Cosmological test of an extended quintessence model

TL;DR

This paper tests an extended quintessence scalar-tensor gravity model against Planck 2018 CMB, BAO, and RSD data, finding it fits the data nearly as well as standard models and constrains the variation of the gravitational constant.

Contribution

It provides observational constraints on an extended quintessence model with a quadratic potential using recent cosmological data, comparing its fit to the standard Lambda-CDM model.

Findings

Model parameters are tightly constrained, with .8 imes 10^5 < .6 < 9.5 imes 10^7.

The extended quintessence model fits the data nearly as well as . Lambda-CDM.

Variation of G since recombination is limited to 0.97-1.03 times G0.

Abstract

We investigate the cosmological observational test of the extended quintessence model, i.e. a scalar-tensor gravity model with a scalar field potential serving as dark energy, by using the Planck 2018 cosmic microwave background (CMB) data, together with the baryon acoustic oscillations (BAO) and redshift-space distortion (RSD) data. As an example, we consider the model with a Brans-Dicke kinetic term $\frac{\omega(\phi)}{\phi} \phi_{;\mu} \phi^{;\mu} $ and a quadratic scalar potential $V (\phi) = A + B (\phi - \phi_0) + \frac{C}{2} (\phi - \phi_0)^2$, which reduces to general relativity (GR) in the limit $\omega(\phi) \to \infty$, and the cosmological constant in the limit $B=C=0$. In such a model the scalar field typically rolls down the potential and oscillates around the minimum of $V (\phi)$. We find that the model parameter estimate for the CMB+BAO+RSD data set is given by $\lg \alpha = -3.6 _{-0.54}^{+0.66}~ (68\%)$, corresponding to $ 3.8 \times 10^5 < \omega_0 < 9.5 \times 10^7~ (68\%)$, and $\lg C = 4.9 \pm 1.4~ (68\%) $. However, the GR $\Lambda$CDM model can fit the data almost as good as this extended quintessence model, and is favored by the Akaike information criterion (AIC). The variation of the gravitational constant since the epoch of Recombination is constrained to be $0.97 < G_{\rm rec}/G_0 < 1.03~ (1 \sigma)$. In light of recent report that the CMB data favors a closed universe, we consider the case with non-flat geometry in our fit, and find that the mean value of $\Omega_k$ shifts a little bit from $-0.049$ to $-0.036$, and the parameters in our model are not degenerate with $\Omega_k$.