Dark matters are Inert, or FIMPy, or WIMPy or UFOy: An inflationary gravitational particle production

Abstract

In this letter, we explore the phenomenological impact of inflationary gravitational particle production in the physics of Dark Matter (DM). Large-scale DM fluctuations generated during inflation behave as gravitational particles upon their post-inflationary horizon reentry and alter the conventional Boltzmann dynamics of DM with a non-conserving source term, thereby producing significant phenomenological consequences. Within this framework, we analyze four distinct types of DM classified according to their production mechanisms. Dark matter may be completely non-interacting with the thermal bath, behaving as Inert Dark Matter. Alternatively, depending on the strength of its interactions with bath particles, DM may exhibit WIMPy, UFOy, or FIMPy behavior, sharing characteristics with their conventional counterparts. The late-time enhancement of the DM number density, driven by the successive horizon reentry of gravitationally produced low-momentum modes, enlarges the viable parameter space for both thermal and non-thermal DM scenarios. Remarkably, this expanded parameter space remains consistent with current constraints from $ΔN_{\rm eff}$ and Lyman-$α$ bound.

Figures (3)

• Figure 1: Figure represents IR mass $m_{\chi}^{\rm IR}$ vs $\delta$ parameter space for purely gravitational dark matter or an inert dark matter. Each $\delta$ gives a single DM mass which satisfies the present relic.
• Figure 2: This is the representative figure comparing three different mechanisms of DM yield ($Y_{\chi},~Y_{\chi}^{\rm eq}$) with respect to varying $x={m_{\chi}}/{T}$. Solid lines represent the DM yield, $Y_{\chi}$, for UFOy (magenta), FIMPy (blue), and WIMPy (brown) DM, along with the equilibrium evolution $Y_{\chi}^{\rm eq}$ in dashed black curves. For WIMPy and FIMPy DM, the DM mass is chosen to be $m_{\chi}=10$ GeV, and for UFOy DM, the mass is chosen to be $m_{\chi}= 10^{-8}$ GeV.
• Figure 3: Figure represents the ($m_{\chi},\,\langle\sigma v\rangle$) DM parameter space for different $\delta$ values exploring three distinct possibilities (WIMPy, FIMPy, UFOy) of the DM production mechanism. Five red dots on the solid colored lines indicate the transition from WIMPy (solid colored lines) to UFOy (dot-dashed colored lines) scenarios, and five black dots indicate the transition from the UFOy to FIMPy (dashed colored lines) scenarios. For $\delta=-3, -2.95, -2.9, -2.85, -2.8$, $Q_{\rm IR}$ gives the total abundance of DM in the non-thermal freeze-in scenario, resulting in the sharp fall of $\langle\sigma v\rangle$ from the black dots at those critical mass values $m_{\chi}^{\rm IR}$(see Eq.(\ref{['eq:mIR']})). The light red shaded region$\left(m_{\chi}\in (m_{\chi}^{\rm UFO},\,4\,{\rm MeV}]\right)$is excluded by the $\Delta N_{\rm eff}$ constraint.