LT-CDM: Late-time friction on cold dark matter and the S8 tension

We introduce a minimal one-parameter extension of ΛCDM, termed Late-time Cold Dark Matter friction (LT-CDM), in which the cold dark matter velocity divergence experiences a drag term scaling as a⁴. This scaling makes the mechanism negligible at early times (preserving the CMB and sound horizon) while suppressing late-time structure growth. Implemented in a modified version of the CLASS Boltzmann code and analyzed via MCMC against BAO data (DESI DR1, BOSS DR12), the Planck acoustic-scale prior, a BBN prior, and the DES Year 3 weak-lensing measurement of S8, the model shows a 2.1σ preference for non-zero friction (α₀ = 2.7 ± 1.3). The mechanism resolves the S8 tension to 0.1σ while leaving the CMB acoustic scale and BAO sound horizon unchanged. This deposit includes the manuscript, the patch against CLASS v3.3.4, the likelihood and MCMC driver scripts, and the full MCMC chains (8 parallel chains, 70,782 accepted samples) for complete reproducibility.我们提出了Λ冷暗物质模型（ΛCDM）的极简单参数扩展形式，命名为延迟型冷暗物质摩擦模型（Late-time Cold Dark Matter friction, LT-CDM）。该模型中，冷暗物质的速度散度会受到一个形如a⁴的拖曳项作用。这种标度关系使得该机制在早期宇宙中可忽略不计，保留了宇宙微波背景（Cosmic Microwave Background, CMB）与声视界的预言，同时抑制晚期宇宙的结构生长。 本研究基于修改版的CLASS玻尔兹曼代码实现该模型，并通过马尔可夫链蒙特卡洛（Markov Chain Monte Carlo, MCMC）方法，以暗能量光谱仪（Dark Energy Spectroscopic Instrument, DESI）DR1、重子振荡光谱巡天（BOSS）DR12的BAO数据，普朗克卫星声学尺度先验、大爆炸核合成（Big Bang Nucleosynthesis, BBN）先验，以及暗能量巡天（Dark Energy Survey, DES）第三年弱引力透镜测得的S8测量结果作为观测约束开展拟合分析。结果显示，该模型在2.1σ置信水平下支持非零摩擦项（α₀ = 2.7 ± 1.3）。该机制可将S8张力缓解至0.1σ，同时保持CMB声学尺度与BAO声视界不变。 本数据集存档包含完整的研究手稿、适配CLASS v3.3.4版本的补丁文件、似然函数与MCMC驱动脚本，以及完整的MCMC采样链（8条并行链，共计70782个接受样本），以保障研究结果可完全复现。