Holographic energy density, dark energy sound speed, and tensions in cosmological parameters: H0 and S8

Interesting discrepancies in cosmological parameters are challenging the success of the ACDM model. Direct measurements of the Hubble constant H0 using Cepheid variables and supernovae turn out to be higher than inferred from the Cosmic Microwave Background (CMB). Weak galaxy lensing surveys consistently report values of the strength of matter clustering a-8 lower than values derived from the CMB in the context of ACDM. In this paper we address these discrepancies in cosmological parameters by considering Dark Energy (DE) as a fluid with evolving equation of state wde(z), constant sound speed squared c2s, and vanishing anisotropic stress a. Our wde(z) is derived from the Holographic Principle and can consecutively exhibit radiation-like, matter-like, and DE-like behaviour, thus affecting the sound horizon and the comoving angular diameter distance, hence H0. Here we show DE sound speed plays a part in the matter clustering behaviour through its effect on the evolution of the gravitational potential. We compute cosmological constraints using several data set combinations including primary CMB, CMB lensing, redshift-space-distortions, local distance-ladder, supernovae, and baryon acoustic oscillations. In our analysis we marginalise over c2s and find c2s = 1 is excluded at >= 3a. For our baseline result including the whole data set we found H0 and a-8 in good agreement (within 2a) with low redshift probes. Our constraint for the baryon energy density cob is however in 3a tension with BBN constraints. We conclude evolving DE also having non-standard clustering properties [e.g., c2s(z, k)] might be relevant for the solution of current discrepancies in cosmological parameters.

Automated summary

Interesting discrepancies in cosmological parameters challenge the success of the ACDM model. The Hubble constant and matter clustering strength derived from various measurements are higher/lower than those inferred from the Cosmic Microwave Background (CMB) in the context of ACDM. In this study, we propose a Dark Energy (DE) model with an evolving equation of state and constant sound speed squared to address these discrepancies. By considering the impact of DE sound speed on gravitational potential evolution, we find good agreement between our results and low redshift probes, but tension with BBN constraints in the case of baryon energy density.