We discuss the ability of a dark fluid becoming relevant around the time of matter radiation equality to significantly relieve the tension between local measurements of the Hubble constant and CMB inference, within the ΛCDM model. We show that the gravitational impact of acoustic oscillations in the dark fluid balance the effects on the CMB and result in an improved fit to CMB measurements themselves while simultaneously raising the Hubble constant. The required balance favors a model where the fluid is a scalar field that converts its potential to kinetic energy around matter radiation equality which then quickly redshifts away. We derive the requirements on the potential for this conversion mechanism and find that a simple canonical scalar with two free parameters for its local slope and amplitude robustly improves the fit to the combined data by ∆χ 2 ≈ 12.7 over ΛCDM. We uncover the CMB polarization signatures that can definitively test this scenario with future data.

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We place observational constraints on the Galileon ghost condensate model, a dark energy proposal in cubic-order Horndeski theories consistent with the gravitational-wave event GW170817. The model extends the cubic covariant Galileon by taking an additional higher-order field derivative X 2 into account. This allows for the dark energy equation of state wDE to access the region −2 < wDE < −1 avoinding ghosts. Indeed, this peculiar evolution of wDE is favored over that of the cosmological constant Λ (wDE = −1) from the joint data analysis of cosmic microwave background (CMB) radiation, baryonic acoustic oscillations, supernovae type Ia and redshift-space distortions. Furthermore, our model exhibits a better compatibility with the CMB data over the Λ-cold-darkmatter (ΛCDM) model by suppressing large-scale temperature anisotropies. We perform a model selection analysis by using several methods and find a statistically significant preference of the Galileon ghost condensate model over ΛCDM.