CDM Fails Again

I'm not surprised, but again and again and again, the evidence against cold dark matter theories piles up. 

The properties of substructure in galaxy clusters, exquisitely probed by gravitational lensing, offer a stringent test of dark matter models. Combining strong and weak lensing data for massive clusters, we map their total mass--dominated by dark matter--over the dynamic range needed to confront small-scale predictions for collisionless cold dark matter (CDM). Using state-of-the-art lens models, we extract four key subhalo properties: the mass function, projected radial distribution, internal density profile, and tidal truncation radius. 

We find that the subhalo mass function and truncation radii are consistent with CDM expectations. In contrast, the inner density profiles and radial distribution of subhalos are strongly discrepant with CDM. The incidence of galaxy-galaxy strong lensing (GGSL) from subhalo cores exceeds CDM predictions by nearly an order of magnitude, requiring inner density slopes as steep as γ≳2.5 within r≲0.01R200 consistent with core-collapsed self-interacting dark matter (SIDM), while the same subhalos behave as collisionless in their outskirts. Additionally, the observed radial distribution of subhalos hosting bright cluster member galaxies, explicitly modeled in the lens reconstructions, remains incompatible with CDM. Together, these small-scale stress tests reveal an intriguing paradox and challenge the dark matter microphysics of purely collisionless CDM and motivate hybrid scenarios, such as a dual-component model with both CDM and SIDM, or entirely new classes of dark matter theories.

Priyamvada Natarajan, Barry T. Chiang, Isaque Dutra, "New CDM Crisis Revealed by Multi-Scale Cluster Lensing" arXiv:2601.07909 (January 12, 2026).

Lava Worlds

Until most of my posts, this isn't notable because it sheds light on any deeper laws of physics. It is just amazing that worlds like this exist.

Lava worlds are rocky planets with dayside skins made molten by stellar irradiation. Tidal heating on these shortest-period planets is more than skin deep. We show how orbital eccentricities of just a few percent (within current observed bounds and maintained secularly by exterior companions) can create deep magma oceans. ``Lava tidal waves'' slosh across these oceans; we compute the multi-modal response of the ocean to tidal forcing, subject to a coastline at the day-night terminator and a parameterized viscous drag. Wave interference produces a dayside heat map that is spatially irregular and highly time-variable; hotspots can wander both east and west of the substellar point, and thermal light curves can vary and spike aperiodically, from orbit to orbit and within an orbit. Heat deposited by tides is removed in steady state by a combination of fluid, mushy, and solid-state convection in the mantle. For Earth-sized planets with sub-day periods, the entire mantle may be tidally liquified.

Mohammad Farhat, Eugene Chiang, "Magma Ocean Waves and Thermal Variability on Lava Worlds" arXiv:2601.07080 (January 11, 2026) (Submitted to AAS Journals).

Baryonic Feedback

One of the ways to overcome the discrepancies between dark matter particle theories and what we observe is to attribute the discrepancies to baryonic feedback effects that are not terribly well understood. An ambitious new paper with many co-authors examines feedback effects in multiple cosmology simulations. The trouble is that the feedback seems to aggravate the discrepancies between what of observed and what simulations predict, rather than resolving them. 

Galaxy cores behave more or less like galaxies without dark matter phenomena, while the dynamics of galactic fringes are dominated by dark matter phenomena. And, more massive galaxies are less proportionately dark matter phenomena driven than less massive galaxies. Yet, these are just the opposite of the effects of baryonic feedback in the simulations considered.

Baryonic processes such as radiative cooling and feedback from massive stars and active galactic nuclei (AGN) directly redistribute baryons in the Universe but also indirectly redistribute dark matter due to changes in the gravitational potential. In this work, we investigate this "back-reaction" of baryons on dark matter using thousands of cosmological hydrodynamic simulations from the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project, including parameter variations in the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE galaxy formation models. 

Matching haloes to corresponding N-body (dark matter-only) simulations, we find that virial masses decrease owing to the ejection of baryons by feedback. Relative to N-body simulations, halo profiles show an increased dark matter density in the center (due to radiative cooling) and a decrease in density farther out (due to feedback), with both effects being strongest in SIMBA (> 450% increase at r < 0.01 Rvir). The clustering of dark matter strongly responds to changes in baryonic physics, with dark matter power spectra in some simulations from each model showing as much as 20% suppression or increase in power at k ~ 10 h/Mpc relative to N-body simulations. 

We find that the dark matter back-reaction depends intrinsically on cosmology (Omega_m and sigma_8) at fixed baryonic physics, and varies strongly with the details of the feedback implementation. These results emphasize the need for marginalizing over uncertainties in baryonic physics to extract cosmological information from weak lensing surveys as well as their potential to constrain feedback models in galaxy evolution.

Matthew Gebhardt, et al., "Cosmological back-reaction of baryons on dark matter in the CAMELS simulations" arXiv:2601.06258 (January 9, 2026).

A new paper suggesting an interacting dark energy model is also intriguing.

Recent DESI baryon acoustic oscillation data reveal deviations from ΛCDM cosmology, conventionally attributed to dynamical dark energy (DE). We demonstrate that these deviations are equally, if not better, explained by interactions between dark matter and dark energy (IDE), without requiring a time-varying DE equation of state. Using a unified framework, we analyze two IDE models--coupled quintessence and coupled fluid--against the latest CMB (Planck, ACT, SPT), DESI BAO, and SN (including DES-Dovekie recalibrated) data. Both IDE scenarios show robust evidence for non-vanishing interactions at the 3-5σ level, with marginalized constraints significantly deviating from the ΛCDM limit. This preference persists even under DES-Dovekie SN recalibration, which weakens dynamical DE evidence. Crucially, for the same number of free parameters, IDE models provide fits to low- and high-redshift data that match or exceed the performance of the CPL dynamical DE parametrization. Our results establish IDE as a physically motivated alternative to dynamical DE, highlighting the necessity of future cosmological perturbation measurements (e.g., weak lensing, galaxy clustering) to distinguish between these paradigms.

Tian-Nuo Li, et al., "Strong Evidence for Dark Sector Interactions" arXiv:2601.07361 (January 11, 2026).

See also a new paper exploring Moffat's modified gravity approach, and a new paper examining the warm dark matter hypothesis.