LAUSR

Recent low-redshift observations have yielded the present-time Hubble parameter value \begin{document}$H_{0}\simeq 74\;\rm{km s}^{-1} \rm{Mpc}^{-1}$\end{document} . This value is approximately 10% higher than the predicted value of \begin{document}$H_{0}=67.4\;\rm{km s}^{-1}\rm{Mpc}^{-1}$\end{document} , based on Planck's observations of the Cosmic Microwave Background radiation (CMB) and the \begin{document}$\Lambda$\end{document} CDM model. Phenomenologically, we show that, by adding an extra component, X, with negative density to the Friedmann equation, it can address the Hubble tension without changing the Planck's constraint on the matter and dark energy densities. To achieve a sufficiently small extra negative density, its equation-of-state parameter must satisfy \begin{document}$1/3\leqslant w_{X}\leqslant 1$\end{document} . We propose a quintom model of two scalar fields that realizes this condition and potentially alleviate the Hubble tension. One scalar field acts as a quintessence, while another “phantom” scalar conformally couples to matter such that a viable cosmological scenario is achieved. The model only depends on two parameters, \begin{document}$\lambda_{\phi}$\end{document} and \begin{document}$\delta$\end{document} , which represent the rolling tendency of the self-interacting potential of the quintessence and the strength of the conformal phantom-matter coupling, respectively. The toy quintom model with \begin{document}$H_{0}=73.4\;\rm{km s}^{-1}\rm{Mpc}^{-1}$\end{document} (Quintom I) yields a good Supernovae-Ia luminosity fit and acceptable \begin{document}$r_{\rm BAO}$\end{document} fit but slightly small acoustic multipole \begin{document}$\ell_{A}=285.54$\end{document} . A full parameter scan revealed that the quintom model was superior to the \begin{document}$\Lambda$\end{document} CDM model in certain regions of the parameter space, \begin{document}$0.02 , while significantly alleviating the Hubble tension, although it is not completely resolved. A benchmark quintom model, Quintom II, is presented as an example.