The existence of an early matter-dominated epoch prior to the Big Bang Nucleosynthesis may lead to a scenario where the thermal dark matter cools faster than plasma before the radiation-dominated era begins. In the radiation-dominated epoch, dark matter free-streams after it decouples both chemically and kinetically from the plasma. In the presence of an early matter-dominated era, chemical decoupling of the dark matter may succeed by a partial kinetic decoupling before reheating ends, depending upon the contributions of different partial wave amplitudes in the elastic scattering rate of the dark matter. We show that the s-wave scattering is sufficient to partially decouple the dark matter from the plasma, if the entropy injection during the reheating era depends on the bath temperature, while p-wave scattering leads to full decoupling in such cosmological backdrop. The decoupling of dark matter before the end of reheating causes an additional amount of cooling, reducing its free-streaming horizon compared to the usual radiation-dominated cosmology. The enhanced matter perturbations for scales entering the horizon prior to the end of reheating, combined with the reduced free-steaming horizon, increase the number density of sub-earth mass halos. The resulting boost in the dark matter annihilation signatures could offer an intriguing probe to differentiate pre-BBN non-standard cosmological epochs. We show that the free-streaming horizon of the dark matter requires to be smaller than a cut-off to ensure a boost in the sub-earth halo populations. As case studies we present two examples: one for a scalar dark matter with $s$-wave elastic scattering and the other one featuring a fermionic dark matter with $p$-wave elastic scattering. We identify regions of parameter space in both models where the dark matter kinetically decouples during reheating, amplifying small-scale structure formation.
Breaking a dark degeneracy: The gamma-ray signature of early matter domination
宇宙早期的热历史没有受到严格的约束，它可能经历了一段早期物质支配的时期，由重粒子或振荡的标量场驱动，在大爆炸核合成开始之前衰变为辐射。这种粒子衰变产生的熵减少了热遗迹暗物质达到观测到的丰度所需的截面。这种暗物质性质和热历史之间的简并性极大地拓宽了可行的暗物质候选者的领域，破坏了限制暗物质身份的努力。幸运的是，早期物质主导的时代也放大了小尺度的密度波动，导致早期微晕的形成，提高了暗物质湮灭率，并使现有的间接探测探测器能够看到更小的截面。采用最近开发的微晕形成和演化的模型，我们开发了一个程序，以获得间接检测早期物质占主导地位的宇宙学中的暗物质湮灭的限制。这个过程适当占微晕为主的信号的独特形态。虽然限制取决于暗物质的自由流动的规模，微晕使人们有可能获得小到$\langleσv\rangle \lesssim 10^{-32}$ cm $^3$ s $^{-1}$使用费米-LAT观测的各向同性伽马射线背景和天龙座矮星系的上限。
