Light Dark Matter: A Common Solution to the Lithium and ${H_0}$ Problems

Abstract

Currently, the standard cosmological model faces some tensions and discrepancies between observations at early and late cosmological time. One of them concerns the well-known $H_0$-tension problem, i.e., a $\sim4.4σ$-difference between the early-time estimate and late-time measurements of the Hubble constant, $H_0$. Another puzzling question rests in the cosmological lithium abundance, where again local measurements differ from the one predicted by Big Bang Nucleosynthesis (BBN). In this work, we show that a mechanism of light dark matter production might hold the answer for these questions. If dark matter particles are sufficiently light and a fraction of them was produced non-thermally in association with photons, this mechanism has precisely what is needed to destroy Lithium without spoiling other BBN predictions. Besides, it produces enough radiation that leads to a larger $H_0$ value, reconciling early and late-time measurements of the Hubble expansion rate without leaving sizable spectral distortions in the Cosmic Microwave Background spectrum.

Figures (2)

• Figure 1: Effect of the dark matter production mechanism on $H_0$ for lifetime $\tau=10^5$ s (green), $\tau=10^6$ s (blue) and $\tau=10^7$ s (red). The contours correspond to 95% CL constraints from Planck 2015 power spectra excluding high $l$ polarisation (light colors) and adding CMB lensing and BAO (dark colors).
• Figure 2: Region of parameter space in which the Lithium abundance is diluted by 20-80% from BBN original prediction (green), and where $H_0=71$ Mpc$^{-1}$km/s (light red) or $H_0=74$ Mpc$^{-1}$km/s (dark red). The blue curves correspond to current (dashed) and projected (dotted) CMB spectral distortion bounds. For $\tau\simeq2\times 10^4$ s one can simultaneously accommodate the Lithium and $H_0$ problems with light dark matter. Notice if one assumes $f\ll 1$, sub-keV dark matter arises. See text for details.