Lavinia Heisenberg, Hector Villarrubia-Rojo, Jann Zosso

In a model independent approach, we derive generic conditions that any late time modification of the $\Lambda$CDM expansion history must satisfy in order to consistently solve both the $H_0$ and the $\sigma_8$ tensions. Our results are fully analytical and the method is merely based on the assumption that the late-time deviations from $\Lambda$CDM remain small. For the concrete case of a dark energy fluid with deviations encoded in the expansion history and the gravitational coupling constant, we present necessary conditions on its equation of state. Solving both the $H_0$ and $\sigma_8$ tensions requires that $w(z)$ must cross the phantom divide if $G_\text{eff}=G$. On the other hand, for $G_\text{eff}=G+\delta G(z)$ and $w(z)\leq -1$, it is required that $\displaystyle \frac{\delta G(z)}{G}<\alpha(z)\frac{\delta H(z)}{H(z)}<0$ at some redshift $z$.

Dominik A. Riechers, Axel Weiss, Fabian Walter, Christopher L. Carilli, Pierre Cox, Roberto Decarli, Roberto Neri

Distortions of the observed cosmic microwave background imprinted by the Sunyaev-Zel'dovich effect toward massive galaxy clusters due to inverse Compton scattering of microwave photons by high-energy electrons provide a direct measurement of the microwave background temperature at redshifts from 0 to 1. Some additional background temperature estimates exist at redshifts from 1.8 to 3.3 based on molecular and atomic line excitation temperatures in quasar absorption line systems, but are model dependent. To date, no deviations from the expected (1+z) scaling behavior of the microwave background temperature have been seen, but the measurements have not extended deeply into the matter-dominated era of the universe at redshifts z>3.3. Here we report the detection of sub-millimeter line absorption from the water molecule against the cosmic microwave background at z=6.34 in a massive starburst galaxy, corresponding to a lookback time of 12.8 Gyr. Radiative pumping of the upper level of the ground-state ortho-H2O(110-101) line due to starburst activity in the dusty galaxy HFLS3 results in a cooling to below the redshifted microwave background temperature, after the transition is initially excited by the microwave background. The strength of this effect implies a microwave background temperature of 16.4-30.2 K (1-sigma range) at z=6.34, which is consistent with a background temperature increase with redshift as expected from the standard CDM cosmology.