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Cuspy halo problem

The cuspy halo problem, also known as the core-cusp problem, refers to the discrepancy between the cuspy central density profiles of dark matter halos predicted by cold dark matter (CDM) simulations and the cored profiles inferred from observations of dwarf galaxies and low-surface-brightness galaxies.^[1] In CDM models, such as the Lambda cold dark matter (ΛCDM) paradigm, N-body simulations predict that dark matter density ρ increases steeply toward the halo center, following profiles like the Navarro-Frenk-White (NFW) form where the logarithmic slope approaches -1 (i.e., ρ ∝ r^{-1}).^[2] By contrast, observational data from rotation curves and stellar kinematics reveal much flatter central slopes, typically between 0 and -0.5, indicating constant-density cores rather than cusps.^[3]^[4]^[5] This tension emerged prominently in the early 2000s through analyses of gas-rich dwarf galaxies, such as those in the THINGS survey, which showed central slopes around -0.2 ± 0.2, inconsistent with the steeper cusps from pure dark matter simulations.^[4] Subsequent studies, including those of low-mass galaxies, have reinforced the evidence for cores, with average observed slopes of approximately -0.22 ± 0.08, while simulations yield values closer to -0.8 to -1.4 depending on resolution and profile fits like NFW or Einasto.^[5] The problem is particularly acute in low-mass systems, where baryonic effects are expected to be minimal, highlighting potential shortcomings in the collisionless CDM framework.^[1] The cuspy halo problem represents one of the key small-scale challenges to the ΛCDM model, alongside issues like the missing satellites and too-big-to-fail problems, and has spurred extensive research into resolution mechanisms. Proposed solutions include baryonic processes, such as supernova feedback that drives gas outflows and flattens cusps, or dynamical friction from massive objects; alternative dark matter candidates like warm or self-interacting dark matter, which naturally produce cores; and modifications to gravity.^[1] Recent high-resolution simulations and observations, including those from 2025, suggest that while some tensions persist—especially in ultra-faint dwarfs—advances in modeling baryonic physics may reconcile predictions with data in many cases.^[6]^[7]

Background and Definition

Core Concept

The cold dark matter (CDM) paradigm posits that the large-scale structure of the universe arises from the gravitational instability of a smooth, initially uniform distribution of non-relativistic, collisionless dark matter particles, leading to the hierarchical formation of dark matter halos with centrally cuspy density profiles. These profiles feature a density \rho that diverges toward the galactic center, scaling asymptotically as \rho \propto r^{-\alpha} with \alpha \approx 1 in the inner regions.^[8]^[9] A cusp describes this steep density gradient, where the dark matter density rises sharply as the radius r approaches zero, in contrast to a core, which is a central region with roughly constant density.^[8]^[10] The cuspy halo problem emerges from the apparent mismatch between these predicted cuspy profiles and evidence for cored central densities, a discrepancy most evident in low-mass galaxies where the inner dynamics are dominated by dark matter.^[10] This theoretical expectation is encapsulated in the Navarro-Frenk-White (NFW) profile, a universal form fitted to halo structures from N-body simulations of CDM collapse:

\rho(r) = \frac{\rho_s}{(r/r_s)(1 + r/r_s)^2},

where \rho_s is a characteristic density and r_s is a scale radius; the profile yields an inner cusp slope of -1.^[8]

