Prospects of Indirect Detection of Dark Matter via Primordial Black Hole Induced Gravitational Waves

TL;DR

This paper investigates DM production in a PBH-dominated early Universe, where PBH evaporation induces reheating and generates a stochastic GW background. It analyzes DM production via PBH evaporation, gravitational scattering, and thermal freeze-in/out, identifying regions that yield the observed relic abundance while respecting BBN, CMB, and Lyman-$\alpha$ constraints. The authors show that GW observations from LISA and ET can uniquely probe feebly interacting DM (freeze-in) and PBH-induced expansion history, while indirect detection constrains larger annihilation cross-sections for freeze-out scenarios; crucially, the DM parameter space probed by GWs and by indirect searches is largely non-overlapping. Together, these results establish gravitational-wave observations as a powerful, independent probe of DM production and the pre-BBN thermal history, offering a new observational window into DM properties. They also highlight the complementary roles of DM indirect searches and GW missions in testing PBH-reheating scenarios across a wide range of DM masses and interaction strengths.

Abstract

Primordial black holes (PBHs), produced in the early Universe, can source a stochastic background of induced gravitational waves (GWs) and provide a non-thermal origin for dark matter (DM). We investigate DM production in a PBH-dominated cosmological framework, including contributions from PBH evaporation, gravitational production, and thermal freeze-in and freeze-out mechanisms, and determine the regions consistent with the observed DM relic abundance. We find that thermal freeze-in can compensate for the underabundance of PBH-sourced DM, while indirect detection remains largely insensitive due to the feeble interaction strength, making future GW observatories such as LISA and the Einstein Telescope (ET) unique probes of this scenario. For freeze-out DM, indirect detection experiments constrain regions with relatively large annihilation cross-sections, whereas GW observations probe complementary regions with heavier DM masses and smaller interaction strengths. Consequently, the same DM parameter space cannot be simultaneously probed by both indirect detection searches and GW missions. These results establish GW observations as a powerful and independent probe of DM production in PBH-dominated cosmologies, opening a new observational window into DM properties and the thermal history of the pre-BBN Universe.

Figures (18)

• Figure 1: Left panel: Evolution of the normalized energy densities,$\Omega_j \equiv \rho_j /(3 M_{P}^2 H^2)$, as functions of the normalized scale factor. Right panel: Dependence of the critical PBH energy fraction $\beta_c$ on the initial PBH mass $M_{\rm in}$. The shaded regions indicate excluded parameter space: the red band corresponds to constraints from CMB on the PBH-formation mass, while the brown band represents bounds arising from BBN.
• Figure 2: Allowed parameter range in the $M_{\rm in}$-$M_{\rm X}$ plane, considering the production of DM only from the decay of PBH. The blue shaded region is excluded due to the overproduction of DM. The grey-hatched areas are ruled out by BBN and CMB constraints. The excluded region from Lyman-$\alpha$ observation is shown by light orange region. Figure reproduced from Ref. Haque:2023awl.
• Figure 3: s-channel Feynman diagram for the production of DM from scattering of the SM particles through the exchange of graviton. $R$ and $X$ are the radiation and DM particles, respectively. $h_{\mu\nu}$ presents the graviton field.
• Figure 4: Allowed parameter range in the $M_{\rm in}$-$M_{\rm X}$ plane, considering the gravitational production of DM from the SM thermal bath together with PBH-sourced DM. The purple shaded region corresponds to overproduction of DM due to gravitational freeze-in, while the blue-shade denotes overproduction from PBH evaporation. Regions excluded from Lyman-$\alpha$ observations are shown by light orange shade, and the grey-hatched areas are ruled out by BBN and CMB constraints. The left panel corresponds to $\beta=10^{-7}$, while the right panel is for $\beta=10^{-9}$. The red-hatched region indicates $\beta<\beta_c$, where the PBH never dominate the energy budget of the Universe.
• Figure 5: Evolution of yield of DM and temperature for PBH reheating scenario. The red curve shows the evolution of temperature. Each panel displays the yield of DM (black solid line) and equilibrium number density (purple dotted line), for $M_{\rm X}=10^{13}$ GeV, $M_{\rm in}=10^7$ g and $\beta=10^{-7}$. The left panel corresponds to PBH-sourced DM production. The right panel shows the freeze-in production, with $\langle \sigma v\rangle=1.1\times 10^{-46}$ GeV$^{-2}$, in addition to PBH-sourced production.