Late-time Universe, H 0 -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 H 0 -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 H 0 -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 d u ∈ [ − 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 d u = 3 / 2 ≡ δ = 1 is ruled out by 6.6 σ . However, there is a pronounced gap between the models and what is demanded by observational data in the q Q -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.