Historical Development

Predictions of cuspy dark matter halo profiles emerged from numerical simulations of structure formation in the cold dark matter (CDM) paradigm in the early 1990s. However, the recognition of the cuspy halo problem, also known as the core-cusp problem, began in 1994 when Flores and Primack, along with Moore, highlighted the tension between these cuspy profiles and observations of cored density distributions in dwarf and low-surface-brightness galaxies. High-resolution N-body simulations conducted by Navarro, Frenk, and White in 1996 revealed that dark matter halos exhibit a universal density profile with a central cusp, where the density ρ scales as ρ ∝ r^{-1} at small radii, as encapsulated in the Navarro-Frenk-White (NFW) profile. This finding solidified the expectation that CDM halos should be cuspy rather than featuring constant-density cores, marking a key milestone in theoretical predictions for dark matter distribution.^[8]^[10]^[11] Initial observational tensions were further detailed in the mid-1990s and early 2000s through analyses of rotation curves in dwarf and low-surface-brightness galaxies, which favored cored profiles over the predicted cusps. For instance, de Blok et al. (2001) derived high-resolution rotation curves from HI observations of such systems, demonstrating that their central dark matter densities are better fit by pseudo-isothermal profiles with flat cores (ρ ≈ constant) rather than NFW cusps. These results highlighted a growing discrepancy between simulations and data, prompting further scrutiny of small-scale structure in the CDM model.^[12] The explicit naming of the "core-cusp problem" appeared in the literature around 2004, framing the mismatch as a fundamental challenge to CDM predictions on galactic scales. Bosma (2004), in an observational overview, referred to the issue as the core/cusp problem, emphasizing its implications for dark matter in low-surface-brightness galaxies.^[13] This terminology encapsulated the ongoing debate over whether observations systematically indicate cores where theory demands cusps. Throughout the 2010s, the debate evolved with refined observations and simulations, intensifying scrutiny of the problem's implications for the ΛCDM model. Reviews such as de Blok (2010) synthesized evidence from gas-rich dwarfs showing persistent core preferences, while higher-resolution data from dwarf spheroidals added layers of complexity, revealing that the tension persists even after accounting for baryonic effects. Similarly, Bullock and Boylan-Kolchin (2017) highlighted how the core-cusp issue, alongside other small-scale challenges, tests the robustness of ΛCDM, spurring proposals for resolutions like self-interacting dark matter.^[14]^[15] Despite these advances, the problem remained unresolved, underscoring uncertainties in both theory and observation.

Theoretical Framework

N-Body Simulations

N-body simulations are computational methods that model the gravitational evolution of a large number of particles representing dark matter in the universe, treating them as a collisionless fluid under Newtonian gravity. These simulations discretize the dark matter distribution into N discrete particles, each with mass m = ρ L³ / N, where ρ is the mean cosmic density and L is the simulation box size, and evolve their positions and velocities over cosmic time using the equations of motion derived from Poisson's equation for the gravitational potential. By solving these equations numerically, the simulations track the growth of structure from tiny initial density fluctuations to the formation of galaxies and clusters.^[16] Key techniques in cosmological N-body simulations include particle-mesh (PM) methods, which compute the gravitational potential on a fixed Cartesian grid using fast Fourier transforms (FFT) to solve Poisson's equation efficiently for long-range forces, achieving O(N log N) scaling but limited resolution due to grid discretization. Tree-based algorithms, such as the Barnes-Hut method introduced in 1986, approximate distant particle interactions by grouping them into a hierarchical octree structure, enabling higher resolution for short-range forces at O(N log N) cost while reducing computational expense compared to direct particle-particle summation. Hybrid approaches like tree-particle-mesh (TPM) combine PM for large-scale modes with tree or particle-particle calculations for small scales, balancing accuracy and efficiency in large-volume runs.^[17]^[18] In these simulations, dark matter halos form through the hierarchical merging of smaller structures originating from primordial density fluctuations in the early universe, seeded by quantum fluctuations during inflation and amplified by gravitational instability as the universe expands. Initial conditions are generated from linear theory power spectra consistent with cosmic microwave background observations, with particles placed according to a Gaussian random field, and the simulation evolves forward from high redshift (e.g., z ≈ 100) to the present, allowing overdensities to collapse into bound halos via nonlinear gravitational dynamics. High-resolution examples include the Millennium Simulation of 2005, which used the GADGET code to follow 10 billion particles in a 500 h⁻¹ Mpc volume under a ΛCDM cosmology, resolving halo structures down to dwarf galaxy scales. Recent high-resolution simulations, such as those from the IllustrisTNG project (as of 2018 and later refinements), continue to support the predicted cuspy profiles with inner slopes around -1, with improved resolution minimizing artificial relaxation effects.^[19]^[20]^[21] A primary limitation of pure N-body simulations is their assumption of collisionless dark matter dynamics, neglecting the effects of baryonic matter such as gas cooling, star formation, and feedback processes that can alter halo structures in realistic galaxy formation scenarios. Early simulations ignored baryons entirely to focus on dark matter clustering, though subsequent hydrodynamical extensions incorporate them to address these shortcomings. These simulations typically predict central density profiles for halos resembling the Navarro-Frenk-White (NFW) form.^[22]

