Late-time Universe, H0-tension, and unparticles

Lambda cold dark matter (Λ CDM) is increasingly facing challenges in late-time cosmology, notably in the Hubble parameter H(z) at redshift zero known as H0-tension. Extensions to Λ CDM have been approached mostly by models with an extra parameter. In this work, we provide a framework for evaluating the viability of models of late-time cosmology by investigating unparticle cosmology as a case study. Unparticle cosmology proposes a scale invariant contribution by a dimensionless parameter δ. We focus on H0-tension in comparison with scale invariant and Λ CDM models. The dynamical behavior of unparticle cosmology with and without a cosmological constant shows that for most values of δ ∈ [ − 6 , 1 ] (corresponding to du ∈ [− 2, 3/2]), the late-time Universe will be Λ and matter dominated, respectively, with negligible contribution of unparticles. It predicts H(z) in the future to be zero or constant, the latter pointing to a stable de Sitter phase for our Universe. Our data analysis shows δ = −2.06 ± 0.46 as the best fit to the data. Consequently, the conventional value of du = 3/2 ≡ δ = 1 is ruled out by 6.6σ. However, there is a pronounced gap between the models and what is demanded by observational data in the qQ-diagram for all δ. We conclude that unparticle cosmology fails to come to the rescue of challenges to late-time Λ CDM, but it provides a pointer to holographic dark energy.