Dark sector production and baryogenesis from not quite black holes

TL;DR

This work investigates primordial thermal 2-2-holes, horizonless ultracompact objects in quadratic gravity, as an alternative to PBHs for dark matter and baryogenesis. By analyzing evaporation-derived production of dark sector particles and the generation of the baryon asymmetry, it maps the viable parameter space in terms of the remnant mass $M_{ m min}$ and the initial mass $M_{ m init}$, incorporating constraints from BBN, CMB, and remnant mergers. The study finds that dark matter and dark radiation can be accommodated across a wide range of $M_{ m min}$, with DM masses spanning many orders of magnitude and DR contributing to $\Delta N_{ m eff}$ in a way that can be probed by current and future observations. For baryogenesis, both direct heavy-particle decays and electroweak baryogenesis are possible in parts of the parameter space, though EWBG is generally disfavored for Planck-like remnants, while leptogenesis remains possible only in a narrow Planck-like window. Overall, the framework yields distinctive, testable predictions that connect quantum-gravity-inspired remnants to cosmological observables and high-energy astrophysical signals.

Abstract

Primordial black holes have been considered as an attractive dark matter candidate, whereas some of the predictions heavily rely on the near-horizon physics that remains to be tested experimentally. As a concrete alternative, thermal 2-2-holes closely resemble black holes without event horizons. Being a probable endpoint of gravitational collapse, they not only provide a resolution to the information loss problem, but also naturally give rise to stable remnants. Previously, we have considered primordial 2-2-hole remnants as dark matter. Due to the strong constraints from a novel phenomenon associated with remnant mergers, only small remnants with close to the Planck mass can constitute all of dark matter. In this paper, we examine the scenario that the majority of dark matter consists of particles produced by the evaporation of primordial 2-2-holes, whereas the remnant contribution is secondary. The products with light enough mass may contribute to the number of relativistic degrees of freedom in the early universe, which we also calculate. Moreover, 2-2-hole evaporation can produce particles that are responsible for the baryon asymmetry. We find that baryogenesis through direct B-violating decays or through leptogenesis can both be realized. Overall, the viable parameter space for the Planck remnant case is similar to primordial black holes with Planck remnants. Heavier remnants, on the other hand, lead to different predictions, and the viable parameter space remains large even when the remnant abundance is small.

Figures (8)

• Figure 1: Constraints on the mass fraction of 2-2-hole remnants $f$ as a function of $M_{\mathrm{min}}$Aydemir:2020xfd. The gray lines present upper bounds from purely gravitational interactions as in the case for PBHs. The colored lines show the constraints on the high-energy particle fluxes special to 2-2-hole remnants. The solid line considers only the on-shell neutrinos and serves as a conservative estimation. The dash and dotted lines include the parton shower effects and may suffer more from the theoretical uncertainties.
• Figure 2: Constraints on the dark matter mass $m_\chi$ as a function of the 2-2-hole initial mass $M_{\mathrm{init}}$ for the benchmark remnant masses $M_{\mathrm{min}}$ in (\ref{['eq:MminBM']}), assuming a single particle component with $\textrm{B}_\chi\, (g_\chi)=0.01\,(1)$, $0.5\, (107)$. The white region is allowed, and the black dotted lines denote $T_\textrm{init}$, the separation between the light mass and heavy mass cases. The red dashed lines show the upper and lower bounds derived from the observed abundance in the non-domination case that terminates at $M_\textrm{HL}$ on the left and $M_\textrm{BBN}$ on the right. For small $M_{\mathrm{min}}$ in the first and second columns, the 2-2-hole domination is allowed for $M_\textrm{DM}\lesssim M_{\mathrm{init}}\lesssim M_\textrm{BBN}$, and the thick lines show the relevant parameter space. For the second column, we show the stronger bounds with $f\leq 10^{-4}$ in addition. For the third column, taking $f\leq 0.1$, $M_\textrm{HL}$ goes beyond $M_\textrm{BBN}$ when $\textrm{B}_\chi=0.01$ and there is no viable parameter space. The blue shaded region is excluded by the free-streaming constraints.
• Figure 3: Several benchmark cases demonstrating contributions to $\Delta N_{\mathrm{eff}}$ from the 2-2-hole evaporation in the early universe, for a given $M_{\mathrm{min}}$. Blue, red, and magenta lines/bands denotes regions for $\mathrm{B}_{\mathrm{DR}}$ ($g_{\mathrm{DR}})=0.01\;(1),\; 0.1\;(12)$ and $0.5\; (107)$, respectively. The parameter space for the domination scenario, which can be realized only for $M_{\mathrm{DM}}\lesssim M_{\mathrm{init}} \lesssim M_{\mathrm{BBN}}$ when $M_{\mathrm{min}}\lesssim M_{\mathrm{min}}^\textrm{D}$, is given as shaded horizontal bands where the upper and lower limits correspond $g_{*,\tau_L}^{\mathrm{SM}}\approx 11$ and 107, respectively. The non-domination case corresponds to the dashed lines and connects to the domination band at $M_{\mathrm{init}}$ where $f_{\mathrm{max}}$ saturates the upper bound. The grey regions denote the excluded parameter space based on the Planck data, with the upper bound being $\Delta N_{\mathrm{eff}}\leqslant 0.28$ (or $0.52$ if the Hubble tension is taken into account). The dotdashed line denotes the projected sensitivity of CMB-S4 measurements. The turquoise region shows the parameter space that could potentially alleviate the Hubble tension.
• Figure 4: Constraints on the CP-violation parameter $\gamma$ required for $\mathcal{B} \gtrsim 10^{-10}$, with respect to $M_{\mathrm{init}}$ for a given $M_{\mathrm{min}}$, assuming $\kappa_X=1$ and $\textrm{B}_X\,(g_X)=0.01\,(1)$, $0.5\,(107)$. The grey region denotes the excluded parameter space with $\mathcal{B}$ being too small, and the solid line shows the region relevant for the domination case. For each $M_{\mathrm{min}}$ value, we take into account the observational constraints on $f$. $M_\textrm{EW}$ denotes the value of $M_{\mathrm{init}}$ for which the background temperature $T_\textrm{bkg}^{\;\tau}=E_\textrm{W}$. The orange ($M_{\mathrm{init}}\lesssim M_\textrm{EW}$) region shows the relevant parameter space for leptogenesis and direct baryogenesis with (B-L) production, while the white ($M_{\mathrm{init}}\gtrsim M_\textrm{EW}$) region is for direct baryogenesis.
• Figure 5: Constraints on the CP-violation parameter $\gamma$ for $\mathcal{B} \gtrsim 10^{-10}$ with respect to the heavy particle mass $m_X$, assuming $\mathrm{B}_X=0.01$. Benchmark values of $(M_{\mathrm{min}},\;M_{\mathrm{init}},\;f)$ are chosen according to Fig. \ref{['gamma-Minit']}. For each contour, the horizontal part denotes the light mass case with $m_X \leqslant T_{\mathrm{init}}$ and $\kappa_X =1$, and the ascending part is for the heavy mass case with $m_X > T_{\mathrm{init}}$ and $\kappa_X <1$. The available parameter space is the upper left region. Only the region corresponding to the orange contour allows leptogenesis.