Dark Matter Recoupling

Abstract

In the late Universe, and on cosmological scales, dark matter is conventionally assumed to be collisionless, as a consequence of the strong existing bounds on dark matter interactions at the Cosmic Microwave Background last-scattering surface. Challenging this lore, here we show that dark matter interactions can be naturally weak at early times, but then grow to observationally relevant strengths at very late times, even significantly after reionization. This is realized if dark matter recouples to a dark radiation species in the range of redshifts probed by the current generation of galaxy surveys. We systematically study, for the first time, the phenomenology of this dark matter recoupling scenario. A combination of Cosmic Microwave Background and Baryon Acoustic Oscillation data show that the interaction needs to be weak at present, if the entirety of dark matter couples to dark radiation. From a complementary perspective, a 4% fraction of dark matter could still be strongly interacting today. Implications for a microscopic model realizing the recoupling dynamics are discussed.

Figures (9)

• Figure 1: Feynman diagrams for elastic $\mathrm{DM}\text{-}\mathrm{DR}$ scattering at tree level. From left to right, the $t$-, $s$- and $u$-channel contributions. The cyan vertex corresponds to the cubic self-coupling $\bm{g_\phi}$ of the DR scalar $\phi$, whereas brown vertices involve the Yukawa coupling $\bm{y_\chi}$ between $\phi$ and the fermionic DM $\chi$.
• Figure 2: Ratio between the momentum transfer rates of DR (blue) and DM (red) and the conformal Hubble parameter $\mathcal{H}$. We set $\Delta N_{\rm eff}= 0.1$ and $a_{\rm D} = 100\;\mathrm{Mpc}^{-1}$. The vertical gray lines correspond to matter-radiation equality (dashed) and recombination (solid).
• Figure 3: Evolution of the DM temperature and sound speed with redshift, for $\Delta N_{\rm eff}= 0.1$ and $a_{\rm D} = 100\;\mathrm{Mpc}^{-1}$. Left: DM temperature $T_\chi$, in Kelvin. For reference, we also show the DR temperature (black dashed curve). Right: Ratio $\mathcal{H}/c_\chi$, which approximately sets the DM Jeans wavenumber $k_{s,\chi}$ during matter domination, assuming $m_\chi = 10$ keV. DM perturbations are suppressed by thermal motions for $k\gtrsim k_{s,\chi}$, which however lies beyond the scales of relevance for this work.
• Figure 4: Left: The DR density perturbation $\delta_{\rm DR}$ as a function of wavenumber $k$, obtained from CLASS (solid) and our analytical approximation in Eq. \ref{['eq:deltaDR_an']} (dashed). Also shown, for comparison, is the DM density perturbation $\delta_\chi$ (dot-dashed). Right: Ratio between the total matter density transfer function in a recoupling model and the corresponding quantity in a $\Lambda$CDM Universe with extra radiation. Solid [dashed] lines correspond to CLASS [our analytical solution in Eq. \ref{['eq:deltam']}]. Thin dot-dashed lines indicate the improved asymptotic ratio in Eq. \ref{['eq:deltam_limit_impr']}, which accounts for $\Lambda$ domination at low redshift. As in previous figures, we set $\Delta N_{\rm eff} = 0.1$ and $a_{\rm D} = 100$ Mpc$^{-1}$.
• Figure 5: Top: Lensed TT CMB power spectrum (left) and CMB lensing power spectrum (right). A reference model with non-interacting free-streaming DR, assuming an energy density corresponding to $\Delta N_{\rm eff} = 0.1$, is shown in red. Other colors show increasing values of $a_{\rm D}$ (in $\mathrm{Mpc}^{-1}$). Bottom: Ratio to the corresponding $\Lambda$CDM Universe.