9. Bayesian analysis of a Unified Dark Matter model with transition: can it alleviate the \(H_0\) tension?

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Context: Cosmological models in which Dark Matter (DM) and Dark Energy (DE) are described by a single component, dubbed Unified Dark Matter (UDM) models, can have an equation state \(<−1\) at late times without violating the null energy condition. I investigated whether this feature can relieve the Hubble tension.

Method: I performed a Bayesian analysis of the model using SNIa data from Pantheon, the CMB distance prior from Planck, and the prior on the absolute magnitude \(M\) of SNIa from SH0ES. I modified the CLASS code to implement the UDM model, and performed a MCMC analysis with MontePython. The statistical analysis and the figures were then obtained with GetDist. We also discuss the importance of using the prior on \(M\) for constraining this model.

Results:

• The data suggests a smooth transition taking place at redshifts \(z \simeq 2.85\)…
• … which provides a value of \(H_0=69.64±0.88\) for the Hubble constant, slightly alleviating the tension by \(\approx 1.5\sigma\).