Inferred Dark Matter Cores Are Indifferent To Baryonic Physics

Lots of dark matter models of galaxies rely on baryonic feedback (i.e. influence from ordinary matter) to account for its distributions. But a new paper states that the inferred core of a dark matter distribution over many galaxies is indifferent to the distribution of baryonic matter, which undermines this approach.

We present the first dark matter density profile derived directly from James Webb Space Telescope JADES Data Release~3 NIRSpec observations of N=587 galaxies spanning the cosmic noon epoch 1.5 ≤ z ≤ 3.5. From each NIRSpec G235M/G395M spectrum we extract the Hα emission-line velocity dispersion σha, stack galaxies in four redshift bins of Δz=0.5, and reconstruct representative group rotation curves within the General Theory of Relativity. Fitting the four-parameter modified exponential model and deriving the exact GTR energy density, we reduce the profile to a compact [2/3] Padé approximant with all coefficients determined in closed form from JWST observations. The profile is cusp-free, fully analytic, and redshift-dependent. Energy conditions, causality, and orbital stability are all satisfied. Most strikingly, the central density ρ(0) varies by less than 15% across z=1.5--3.5 despite a 35% decline in the asymptotic rotation velocity, revealing a universal dark matter core saturation density at cosmic noon decoupled from baryonic evolution.

Aritra Sanyal, Farook Rahaman "The New Dark Matter Density Profile from JWST JADES Galaxies" arXiv:2609.14071 (September 12, 2026).

Another paper also looks at inferred dark matter halo distributions, and, yet again, the distributions are not a good fit to the theoretically mandated NFW distribution for collisionless dark matter which is widely used by astrophysicists nonetheless.

We fit the circular velocity data, derived from weak lensing observations of isolated galaxies selected from the KiDS survey in four baryonic mass bins, using three different dark matter profiles. These include NFW, Burkert, and pseudo-isothermal profile. We find that the NFW and Burkert profiles cannot adequately fit the circular velocity data for all the four baryonic mass bins, whereas only the pseudo-isothermal profile can fit the data for all the four bins.

Sri Devaki Meduri, Shantanu Desai, "A comprehensive assessment of weak-lensing inferred circular velocity profiles of isolated galaxies" arXiv:2609.14085 (September 12, 2026).

Neutrinoless Quadruple Beta Decay

While detection of neutrinoless double beta decay is a sign that neutrinos are Majorana particles, neutrinoless quadruple beta decay in the absence of neutrinoless double beta decay is a sign that neutrinos have Dirac mass. So far, neither form of decay has been observed.

The observation of neutrinoless quadruple beta decay (0ν4β) in the absence of neutrinoless double beta decay (0ν2β) has been argued to provide a strong indication that neutrinos are Dirac particles. We report a search for 0ν4β decay of 136Xe using a total 136Xe exposure of 148.4 kg⋅yr, collected during the commissioning and the first science runs of the PandaX-4T experiment. No significant excess of events over the background is observed. A lower limit on the 0ν4β decay half-life of 136Xe is set at 6.01 x 10^24 yr at the 90% confidence level. This result establishes the most stringent constraint on this process in xenon, demonstrating the unique capability of the PandaX-4T detector in probing lepton number violation and shedding light on the fundamental nature of neutrinos.

PandaX Collaboration, "Search for neutrinoless quadruple beta decay of 136Xe in PandaX-4T detector" arXiv:2609.10048 (September 9, 2026).