# A Simple Phenomenological Emergent Dark Energy Model can Resolve the Hubble Tension

## TL;DR

This paper proposes a simple emergent dark energy model that appears at late times, effectively resolving the Hubble tension by fitting multiple cosmological observations better than the standard b4CDM model.

## Contribution

It introduces a phenomenological emergent dark energy model with no early-time presence, improving data fit and potentially ruling out the cosmological constant with future precise measurements.

## Key findings

- The model fits cosmological data better than b4CDM with b4 b4 b4 b4 -41.08.
- It can resolve the Hubble tension by accommodating local b4H_0b4 measurements.
- The model suggests a late-time emergence of dark energy from zero to b4 -1.

## Abstract

Motivated by the current status of the cosmological observations and significant tensions in the estimated values of some key parameters assuming the standard $\Lambda$CDM model, we propose a simple but radical phenomenological emergent dark energy model where dark energy has no effective presence in the past and emerges at the later times. Theoretically, in this phenomenological dark energy model with zero degree of freedom (similar to a $\Lambda$CDM model), one can derive that the equation of state of dark energy increases from $-\frac{2}{3 {\rm{ln}}\, 10} -1$ in the past to $-1$ in the future. We show that by setting a hard-cut 2$\sigma$ lower bound prior for the $H_0$ that associates with $97.72\%$ probability from the recent local observations~\citep{riess2019large}, this model can satisfy different combinations of cosmological observations at low and high redshifts (SNe Ia, BAO, Ly{$\alpha$} BAO and CMB) substantially better than the concordance $\Lambda$CDM model with $\Delta \chi^2_{bf} \sim -41.08$ and $\Delta\,{\rm{DIC}} \sim-35.38$. If there are no substantial systematics in SN Ia, BAO or Planck CMB data and assuming reliability of the current local $H_0$ measurements, there is a very high probability that with slightly more precise measurement of the Hubble constant our proposed phenomenological model rules out the cosmological constant with decisive statistical significance and is a strong alternative to explain combination of different cosmological observations. This simple phenomenologically emergent dark energy model can guide theoretically motivated dark energy model building activities.