Predicted Density Profiles

Simulations of cold dark matter (CDM) halos in a ΛCDM cosmology predict a universal density profile characterized by a cuspy inner region and a steeper outer decline. The Navarro-Frenk-White (NFW) profile, derived from high-resolution N-body simulations, describes this structure with an inner density slope of approximately -1 (where \rho \propto r^{-\alpha} and \alpha \approx 1) transitioning to an outer slope of approximately -3. This form arises from hierarchical structure formation, where smaller subhalos merge to build larger halos, leading to a central concentration of dark matter.^[23] While the NFW profile provides a simple two-parameter fit (scale radius r_s and characteristic density \rho_s), variations in halo properties introduce deviations. An alternative, the Einasto profile, offers a better fit across a wider range of radii, given by

\rho(r) = \rho_s \exp\left[-\frac{2}{\alpha} \left( \left(\frac{r}{r_s}\right)^\alpha - 1 \right)\right],

with the shape parameter \alpha \approx 0.17 for Milky Way-mass halos. This profile captures a smoother transition without sharp breaks, reflecting the cumulative effects of multiple accretion episodes in halo assembly. Simulations predict an inner slope close to -1 that is nearly independent of halo mass and formation redshift, consistent with the universal NFW profile. While the concentration parameter shows dependence on mass and accretion history, the central cusp slope remains around α ≈ 1 across a wide range of halo masses. Numerical convergence remains a challenge for resolving the innermost regions. Early simulations suggested steeper inner slopes (\alpha \approx 1.5), but higher-resolution runs with more particles reveal a convergence toward \alpha \approx 1 beyond a minimum resolved radius proportional to the particle mass. This resolution dependence highlights the need for simulations with at least $10^6 particles per halo to reliably predict cuspy profiles, as artificial two-body relaxation can flatten or steepen the core artificially.^[24]

Observational Constraints

Evidence from Dwarf Galaxies

Dwarf spheroidal galaxies, such as Draco and Sculptor, serve as ideal probes of dark matter distributions due to their high mass-to-light ratios, often exceeding 100 in solar units, indicating dominance by dark matter over baryonic components throughout their extent. Kinematic studies of these systems rely on measurements of stellar velocity dispersions, which reveal flat or slowly varying profiles in the central regions, consistent with constant dark matter densities rather than steeply rising ones.^[25] In the seminal analysis by Walker et al. (2009), application of the Jeans equation to data from eight classical dwarf spheroidals, including Draco and Sculptor, yielded mass profiles consistent with cored dark matter halos with scale radii around 150 pc, as well as cuspy Navarro–Frenk–White (NFW) profiles with larger scale radii (~800 pc).^[25] Subsequent analyses have reinforced these findings, estimating core radii in the range of 280 pc to 1.3 kpc for several Milky Way satellites by modeling velocity dispersion profiles with isothermal or similar distributions.^[26] For instance, in Draco and Sculptor, the inferred central dark matter densities remain roughly constant out to these scales, supporting the presence of sizable cores.^[26] More recent studies using Gaia proper motions, such as those from 2022, have tightened constraints, finding evidence for cuspy profiles in massive dwarf spheroidals while cores remain favored in lower-mass systems.^[27] Complementary evidence comes from neutral hydrogen (HI) observations of gas-rich dwarf irregular galaxies, where rotation curves rise slowly in the inner regions, implying flat dark matter density profiles with core sizes on the order of hundreds of parsecs.^[28] These kinematic signatures from both stellar and gas tracers contrast with the cuspy central densities predicted by cold dark matter simulations.^[28]

