WIMPZILLAS!

TL;DR

The paper explores nonthermal dark matter candidates, WIMPZILLAS, with masses far above conventional thermal WIMPs, whose existence and abundance can be rooted in early-universe dynamics tied to inflation. It reviews four production channels—gravitational production, reheating, preheating, and bubble collisions—showing that each can yield the correct relic density without requiring strong couplings, with the mass scale set by the inflationary Hubble parameter $H_I$. A key insight is that these superheavy relics can achieve $\Omega_X h^2 \sim 0.3$ under plausible conditions, circumventing the unitarity and equilibrium constraints that limit thermal WIMPs. The work also highlights observational links, suggesting that measuring tensor perturbations could reveal $H_I$ and thereby anchor the WIMPZILLA mass scale, while noting the potential for indirect or direct detection if these particles carry color or electric charge. Overall, the study broadens the landscape of dark-matter candidates by showing how extremely massive, nonthermal relics could constitute the cosmic dark matter.

Abstract

There are many reasons to believe the present mass density of the universe is dominated by a weakly interacting massive particle (WIMP), a fossil relic of the early universe. Theoretical ideas and experimental efforts have focused mostly on production and detection of thermal relics, with mass typically in the range a few GeV to a hundred GeV. Here, I will review scenarios for production of nonthermal dark matter. Since the masses of the nonthermal WIMPS are in the range 10^{12} to 10^{16} GeV, much larger than the mass of thermal wimpy WIMPS, they may be referred to as WIMPZILLAS. In searches for dark matter it may be well to remember that ``size does matter.''

Figures (5)

• Figure 1: A thermal relic starts in lte at $T\gg M_X$. When the rates keeping the relic in chemical equilibrium become smaller than the expansion rate, the density of the relic relative to the entropy density freezes out.
• Figure 2: The contribution of gravitationally produced wimpzillas to $\Omega_Xh^2$ as a function of $M_X/H_I$. The shaded area is where thermalization may occur if the annihilation cross section is its maximum value. Also shown is the contribution assuming that the wimpzilla is present at the end of inflation with a temperature $T=H_I/2\pi$.
• Figure 4: The evolution of energy densities and $T/M_X$ as a function of the scale factor. Also shown is $X/X_{EQ}$.
• Figure 5: A graph of $\Omega_Xh^2$ versus $M_X/H_I$ for $gM_{Pl}/H_I= 10^6$. The solid curve is a numerical result, while the dashed and dotted curves are analytic approximations.
• Figure 7: Dark matter may be much more massive than usually assumed, much more massive than wimpy wimps, perhaps in the wimpzilla class.