Current and future constraints on the expansion history of the GREA model

Abstract

In this work, we investigate the General Relativistic Entropic Acceleration (GREA) framework, in which late-time acceleration emerges from entropy production associated with the cosmological horizon, and compare its performance with the standard $Λ$CDM description of the Universe. We first confront GREA with current background observations, including baryon acoustic oscillations, type Ia supernovae, compressed CMB information, and cosmic chronometers, with particular emphasis on the geometric horizon parameter $\sqrt{-k}η_0$. We then introduce a phenomenological extension of the theory by allowing for an additional dark energy component, $Ω_{de}$, enabling the recovery of a $Λ$CDM-like expansion history as a limiting case. We perform a Bayesian parameter inference and model comparison analysis using both current data and mock datasets representative of future surveys, including SKAO, LSST, and ET. While current data statistically prefer $Λ$CDM when compressed CMB information is included, GREA remains competitive for low-redshift combinations. Forecasts indicate that gravitational wave standard sirens are expected to enhance the ability to discriminate between entropic-driven and dark-energy-driven expansion scenarios, and to identify the underlying cosmological model favored by the data.

Figures (7)

• Figure 1: Comparison between $\Lambda$CDM model (black dashed lines) and GREA framework for different values of the parameter $\sqrt{-k}\eta_0$. Top left: Hubble expansion rate $H(z)$ normalized to $\Lambda$CDM, compared with cosmic chronometers data. Top right: transverse comoving distance $D_M(z)/r_d$, normalized to $\Lambda$CDM, with BAO measurements shown as data points. Bottom left: BAO distance $D_H(z)/r_d$ normalized to $\Lambda$CDM, with corresponding BAO data. Bottom right: Difference in apparent magnitude $\Delta m_B(z)$ with respect to $\Lambda$CDM, compared with Pantheon+ supernova data. All quantities are shown as ratios (or differences, in case of $m_B$) relative to $\Lambda$CDM to highlight deviations induced by the GREA model. Increasing the values of $\sqrt{-k}\eta_0$ leads to progressively larger departures from $\Lambda$CDM, especially at low redshift, while converging to the standard cosmology at early times. Both models are plotted assuming the following cosmological parameters: $H_0=67.4$ km/s/Mpc , $\Omega_bh^2=0.02237$, $\Omega_ch^2=0.12$. Further details on datasets are provided in \ref{['sec:dataset']}.
• Figure 2: Comparison between $\Lambda$CDM model (black dashed lines), GREA (red dashed lines) and modified GREA for different values of the parameter $\Omega_{de}h^2$. Top left: Hubble expansion rate $H(z)$ normalized to $\Lambda$CDM, compared with cosmic chronometers data. Top right: transverse comoving distance $D_M(z)/r_d$, normalized to $\Lambda$CDM, with BAO measurements shown as data points. Bottom left: BAO distance $D_H(z)/r_d$ normalized to $\Lambda$CDM, with corresponding BAO data. Bottom right: Difference in apparent magnitude $\Delta m_B(z)$ with respect to $\Lambda$CDM, compared with Pantheon+ supernova data. All quantities are shown as ratios (or differences, in case of $m_B$) relative to $\Lambda$CDM to highlight deviations induced by the modified GREA. Increasing the values of $\Omega_{de}h^2$ leads to progressively larger departures from $\Lambda$CDM, especially at low redshift, while converging to the standard cosmology at early times. All models are plotted assuming the following cosmological parameters: $H_0=67.4$ km/s/Mpc , $\Omega_bh^2=0.02237$, $\Omega_ch^2=0.12$, $\sqrt{-k}\eta_0=3.6$. Further details on datasets are provided in \ref{['sec:dataset']}.
• Figure 3: Left panel: posterior distribution of $H_0$, $H(0)$ and $\Omega_m$ from DESI+P-ACT+CC+Pantheon+ obtained with $\Lambda$CDM (red) and GREA (blue). Right panel: posterior distribution of $\sqrt{-k}\eta_0$, $\alpha$ and $\Omega_{GREA}$ from DESI+P-ACT+CC+Pantheon+ (pink) and DESI+CC+Pantheon+ (green) obtained with GREA theory.
• Figure 4: Posterior distribution of $H(0)$, $\Omega_m$ and $\Omega_{de}$ from DESI+P-ACT+CC+Pantheon+ obtained with modified GREA. In the orange case we use a flat prior on $\sqrt{-k}\eta_0$ between $[1.0;5.0]$, in the purple case we use a flat prior on $\sqrt{-k}\eta_0$ between $[1.0;10.0]$, allowing for the $\Lambda$CDM limit.
• Figure 5: Left: posterior distribution of GREA parameters from GREA SKAO+LSST (pink), GREA SKAO+LSST+ET (blue) obtained with GREA. The dashed lines correspond to the parameters value that we used to generate the mock datasets. Right: posterior distribution of $\Lambda$CDM parameter from GREA SKAO+LSST (yellow), GREA SKAO+LSST+ET (green) obtained with $\Lambda$CDM analysis. The dashed lines correspond to the parameters values that we used to generate the mock datasets.