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Holographic energy density, dark energy sound speed, and tensions in cosmological parameters: H₀ and S₈
Wilmar Cardona, M. A. Sabogal
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Interesting discrepancies in cosmological parameters are challenging the success of the ΛCDM 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 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 w_(de)(z), constant sound speed squared ĉₛ², and vanishing anisotropic stress σ. Our w_(de)(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 ω_b 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.
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