Our ignorance of the period between the end of inflation and the beginning of Big Bang Nucleosynthesis limits our understanding of the origins and evolution of dark matter. One possibility is that the Universe's energy density was dominated by a fast-rolling scalar field while the radiation bath was hot enough to thermally produce dark matter. We investigate the evolution of the dark matter density and derive analytic expressions for the dark matter relic abundance generated during such a period of kination. Kination scenarios in which dark matter does not reach thermal equilibrium require σv < 2.7 × 10 −38 cm 3 s −1 to generate the observed dark matter density while allowing the Universe to become radiation dominated by a temperature of 3 MeV. Kination scenarios in which dark matter does reach thermal equilibrium require σv > 3 × 10 −26 cm 3 s −1 in order to generate the observed dark matter abundance. We use observations of dwarf spheroidal galaxies by the Fermi Gamma-Ray Telescope and observations of the Galactic Center by the High Energy Stereoscopic System to constrain these kination scenarios. Combining the unitarity constraint on σv with these observational constraints sets a lower limit on the temperature at which the Universe can become radiation dominated following a period of kination if σv > 3 × 10 −31 cm 3 s −1 . This lower limit is between 0.05 GeV and 1 GeV, depending on the dark matter annihilation channel.

Recent observations show that small, young, stellar groupings of ∼ 10 to 40 members tend of have a centrally-located most massive member, reminiscent of mass segregation seen in large clustered systems. Here, we analyze hydrodynamic simulations which form small clusters and analyze their properties in a manner identical to the observations. We find that the simulated clusters possess similar properties to the observed clusters, including a tendency to exhibit mass segregation. In the simulations, the central location of the most massive member is not due to dynamical evolution, since there is little interaction between the cluster members. Instead, the most massive cluster member appears to form at the center. We also find that the more massive stars in the cluster form at slightly earlier times.

If the Universe's energy density was dominated by a fast-rolling scalar field while the radiation bath was hot enough to thermally produce dark matter, then dark matter with larger-than-canonical annihilation cross sections can generate the observed dark matter relic abundance. To further constrain these scenarios, we investigate the evolution of small-scale density perturbations during such a period of kination. We determine that once a perturbation mode enters the horizon during kination, the gravitational potential drops sharply and begins to oscillate and decay. Nevertheless, dark matter density perturbations that enter the horizon during an era of kination grow linearly with the scale factor prior to the onset of radiation domination. Consequently, kination leaves a distinctive imprint on the matter power spectrum: scales that enter the horizon during kination have enhanced inhomogeneity. We also consider how matter density perturbations evolve when the dominant component of the Universe has a generic equation-of-state parameter w. We find that matter density perturbations do not grow if they enter the horizon when 0 < w < 1/3. If matter density perturbations enter the horizon when w > 1/3, their growth is faster than the logarithmic growth experienced during radiation domination. The resulting boost to the small-scale matter power spectrum leads to the formation of enhanced substructure, which effectively increases the dark matter annihilation rate and could make thermal dark matter production during an era of kination incompatible with observations.

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