A $w-M$ phantom transition at $z_t<0.1$ as a resolution of the Hubble tension

TL;DR

This paper proposes a rapid late-time transition in dark energy and supernova absolute magnitude to resolve the Hubble tension, supported by improved fits to cosmological data and implications for modified gravity models.

Contribution

It introduces a novel late $w-M$ phantom transition model that better fits data and addresses multiple cosmological tensions without screening mechanisms.

Findings

Better fit to cosmological data compared to smooth models

Addresses both Hubble and growth tensions simultaneously

Predicts a significant reduction in effective Newton constant at low redshift

Abstract

A rapid phantom transition of the dark energy equation of state parameter $w$ at a transition redshift $z_t<0.1$ of the form $w(z)=-1+\Delta w\;\Theta (z_t-z)$ with $\Delta w<0$ can lead to a higher value of the Hubble constant while closely mimicking a Planck18/$\Lambda$CDM form of the comoving distance $r(z)=\int_0^z\frac{dz'}{H(z')}$ for $z>z_t$. Such a transition however would imply a significantly lower value of the SnIa absolute magnitude $M$ than the value $M_C$ imposed by local Cepheid calibrators at $z<0.01$. Thus, in order to resolve the $H_0$ tension it would need to be accompanied by a similar transition in the value of the SnIa absolute magnitude $M$ as $M(z)=M_C+\Delta M \;\Theta (z-z_t)$ with $\Delta M<0$. This is a Late $w-M$ phantom transition ($LwMPT$). It may be achieved by a sudden reduction of the value of the normalized effective Newton constant $\mu=G_{\rm{eff}}/G_{\rm{N}}$ by about $6\%$ assuming that the absolute luminosity of SnIa is proportional to the Chandrasekhar mass which varies as $\mu^{-3/2}$. We demonstrate that such an ultra low $z$ abrupt feature of $w-M$ provides a better fit to cosmological data compared to smooth late time deformations of $H(z)$ that also address the Hubble tension. For $z_t=0.02$ we find $\Delta w\simeq -4$, $\Delta M \simeq -0.1$. This model also addresses the growth tension due to the predicted lower value of $\mu$ at $z>z_t$. A prior of $\Delta w=0$ (no $w$ transition) can still resolve the $H_0$ tension with a larger amplitude $M$ transition with $\Delta M\simeq -0.2$ at $z_t\simeq 0.01$. This implies a larger reduction of $\mu$ for $z>0.01$ (about $12\%$). The $LwMPT$ can be generically induced by a scalar field non-minimally coupled to gravity with no need of a screening mechanism since in this model $\mu=1$ at $z<0.01$.