within our own Milky Way. Observational signature effects of BEC-DM substructure depend upon its unique quantum-mechanical features and have the potential to reveal its presence. Ongoing efforts to determine the dark matter substructure in our Milky Way will continue and expand considerably over the next years. In this contribution, we will discuss some of the existing constraints and potentially new ones with respect to the impact of BEC-DM onto baryonic tracers. Studying dark matter substructure in our Milky Way will soon resolve the question, whether dark matter behaves classical or quantum on scales of≲1 kpc.

Angular momentum and the absence of vortices in the cores of fuzzy dark matter haloes

ABSTRACT Scalar field dark matter (SFDM), comprised of ultralight (≳ 10−22 eV) bosons, is distinguished from massive (≳GeV), collisionless cold dark matter (CDM) by its novel structure-formation dynamics as Bose–Einstein condensate (BEC) and quantum superfluid with wave-like properties, described by the Gross-Pitaevskii and Poisson (GPP) equations. In the free-field (‘fuzzy’) limit of SFDM (FDM), structure is inhibited below the de Broglie wavelength λdeB, but resembles CDM on larger scales. Virialized haloes have ‘solitonic’ cores of radius ∼λdeB that follow the ground-state attractor solution of GPP, surrounded by CDM-like envelopes. As superfluid, SFDM is irrotational (vorticity-free) but can be unstable to vortex formation. We previously showed this can happen in halo cores, from angular momentum arising during structure formation, when repulsive self-interaction (SI) is present to support them out to a second length scale λSI with λSI &amp;gt; λdeB (the Thomas–Fermi regime), but only if SI is strong enough. This suggested FDM cores ($ {\rm without}$ SI) would not form vortices. FDM simulations later found vortices, but only outside halo cores, consistent with our previous suggestion based upon TF-regime analysis. We extend that analysis now to FDM, to show explicitly that vortices should not arise in solitonic cores from angular momentum, modelling them as either Gaussian spheres, or ( n = 2)-polytropic, irrotational Riemann-S ellipsoids. We find that, for typical halo spin parameters, angular momentum per particle is below ℏ, the minimum required even for one singly-quantized vortex in the centre. Even for higher angular momentum, however, vortex formation is not energetically favoured.

Semiclassical regime for dark matter self-interactions

Many particle physics models for dark matter self-interactions - motivated to address long-standing challenges to the collisionless cold dark matter paradigm - fall within the semi-classical regime, with interaction potentials that are long-range compared to the de Broglie wavelength for dark matter particles. In this work, we present a quantum mechanical derivation and new analytic formulas for the semi-classical momentum transfer and viscosity cross sections for self-interactions mediated by a Yukawa potential. Our results include the leading quantum corrections beyond the classical limit and allow for both distinguishable and identical dark matter particles. Our formulas supersede the well-known formulas for the momentum transfer cross section obtained from the classical scattering problem, which are often used in phenomenological studies of self-interacting dark matter. Together with previous approximation formulas for the cross section in the quantum regime, our new results allow for nearly complete analytic coverage of the parameter space for self-interactions with a Yukawa potential. We also discuss the phenomenological implications of our results and provide a new velocity-averaging procedure for constraining velocity-dependent self-interactions. Our results have been implemented in the newly released code CLASSICS.