Rotation Curve Measurements

Rotation curves, which measure the orbital velocities v(r) of gas or stars as a function of galactocentric radius r, provide key observational constraints on the inner dark matter density profiles of galaxies. These velocities are typically derived from the 21 cm emission line of neutral hydrogen (H I) using radio interferometry, allowing mapping of the velocity fields in the gaseous disks of spiral, low-surface-brightness (LSB), and dwarf irregular galaxies. In the context of the cuspy halo problem, rotation curves reveal discrepancies between predicted cuspy profiles from cold dark matter (CDM) simulations and observed behaviors suggesting cored distributions.^[29] High-resolution observations, such as those from The H I Nearby Galaxy Survey (THINGS) conducted with the Very Large Array, have targeted LSB and dwarf irregular galaxies to probe the central regions where dark matter dominance is expected. The THINGS survey, analyzing 19 such galaxies with spatial resolutions down to sub-kpc scales and velocity resolutions of about 5 km/s, finds that inner rotation curves often exhibit a shallow, nearly linear rise in v(r) at small radii, rather than the steeper \sqrt{r} increase anticipated from cuspy profiles like the Navarro-Frenk-White (NFW) model with \rho \propto r^{-1}. This linear rise implies a nearly constant central density \rho \approx constant, consistent with cored profiles such as the pseudo-isothermal (ISO) halo, where \rho(r) = \rho_0 / [1 + (r/R_c)^2] and R_c is the core radius typically exceeding 1 kpc in low-mass systems. For instance, in galaxies like DDO 154 and IC 2574, ISO models fit the data significantly better than NFW profiles, with core radii indicating flat density cores that encompass the observed rotation curve extents.^[29] However, interpreting these central flatnesses requires accounting for potential systematic errors in the data. Beam smearing, arising from the finite resolution of radio telescopes (e.g., 6-12 arcseconds in THINGS), can artificially flatten the inner rotation curves by averaging velocities across the beam, leading to underestimates of the true slope by up to 6 km/s in some cases. Additionally, non-circular motions, such as those induced by bars or spirals, introduce asymmetries in the velocity fields that must be modeled using techniques like tilted-ring fits or Hermite polynomial expansions to derive reliable circular velocities. Despite these corrections, the persistence of evidence for cored profiles across multiple THINGS galaxies underscores the tension with CDM predictions.^[29]

Nature of the Discrepancy

Core-Cusp Tension

The core-cusp tension arises from the mismatch between the steep inner density profiles predicted by cold dark matter (CDM) simulations and the shallower profiles inferred from observations in low-mass galaxies. In standard N-body simulations of CDM halos, the inner density follows a cuspy profile with slope γ ≈ 1, as characterized by the Navarro-Frenk-White (NFW) model where the density ρ ∝ r^{-γ} near the center. In contrast, kinematic data from dwarf galaxies reveal much flatter inner slopes of γ ≈ 0.2–0.5, indicating central constant-density cores rather than cusps. This discrepancy is most pronounced in low-mass dark matter halos with masses below 10^{10} M_⊙, where the dominance of dark matter minimizes the influence of baryonic processes, yet observations consistently favor cores over the expected cusps. Recent studies reveal diversity in dwarf galaxy profiles, with some exhibiting cusps consistent with CDM while others show cores, suggesting the tension may vary by galaxy mass and environment.^[7] Meta-analyses of stellar kinematics and rotation curves across multiple datasets underscore the statistical significance of this mismatch in samples of Milky Way satellites and field dwarfs. For instance, compilations of central dark matter densities at 150 pc reveal an anticorrelation with stellar mass that deviates substantially from CDM expectations. The issue remains unresolved even in probes of the Milky Way's center, where recent stellar dynamics and dynamical modeling have detected evidence for a cored profile (e.g., core size of 282^{+34}_{-31} pc at 68% CL), though uncertainties remain and do not fully resolve discrepancies in lower-mass dwarf galaxies.^[30]

