Probing New Physics with High-Redshift Quasars: Axions and Non-standard Cosmology

The Hubble diagram of quasars, as candidates to ``standardizable" candles, has been used to measure the expansion history of the Universe at late times, up to very high redshifts ($z \sim 7$). It has been shown that this history, as inferred from the quasar dataset, deviates at $\gtrsim 3 \sigma$ level from the concordance ($\Lambda$CDM) cosmology model preferred by the cosmic microwave background (CMB) and other datasets. In this article, we investigate whether new physics beyond $\Lambda$CDM (B$\Lambda$CDM) or beyond the Standard Model (BSM) could make the quasar data consistent with the concordance model. We first show that an effective redshift-dependent relation between the quasar UV and X-ray luminosities, complementing previous phenomenological work in the literature, can potentially remedy the discrepancy. Such a redshift dependence can be realized in a BSM model with axion-photon conversion in the intergalactic medium (IGM), although the preferred parameter space is {in tension with various other astrophysical constraints on axions, at a level} depending on the specific assumptions made regarding the IGM magnetic field. We briefly discuss a variation of the axion model that could evade these astrophysical constraints. On the other hand, we show that models beyond $\Lambda$CDM such as one with a varying dark energy equation of state ($w$CDM) or the phenomenological cosmographic model with a polynomial expansion of the luminosity distance, cannot alleviate the tension. The code for our analysis, based on \texttt{emcee}~\cite{Foreman_Mackey_2013} and \texttt{corner.py}~\cite{corner}, is publicly available at \href{https://github.com/ChenSun-Phys/high\_z\_candles.git}{{\tt github.com/ChenSun-Phys/high\_z\_candles}}.