Phantom matter: a challenging solution to the cosmological tensions

Abstract

The idea of composite dark energy (DE) is quite natural since on general grounds we expect that the vacuum energy density (associated with the cosmological term $\Lambda$) may appear in combination with other effective forms of DE, denoted X. Here we deal with model $w$XCDM, a simplified version of the old $\Lambda$XCDM model (Grande et al., 2006), and exploit the possibility that X behaves as `phantom matter' (PM), which appears in stringy versions of the running vacuum model (RVM). Unlike phantom DE, the PM fluid satisfies the strong energy condition like usual matter, hence bringing to bear positive pressure at the expense of negative energy. Bubbles of PM may appear in the manner of a transitory `phantom vacuum' tunneled into the late universe before it heads towards a new de Sitter era, thereby offering a crop field for the growing of structures earlier than expected. Using SNIa, cosmic chronometers, transversal BAO (BAO 2D), LSS data and the full CMB likelihood from Planck 2018, we find that the $H_0$ and growth tensions virtually disappear, provided that BAO 2D are the only source of BAO data used in the fit. In contrast, our preliminary analysis using exclusively anisotropic BAO (BAO 3D) indicates that the ability to ease the $H_0$ tension is significantly reduced as compared to the scenario with BAO 2D, despite the fact that the overall fit to the cosmological data is still better than in the $\Lambda$CDM. Finally, our approach with BAO 2D favors quintessence-like behavior of the DE below $z\simeq 1.5$ at $\gtrsim 3\sigma$ CL, which is compatible with the recent DESI measurements.