Implications for Dark Matter Models

The cuspy halo problem, arising from the core-cusp tension where N-body simulations predict steep central density profiles (ρ ∝ r^{-1}) in dark matter halos while observations favor flatter cores (ρ ≈ constant), poses a significant challenge to the standard ΛCDM model by questioning its ability to accurately describe hierarchical structure formation on small scales.^[31] This discrepancy highlights potential shortcomings in the collisionless cold dark matter paradigm for low-mass systems, such as dwarf galaxies, where the predicted cusps fail to match measured rotation curves and stellar kinematics.^[31] The issue interconnects with other small-scale crises in ΛCDM, including the missing satellites problem—where simulations overpredict the number of low-mass subhalos compared to observed satellite galaxies—and the too-big-to-fail problem, which notes that the most massive simulated subhalos around the Milky Way are too dense to host the faint dwarf galaxies we observe.^[31] These linked tensions collectively suggest that ΛCDM may require modifications to its dark matter component or inclusion of additional physics to reconcile theory with observations on galactic and subgalactic scales.^[31] Astrophysically, cuspy halo profiles would enhance indirect detection signals from dark matter annihilation, as the high central densities boost gamma-ray and other particle fluxes by factors of up to 50 or more in the Galactic center compared to cored profiles, potentially aiding searches by telescopes like Fermi-LAT.^[32] Similarly, the predicted substructure in cuspy halos influences strong gravitational lensing, where low-mass dark matter clumps (10^6–10^9 M_⊙) perturb lensing arcs and flux ratios, providing indirect evidence for CDM-like abundance even if central profiles are unresolved.^[31] However, core profiles derived from observations reduce predicted lensing frequencies for massive ellipticals, implying that baryonic effects transforming cusps to cores are not universal across galaxy types.^[33] On cosmological scales, the cuspy halo problem has minimal impact on large-scale structure formation, cosmic microwave background anisotropies, or galaxy clustering, which remain robust successes of ΛCDM.^[31] Nonetheless, it serves as a critical probe of dark matter properties, such as self-interaction cross-sections or warmth, constraining models that must reproduce observed cores without altering successful large-scale predictions.^[31]

Proposed Explanations

Baryonic Physics Effects

Baryonic physics, particularly feedback from star formation, plays a crucial role in alleviating the cusp-core discrepancy by modifying the inner density profiles of dark matter halos through dynamical interactions. Supernova-driven outflows, triggered by bursty star formation in dwarf galaxies, expel central gas and impart energy to the dark matter particles, leading to the formation of constant-density cores. This mechanism was first demonstrated in simulations where repeated outflows from supernova explosions in low-mass halos (with virial masses around $10^{9}-10^{10} M_\odot) flatten initially cuspy profiles by evacuating baryons and heating the dark matter distribution.^[34] Hydrodynamical simulations incorporating realistic baryonic physics have shown that these feedback processes can transform cusps into cores on scales relevant to observed dwarfs. In the Feedback In Realistic Environments (FIRE) project, cosmological zoom-in simulations of low-mass galaxies (M_* \simeq 10^{6.5} M_\odot) reveal that bursty star formation, coupled with supernova feedback, creates large dark matter cores of approximately 1 kpc in radius by driving rapid gas outflows that disrupt the central potential multiple times. Higher-resolution FIRE-2 runs further indicate that feedback efficiency leads to core formation down to resolutions of about 100 pc in ultra-faint dwarfs, with the inner slopes flattening from \gamma \approx -1 (cusp-like) to \gamma \approx 0 (core-like). These effects are most pronounced in halos where the stellar-to-halo mass ratio is around $10^{-3}, highlighting the sensitivity to baryonic processes.^[35] While the gravitational contraction of dark matter due to central baryonic concentrations can initially deepen cusps, counteracting processes such as dynamical friction from sinking gas clumps and orbital heating from potential fluctuations mitigate this effect. In models of supernova feedback, rapid changes in the central potential—caused by gas expulsion on timescales shorter than the dynamical time—irreversibly energize dark matter particles, expanding their orbits and preventing recollapse. Dynamical friction contributes modestly by allowing transient baryonic clumps to transfer angular momentum, but the dominant heating arises from resonant energy transfer during outflow episodes. Quantitative analytic models predict that core sizes scale with the injected feedback energy, with the core radius r_c roughly proportional to (E_{\rm fb}/\Sigma)^{1/3}, where E_{\rm fb} is the total supernova energy and \Sigma is the central surface density; for typical dwarf parameters, this yields cores of \sim 100-200 pc that persist to low redshifts.^[36]

