Papers › The impact of the cosmic variance on H₀ on cosmological analyses
The impact of the cosmic variance on H₀ on cosmological analyses
David Camarena, Valerio Marra
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The current 3.8 σ tension between local (arXiv:1804.10655) and global (arXiv:1605.02985) measurements of H₀ cannot be fully explained by the concordance ΛCDM model. It could be produced by unknown systematics or by physics beyond the standard model. On the other hand, it is well known that linear perturbation theory predicts a cosmic variance on the Hubble parameter H₀, which leads to systematic errors on its local determination. Here, we study how including in the likelihood the cosmic variance on H₀ affects statistical inference. In particular we consider the γCDM, wCDM and γwCDM parametric extensions of the standard model, which we constrain with the latest CMB, BAO, SNe Ia, RSD and H₀ data. We learn two important lessons. First, the systematic error from cosmic variance is - independently of the model - approximately σ_(cv)≈0.88 km s⁻¹ Mpc⁻¹ (1.2% H₀ˡᵒᶜ) when considering the redshift range 0.0233 ≤z ≤0.15, which is relative to the main analysis of (arXiv:1804.10655), and σ_(cv)≈1.5 km s⁻¹ Mpc⁻¹ (2.1% H₀ˡᵒᶜ) when considering the wider redshift range 0.01 ≤z ≤0.15. Although σ_(cv) affects the total error budget on local H₀, it does not significantly alleviate the tension which remains at ≈3 σ. Second, cosmic variance, besides shifting the constraints, can change the results of model selection: much of the statistical advantage of non-standard models is to alleviate the now-reduced tension. We conclude that, when constraining non-standard models it is important to include the cosmic variance on H₀ if one wants to use the local determination of the Hubble constant by Riess et al. (arXiv:1804.10655). Doing the contrary could potentially bias the conclusions.
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