Cosmological bounds on dark matter-photon coupling

TL;DR

This work investigates a cosmological extension where dark matter decays into photons, introducing photon number nonconservation via a coupling parameter $\Gamma_{\gamma}$. The authors implement a DM-photon coupling model in the Boltzmann framework, derive background and perturbation equations, and analyze CMB, BAO, LSS, and local $H_0$ measurements with four data combinations using CLASS and MontePython, allowing $N_{\rm eff}$ and the sum of neutrino masses $\sum m_{\nu}$ to vary. They find a tight upper bound $\Gamma_{\gamma} \leq 1.3\times10^{-5}$ (95% C.L.) and that the model can accommodate higher $H_0$ values consistent with local measurements, while slightly reducing $\sigma_8$ in LSS data; $\sum m_{\nu}$ bounds range from $<0.38$ eV to $<0.83$ eV depending on data sets, and $N_{\rm eff}$ can shift from the standard value but remains statistically compatible with standard predictions, showing no strong evidence for dark radiation. Overall, the DM-photon coupling offers a natural mechanism to alleviate the $H_0$ tension within a controlled extended framework, motivating further exploration of photon nonconservation and related cosmological observables.

Abstract

We investigate an extension of the $Λ$CDM model where the dark matter (DM) is coupled to photons, inducing a nonconservation of the numbers of particles for both species, where the DM particles are allowed to dilute throughout the cosmic history with a small deviation from the standard evolution decaying into photons, while the associated scattering processes are assumed to be negligible. In addition, we consider the presence of massive neutrinos with the effective number of species $N_{\rm eff}$ as a free parameter. The effects of the DM-photon coupling on the cosmic microwave background (CMB) and matter power spectra are analyzed. We derive the observational constraints on the model parameters by using the data from CMB, baryonic acoustic oscillation (BAO) measurements, the recently measured new local value of the Hubble constant from the Hubble Space Telescope, and large scale structure (LSS) information from the abundance of galaxy clusters. The DM-photon coupling parameter $Γ_{γ}$ is constrained to $Γ_{γ} \leq 1.3 \times10^{-5}$ (at 95\% C.L.) from the joint analysis carried out by using all the mentioned data sets. The neutrino mass scale $\sum m_ν$ upper bounds at 95\% C.L. are obtained as $\sum m_ν \sim 0.9$ eV and $\sum m_ν \sim 0.4$ eV with and without the LSS data, respectively. We observe that the DM-photon coupling cause significant changes in the best fit value of $N_{\rm eff}$ but yields statistical ranges of $N_{\rm eff}$ compatible with the standard predictions, and we do not find any evidence of dark radiation. Due to nonconservation of photons in our model, we also evaluate and analyze the effects on the BAO acoustic scale at the drag epoch. The DM-photon coupling model yields high values of Hubble constant consistent with the local measurement, and thus alleviates the tension on this parameter.

Figures (5)

• Figure 1: Theoretical prediction and relative deviations of the CMB TT power spectrum from the base line Planck 2015 $\Lambda$CDM model for some values of $\Gamma_{\gamma}$ while the other parameters are fixed to their best-fit mean values as given in Table \ref{['Table_M1']}.
• Figure 2: Theoretical prediction and relative deviations of the matter power spectrum from the base line Planck 2015 $\Lambda$CDM model for some values of $\Gamma_{\gamma}$ while the other parameters are fixed to their best-fit mean values as given in Table \ref{['Table_M1']}.
• Figure 3: One-dimensional marginalized distribution for $\Gamma_{\gamma}$.
• Figure 4: 68% C.L. and 95% C.L. regions for $\sigma_8$ and $H_0$. The horizontal yellow band corresponds to $H_0=73.24\pm 1.74$ km s${}^{-1}$ Mpc${}^{-1}$ whereas the vertical light red band corresponds to $\sigma_8= 0.75\pm 0.03$.
• Figure 5: One-dimensional marginalized distribution of $N_{\rm eff}$ and $r_{\rm drag}$ (measured in Mpc).