Alternative Dark Matter Candidates

The cuspy halo problem has motivated exploration of dark matter models beyond the standard cold dark matter (CDM) paradigm, where modifications to the dark matter particle's properties inherently lead to cored density profiles in galactic halos without relying on baryonic feedback. These alternatives include warm dark matter (WDM), self-interacting dark matter (SIDM), and fuzzy dark matter (FDM), each addressing small-scale structure discrepancies through distinct physical mechanisms.^[37] Warm dark matter consists of particles with non-negligible thermal velocities at early times, such as keV-scale sterile neutrinos, which introduce a characteristic free-streaming length that suppresses the formation of small-scale density perturbations. This smoothing effect prevents the development of steep cusps in low-mass halos, resulting in cored profiles that align better with observations of dwarf galaxies. Early simulations demonstrated that WDM halos exhibit finite central densities due to this free-streaming, with core sizes scaling inversely with particle mass.^[37]^[38] Self-interacting dark matter posits that dark matter particles experience velocity-dependent scattering interactions, allowing momentum transfer that heats the central regions of halos and flattens cusps into cores. Proposed to resolve multiple CDM tensions, including the core-cusp issue, SIDM models predict core formation through repeated scatterings that redistribute energy, with the interaction strength tuned to avoid excessive heating in massive systems. Numerical tests confirm that intermediate cross-sections produce stable cores in dwarf galaxies while preserving cuspy profiles in clusters.^[39] Fuzzy dark matter, composed of ultralight scalar particles with masses around $10^{-22} eV, behaves as a coherent wave on galactic scales due to its large de Broglie wavelength, which resists gravitational collapse and forms solitonic cores at halo centers. These quantum pressure effects stabilize the density profile against cusp formation, yielding flat cores with radii proportional to the particle mass. Simulations show that FDM halos transition from a central soliton to an surrounding Navarro-Frenk-White-like envelope, providing a natural resolution to the cusp-core tension.^[40]^[41] In SIDM models, cross-sections per unit mass of \sigma/m \sim 1 cm²/g effectively generate cores in Milky Way-sized and dwarf halos that match rotation curve data, while velocity-dependent interactions ensure minimal core overproduction in galaxy clusters, maintaining consistency with lensing observations.^[39]

Observational and Simulation Biases

Observational biases in deriving dark matter density profiles from galaxy data often stem from instrumental limitations that obscure the innermost regions. For rotation curve measurements in dwarf galaxies, the spatial resolution of observations, typically limited to scales greater than 0.1–0.5 kpc due to beam sizes in radio interferometry (e.g., VLA or Westerbork arrays), prevents probing the central kiloparsec where cuspy profiles would manifest as steeply rising velocities. This coarse resolution can smear out the expected cusp, making profiles appear flatter and more core-like, thereby exaggerating the core-cusp tension.^[42]^[43] In dwarf spheroidal galaxies, Jeans modeling of stellar velocity dispersions introduces a similar bias through an imposed floor on the minimum dispersion, often around 1–3 km/s, arising from unresolved binary star motions and spectroscopic errors. This floor artificially caps low central dispersions, leading to inferences of constant-density cores rather than cusps, even when the underlying profile may be steeper. Improved Jeans analyses that account for velocity anisotropy and non-spherical geometry have shown that such assumptions can reverse apparent cores into cuspy profiles in mock data from simulations.^[44] Simulation artifacts further complicate comparisons by altering predicted halo profiles in ways that amplify discrepancies. In low-resolution N-body runs (particle counts below ~10^6), two-body relaxation effects dominate the central dynamics, artificially heating particles and flattening cusps over timescales comparable to a few halo relaxation times (~1 Gyr for dwarf-scale halos), which underestimates the true steepness and partially masks the cuspy nature expected in higher-resolution limits. Conversely, standard adiabatic contraction models, which approximate baryon-driven compression of dark matter halos, overestimate the final central densities by up to a factor of 2–3 in the inner regions, especially for low-concentration halos, leading to excessively steep profiles that exaggerate the cusp relative to uncontracted simulations.^[45]^[46]^[47] Key corrections mitigate these issues by refining both observational inferences and simulation techniques. Enhanced Jeans methods incorporating higher-order moments of the velocity distribution and tidal stripping in subhalo modeling better recover cuspy profiles from dispersion data, reducing biases from assumed isotropy. Including dynamical tidal effects, such as mass loss and heating in subhalos orbiting host galaxies, prevents overestimation of central densities by accounting for profile evolution post-infall, where stripping can create apparent cores in otherwise cuspy structures. Recent high-resolution data, such as microlensing events from the OGLE survey, suggest milder central slopes in the Milky Way's dark matter halo (inner density profile closer to a core of ~0.3 kpc radius than a pure NFW cusp), indicating that past biases may have overstated the tension.^[48]^[49]^[30]

