Nonthermal WIMPs and primordial black holes

Julian Georg, Gizem Şengör, and Scott Watson

Phys. Rev. D 93, 123523 – Published 23 June 2016

Abstract

Nonthermal histories for the early universe have received notable attention as they are a rich source of phenomenology, while also being well motivated by top-down approaches to beyond the Standard Model physics. The early (pre-big bang nucleosynthesis) matter phase in these models leads to enhanced growth of density perturbations on sub-Hubble scales. Here, we consider whether primordial black hole formation associated with the enhanced growth is in conflict with existing observations. Such constraints depend on the tilt of the primordial power spectrum, and we find that nonthermal histories are tightly constrained in the case of a significantly blue spectrum. Alternatively, if dark matter is taken to be of nonthermal origin, we can restrict the primordial power spectrum on scales inaccessible to cosmic microwave background and large scale structure observations. We establish constraints for a wide range of scalar masses (reheat temperatures) with the most stringent bounds resulting from the formation of $10^{15} g$ black holes. These black holes would be evaporating today and are constrained by FERMI observations. We also consider whether the breakdown of the coherence of the scalar oscillations on subhorizon scales can lead to a Jean’s pressure preventing black hole formation and relaxing our constraints. Our main conclusion is that primordial black hole constraints, combined with existing constraints on nonthermal weakly interacting massive particles, favor a primordial spectrum closer to scale invariance or a red tilted spectrum.

Received 18 April 2016

DOI:https://doi.org/10.1103/PhysRevD.93.123523

Physics Subject Headings (PhySH)

Big bang nucleosynthesis Dark matter

Astronomical black holes Weakly interacting massive particles

Particles & Fields

Authors & Affiliations

Julian Georg^*, Gizem Şengör^†, and Scott Watson^‡

Department of Physics, Syracuse University, Syracuse, New York 13244, USA

^*jsgeorg@syr.edu
^†gsengor@syr.edu
^‡gswatson@syr.edu

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Vol. 93, Iss. 12 — 15 June 2016

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Images

Figure 1
Evolution of the background energy densities compared to the total density $ρ$ for a nonthermal history. In the figure, we take $m_{X} = 500 GeV$ , $B_{X} = 1 / 3$ , $g_{*} = 30$ , and $⟨ σ v ⟩ = 3 \times 10^{- 8} {GeV}^{- 2}$ . We also chose the initial dimensionless decay rate as $Γ_{σ} / H_{0} ≃ 2 \times 10^{- 7}$ and the scalar mass $m_{σ} = 10^{6} GeV$ . The universe becomes radiation dominated at $\log a ≃ 11$ and at a temperature of around 700 MeV. We refer the reader to Ref. [5] for more details.
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Figure 2
The left (right) plot gives the constraints for the low (high) reheat model discussed in the text (see also Table 1). The restricted region is at the top (red), and the light (green) region represents models with a spectral tilt of $n = 1.4$ , whereas the darker lower region (blue) is nearly scale invariant at $n = 1.1$ . We see that the stronger constraints are for high reheat models (right panel) where the matter dominated phase is shorter. This is because the evolution in a matter dominated universe softens PBH constraints as the density of black holes and/or evaporation products will scale more slowly compared to that in a radiation dominated universe. This effect is more important than the fact that PBHs have more time to form during a longer matter phase. The observational constraints are comprised of a number of different observations and taken from the review [10].
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Figure 3
This plot gives the constraints relevant for the wino DM scenario discussed in the text. The restricted region is at the top (red), and the light (green) region represents models with a spectral tilt of $n = 1.4$ , whereas the darker lower region (blue) is nearly scale invariant at $n = 1.1$ . We have considered a GeV reheat temperature with $m_{σ} = 1.4 \times 10^{3} TeV$ and $Γ = 4.5 \times 10^{- 19} GeV$ . The allowed PBH mass range then has a minimal mass of $M_{\min} = 2.2 \times 10^{7} g$ , whereas the maximal masses are given by $M_{\max} = 5.7 \times 10^{26} g$ and $M_{\max} = 6.8 \times 10^{28} g$ for $n = 1.1$ and $n = 1.4$ , respectively. We see that the formation of PBHs with masses around $M ≃ 10^{14} g$ are in tension with the data.
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Figure 4
In this figure, we combine our PBH constraints with existing indirect detection bounds on nonthermal SUSY WIMP models. Reheat temperatures below around 1 GeV are ruled out by the indirect detection bounds given in Ref. [5]. Above around 1000 TeV, the WIMPs would necessarily be thermal since masses in excess of that scale would violate unitarity constraints, and so in that region $T > m_{X}$ —DM is thermal. The top red region represents the PBH constraints from a variety of observations including gamma rays, CMB, and relic constraints all taken from the review [10].
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