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

Abstract

Interesting discrepancies in cosmological parameters are challenging the success of the ΛCDM model. Direct measurements of the Hubble constant H 0 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 σ8 lower than values derived from the CMB in the context of ΛCDM. 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 ĉs2, and vanishing anisotropic stress σ. 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 ĉs2 and find ĉs2 = 1 is excluded at ≳ 3σ. For our baseline result including the whole data set we found H0 and σ8 in good agreement (within ≈ 2σ) with low redshift probes. Our constraint for the baryon energy density ωb is however in ≈ 3σ tension with BBN constraints. We conclude evolving DE also having non-standard clustering properties [e.g., ĉs2(z,k)] might be relevant for the solution of current discrepancies in cosmological parameters.