Current Status and Future Directions

Recent Observational Advances

Recent observations from the James Webb Space Telescope (JWST) have provided deeper imaging of ultra-faint dwarf galaxies, revealing structures that inform the core-cusp debate. As of November 2025, JWST Cycle 3 observations continue to target resolved stellar populations in these systems, potentially clarifying dark matter profiles.^[50] Gaia Data Release 3 (DR3), released in 2022, has enabled detailed mapping of the Milky Way's inner stellar halo using blue horizontal branch stars and RR Lyrae variables, showing a density profile that flattens toward the Galactic center. Studies of the halo's spatial distribution indicate a transition to shallower profiles in the inner regions when accounting for tidal debris and accretion history.^[51] Strong lensing studies of galaxy clusters have continued to probe subhalo dark matter profiles, with 2023–2025 analyses indicating varied structures. For instance, a 2025 non-parametric reconstruction of the lensing system JVAS B1938+666 reveals a dense dark matter core in a subhalo with a central concentration, providing evidence for core-like features and highlighting debates in cluster-scale profiles.^[52] These observations provide independent constraints on dark matter granularity at intermediate scales. A 2025 analysis of rotation curves from the SPARC catalog, incorporating bias corrections for observational systematics, addresses the core-cusp tension in dwarf galaxies by demonstrating that cold collisionless dark matter models with statistical mechanics-based profiles can reproduce observed cores without invoking feedback or modified physics. This update emphasizes the role of diversity in dwarf galaxy dynamics, with the analysis applied to the SPARC sample of late-type galaxies.^[53]

Ongoing Theoretical Developments

Recent theoretical efforts in the 2020s have focused on hybrid models that integrate self-interacting dark matter (SIDM) with baryonic physics to address the core-cusp tension more dynamically. These models demonstrate that baryonic processes, such as feedback and dissipation, interact with SIDM self-interactions to produce time-dependent core formation in dark matter halos. For instance, 2024 simulations reveal that the central density profiles evolve over cosmic time, with initial cusps flattening into cores that can partially reform under varying baryonic potentials, offering a nuanced resolution to discrepancies in dwarf galaxy observations.^[54] Machine learning techniques have emerged as powerful tools for extracting dark matter halo profiles from complex simulations, helping to resolve longstanding debates on numerical convergence in core-cusp studies. In 2023, graph neural networks were applied to infer density profiles in dwarf galaxies, enabling precise differentiation between cuspy and cored structures by analyzing simulation data with reduced bias from resolution limits. These AI-driven fits confirm that apparent cores in some simulations arise from incomplete convergence rather than physical effects, providing clearer benchmarks for validating theoretical models against data.^[55] Investigations into tidal evolution highlight how subhalo stripping can mimic cored profiles in otherwise cuspy dark matter distributions. A 2024 study in Physical Review D developed semi-analytic models showing that tidal interactions in host halos strip outer layers of cuspy subhalos, creating flat inner density gradients that persist over gigayears. This mechanism explains observed core-like features in satellite galaxies without invoking modified dark matter physics, emphasizing the role of environmental dynamics in the cusp-core problem.^[49] Looking ahead, theoretical predictions suggest that gravitational wave observatories like LISA could probe core-cusp signatures through supermassive black hole mergers influenced by halo profiles. In SIDM scenarios, density spikes around black holes alter merger rates and eccentricity, producing distinct stochastic gravitational wave backgrounds compared to cuspy cold dark matter predictions. These forecasts indicate that LISA detections in the millihertz band could distinguish between core and cusp models, offering a novel test of dark matter properties in galactic centers.^[56]

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[PDF] arXiv:astro-ph/0601669v3 23 Jun 2006
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[2505.09219] Probing Self-Interacting Dark Matter via Gravitational ...
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