21-cm Constraints on Dark Matter Annihilation after an Early Matter-Dominated Era

TL;DR

The paper investigates how an early matter-dominated era (EMDE) enhances small-scale dark matter structure, forming dense microhalos that boost annihilation and heat the IGM. By computing the EMDE-induced boost factor with the Peak-to-Halo method and propagating energy injection through deposition efficiencies, the authors forecast 21-cm global and power-spectrum signals and compare them to IGRB constraints. They find that global 21-cm measurements at $z\sim17$ can exceed IGRB bounds for light DM, but the 21-cm power spectrum, especially at $z\sim14$, can provide stronger and distinctive constraints by leveraging the homogeneous heating from EMDE-driven annihilation. The work also shows that DM heating imprints in the 21-cm power spectrum can help distinguish DM heating from astrophysical X-ray heating, highlighting the 21-cm power spectrum as a promising probe of pre-BBN cosmology and DM properties in EMDE scenarios.

Abstract

Although it is commonly assumed that relativistic particles dominate the energy density of the universe quickly after inflation, a variety of well-motivated scenarios predict an early matter-dominated era (EMDE) before the onset of Big Bang nucleosynthesis. Subhorizon dark matter density perturbations grow faster during an EMDE than during a radiation-dominated era, leading to the formation of "microhalos" far earlier than in standard models of structure formation. This enhancement of small-scale structure boosts the dark-matter annihilation rate, which contributes to the heating of the intergalactic medium (IGM). We compute how the dark matter annihilation rate evolves after an EMDE and forecast how well measurements of the 21-cm background can detect dark matter annihilation in cosmologies with EMDEs. We find that future measurements of the global 21-cm signal at a redshift of $z\sim 17$ are unlikely to improve on bounds derived from observations of the isotropic gamma-ray background, but measurements of the 21-cm power spectrum have the potential to detect dark matter annihilation following an EMDE. Moreover, dark matter annihilation and astrophysical X-rays produce distinct heating signatures in the 21-cm power spectrum at redshifts around 14, potentially allowing differentiation between these two IGM heating mechanisms.

Figures (9)

• Figure 1: The dimensionless power spectrum $\mathcal{P}(k)$ of DM density perturbations $\delta(k,a)/a^{0.901}$ during the standard matter-dominated era following EMDEs with various $T_\text{RH}$ and $R_\text{cut} \equiv k_{\text{cut}}/k_{\text{RH}}$. The dashed line shows $\mathcal{P}(k)$ without an EMDE. Vertical lines indicate the corresponding $k_\text{RH}$ for each reheat temperature. Modes with $k>k_\text{RH}$ enter the horizon during an EMDE, while modes with $k>k_\text{cut}$ are suppressed due to the small-scale cutoff to $\mathcal{P}(k)$.
• Figure 2: Probability density of collapse redshifts $z_{ec}$ of peaks in the initial density field, calculated from a sample of $N=10^8$ peaks. Dotted lines mark the redshift at which $50\%$ of peaks have collapsed. For an EMDE cosmology with $R_\text{cut} = 10$ and $T_\text{RH} = 1$ GeV, $z_\text{50,peak} = 72$, so it is not displayed in the top panel.
• Figure 3: The boost factor $\mathcal{B}(z)(1+z)^3$ as a function of redshift for various EMDE cosmologies. The shaded region indicates where $\mathcal{B}(z) \leq 0.1$. The standard boost shows Eq. (\ref{['eq:erf']}) with parameters $z_h = 30$, $b_h = 10^6$, and $\beta = 3$.
• Figure 4: Evolution of the IGM temperature due to heating from DM annihilating into $b\bar{b}$ quarks. DM annihilation within the microhalos that form after an EMDE injects energy into the IGM and increases the IGM temperature after it decouples from the CMB temperature at $z\simeq300$. Energy injection from astrophysical sources is not included here.
• Figure 5: The upper bounds on the velocity-averaged cross section for annihilating DM for various EMDE cosmologies and annihilation channels. Assuming that $T_K=T_S$, the left panel adopts $\delta T_{21}\leq-200$ mK ($T_K \leq 7.3$ K at $z = 17$), while the right panel adopts the weaker limit $\delta T_{21}\leq-100$ mK ($T_K \leq 12.8$ K at $z = 17$). In the left panel, we fix the $b\bar{b}$ annihilation channel but consider a range of $R_\mathrm{cut}$, while in the right panel, we fix $R_\mathrm{cut}=40$ but consider a range of channels. In both panels, the shaded region indicates that $\langle \sigma v \rangle > m_\chi^{-2}$, which violates unitarity. The dashed lines in the right panel are the effective annihilation cross section bounds derived using Eq. (\ref{['eq:effectiveSigmav']}) and the lower bounds on the lifetime of decaying DM from Ref. Clark2018. The concordance between the dashed and solid lines demonstrates how DM annihilation following an EMDE mimics decaying DM.