We perform a detailed study of dark matter production via freeze-in under the assumption that some fluid dominates the early Universe before depositing its energy to the plasma causing entropy injection. As a dark matter candidate we consider a fermionic singlet that is produced through its interactions with a scalar particle in the thermal plasma. The fluid alters the expansion rate of the Universe, as well as the scaling of the temperature, which significantly affects the evolution of both the number density and the mean momentum of the dark matter particle. We identify and discuss in detail the effects of the evolution of these quantities by considering several examples representing dark matter production at different stages of expansion and entropy injection. We find that, since the dark matter density is reduced when the entropy injection to the plasma continues after freeze-in, in order to reproduce its observational value an enhanced rate of dark matter production is required relative to standard cosmology. Furthermore, the impact of the assumed non-standard cosmological history on the dark matter mean momentum can result in either a relaxed or a tightened bound on the dark matter mass from large structure formation data.

我们假设一些流体在将其能量沉积到等离子体引起熵注入之前主宰了早期宇宙，我们对通过冻结产生的暗物质进行了详细研究。作为暗物质候选者，我们考虑通过与热等离子体中的标量粒子相互作用产生的费米子单线态。流体改变了宇宙的膨胀率，以及温度的尺度，这显着影响了暗物质粒子的数密度和平均动量的演变。我们通过考虑几个代表暗物质在膨胀和熵注入的不同阶段产生的例子来详细确定和讨论这些量的演变的影响。我们发现，由于在冻结后继续向等离子体注入熵时暗物质密度会降低，因此为了重现其观测值，需要相对于标准宇宙学提高暗物质产生率。此外，假设的非标准宇宙学历史对暗物质平均动量的影响可能会导致大型结构形成数据对暗物质质量的约束放松或收紧。