Speaker
Description
We investigate whether a minimal effective pressure in the dark matter sector can generate observable deviations from standard cold dark matter ($\Lambda$CDM) predictions at nonlinear scales. We model dark matter as a polytropic fluid with equation of state $P = K \rho^{3/2}$, interpreted as an effective coarse-grained closure of the collisionless Jeans hierarchy in virialized halos.
For this choice, equilibrium configurations correspond to the $n = 2$ Lane--Emden solution, producing finite-density cores with mass-dependent scaling. Embedding these solutions within $\Lambda$CDM halo populations, we obtain kiloparsec-scale core radii with weak mass dependence across dwarf-to-galaxy scales, while preserving the background expansion history and linear perturbation growth.
We compute projected surface densities and weak-lensing convergence profiles for mass-matched halos. Relative to Navarro--Frenk--White profiles, the model predicts a systematic suppression of central convergence within $R \lesssim \mathrm{few}\, R_c$, with deviations confined to nonlinear scales. The convergence power spectrum exhibits scale-dependent suppression at high multipoles, providing a potential observational signature for upcoming high-resolution weak-lensing surveys.
This framework introduces a single phenomenological parameter governing nonlinear pressure support and continuously reduces to collisionless cold dark matter in the limit $K \to 0$. It therefore provides a minimal and testable extension of $\Lambda$CDM linking halo core structure to observable lensing signatures.
| Other topic / keywords: | Dark matter; halo structure; polytropic fluid; weak gravitational lensing; large-scale structure |
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