Composite Dark Energy and the Cosmological Tensions

The standard cosmological model currently in force, aka $\Lambda$CDM, has been plagued with a variety of tensions in the last decade or so, which puts it against the wall. At the core of the $\Lambda$CDM we have a rigid cosmological term, $\Lambda$, for the entire cosmic history. Recently, the results from the DESI collaboration suggested the possibility that dark energy (DE) should be dynamical rather than just a cosmological constant. Using a generic $w_0w_a$CDM parameterization, DESI favors quintessence behavior at $\sim 3\sigma$ c.l. However, to alleviate the tensions the DE needs more features. In the proposed $w$XCDM model of [45] the DE is actually a composite cosmic fluid with two components $(X,Y)$ acting sequentially: first $X$ (above a transition redshift $z_t$) and second $Y$ (below $z_t$). Fitting the model to the data, we find that the late component $Y$ behaves as quintessence, like DESI. However, to cure the $H_0$ and growth tensions, $X$ must behave as `phantom matter' (PM), which in contrast to phantom DE provides positive pressure at the expense of negative energy density. Using the SNIa (considering separately Pantheon$+$ and DESY5), cosmic chronometers, transversal BAO, LSS data, and the full CMB likelihood from Planck 2018, we find that both tensions can be completely cut down. We also compare the $w$XCDM with our own results using the standard $w$CDM and $w_0w_a$CDM parameterizations of the DE. In all cases, model $w$XCDM performs much better. Finally, we have repeated our analysis with BAO 3D data (replacing BAO 2D), and we still find that the main dynamical DE models (including composite ones) provide a much better fit quality compared to $\Lambda$CDM. The growth tension is alleviated again, but in contrast, the $H_0$-tension remains significant, which is most likely reminiscent of the internal conflict in the BAO sector.