Revisiting a negative cosmological constant from low-redshift data

TL;DR

This study probes whether a negative cosmological constant $\Omega_{\rm cc}<0$ plus a secondary dark-energy component with constant equation of state $w_\phi$ can be compatible with late-time cosmological data and address the $H_0$ tension. It implements a toy string-inspired model, $c$CDM, alongside $\Lambda$CDM and $w$CDM, and constrains them with BAO and Pantheon Type Ia SNe data under two BAO anchoring schemes: Planck's $r_{\rm drag}$ and SH0ES' $H_0$, via Markov Chain Monte Carlo and Akaike information criterion. The analysis finds no evidence for a negative $\Omega_{\rm cc}$ (only a loose bound $\Omega_{\rm cc}\gtrsim -14$) and a mild preference for $w_\phi<-1$, but $\Lambda$CDM is statistically favored by the AIC across anchors. The results suggest that current data do not require extending $\Lambda$CDM, though future data and more realistic string-inspired models could revisit the possibility of a negative cosmological constant.

Abstract

Persisting tensions between high-redshift and low-redshift cosmological observations suggest the dark energy sector of the Universe might be more complex than the positive cosmological constant of the $Λ$CDM model. Motivated by string theory, wherein symmetry considerations make consistent AdS backgrounds (\textit (i.e.) maximally symmetric spacetimes with a negative cosmological constant) ubiquitous, we explore a scenario where the dark energy sector consists of two components: a negative cosmological constant, with a dark energy component with equation of state $w_φ$ on top. We test the consistency of the model against low-redshift Baryon Acoustic Oscillation and Type Ia Supernovae distance measurements, assessing two alternative choices of distance anchors: the sound horizon at baryon drag determined by the \textit{Planck} collaboration, and the Hubble constant determined by the SH0ES program. We find no evidence for a negative cosmological constant, and mild indications for an effective phantom dark energy component on top. A model comparison analysis reveals the $Λ$CDM model is favoured over our negative cosmological constant model. While our results are inconclusive, should low-redshift tensions persist with future data, it would be worth reconsidering and further refining our toy negative cosmological constant model by considering realistic string constructions.