Implications of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> tension for decaying dark matter with warm decay products
Guillermo Franco Abellán, Riccardo Murgia, Vivian Poulin, Julien Lavalle
Abstract
Recent weak-lensing surveys have revealed that the direct measurement of the parameter combination ${S}_{8}\ensuremath{\equiv}{\ensuremath{\sigma}}_{8}({\mathrm{\ensuremath{\Omega}}}_{m}/0.3{)}^{0.5}$---where ${\ensuremath{\sigma}}_{8}$ is a measure of the amplitude of matter fluctuations on $8\text{ }\text{ }{h}^{\ensuremath{-}1}\text{ }\mathrm{Mpc}$ scales---is $\ensuremath{\sim}3\ensuremath{\sigma}$ discrepant with the value reconstructed from cosmic microwave background (CMB) data assuming the $\mathrm{\ensuremath{\Lambda}}$-cold dark matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) model. In this article, we show that it is possible to resolve the tension if dark matter (DM) decays with a lifetime of ${\mathrm{\ensuremath{\Gamma}}}^{\ensuremath{-}1}\ensuremath{\simeq}55\text{ }\text{ }\mathrm{Gyrs}$ into one massless and one massive product, and transfers a fraction $ϵ\ensuremath{\simeq}0.7%$ of its rest mass energy to the massless component. The velocity kick received by the massive daughter leads to a suppression of gravitational clustering below its free-streaming length, thereby reducing the ${\ensuremath{\sigma}}_{8}$ value as compared to that inferred from the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model, in a similar fashion to massive neutrino and standard warm DM. Contrarily to the latter scenarios, the time dependence of the power suppression and the free-streaming scale allows the 2-body decaying DM scenario to accommodate CMB, baryon acoustic oscillation, growth factor and uncalibrated supernova Ia data. We briefly discuss implications for DM model building, galactic small-scale structure problems and the recent Xenon-1T excess. Future experiments measuring the growth factor to high accuracy at $0\ensuremath{\lesssim}z\ensuremath{\lesssim}1$ can further test this scenario.