Model independent evidence for dark energy evolution from Baryon Acoustic Oscillations

TL;DR

This paper tests whether dark energy is the cosmological constant by exploiting BAO-based measurements of the expansion history with an enhanced Om diagnostic. By constructing $Omh^2$ from independent $H(z)$ values at $z=0$, 0.57, and 2.34, the paper finds $Omh^2 \approx 0.122 \pm 0.01$, which is in tension with the Planck+WP value $\Omega_{0m} h^2 = 0.1426 \pm 0.0025$ expected for ΛCDM. The tension persists across plausible $H_0$ variations, suggesting dark energy may evolve; screened/compensated models provide a better fit and predict a pole in the high-z effective equation of state $w(z)$ as a clear signature. These findings have significant implications for early-universe physics, growth of structure, and potential modified gravity explanations, and motivate future high-precision BAO and growth measurements to test evolving dark energy scenarios.

Abstract

Baryon Acoustic Oscillations (BAO) allow us to determine the expansion history of the Universe, thereby shedding light on the nature of dark energy. Recent observations of BAO's in the SDSS DR9 and DR11 have provided us with statistically independent measurements of $H(z)$ at redshifts of 0.57 and 2.34, respectively. We show that these measurements can be used to test the cosmological constant hypothesis in a model independent manner by means of an improved version of the $Om$ diagnostic. Our results indicate that the SDSS DR11 measurement of $H(z) = 222 \pm 7$ km/sec/Mpc at $z = 2.34$, when taken in tandem with measurements of $H(z)$ at lower redshifts, imply considerable tension with the standard $Λ$CDM model. Our estimation of the new diagnostic $Omh^2$ from SDSS DR9 and DR11 data, namely $Omh^2 \approx 0.122 \pm 0.01$, which is equivalent to $Ω_{0m}h^2$ for the spatially flat $Λ$CDM model, is in tension with the value $Ω_{0m}h^2 = 0.1426 \pm 0.0025$ determined for $Λ$CDM from Planck+WP. This tension is alleviated in models in which the cosmological constant was dynamically screened (compensated) in the past. Such evolving dark energy models display a pole in the effective equation of state of dark energy at high redshifts, which emerges as a smoking gun test for these theories.

Figures (1)

• Figure 1: The Hubble parameter (left panel) and the effective equation of state of dark energy (right panel) are shown for the Braneworld model described by (\ref{['eq:hubble_brane']}) (solid red) and $\Lambda$CDM (dotted green). Also shown is the matter contribution: $H_0\sqrt{\Omega_{0m} (1+z)^3}$ where $H_0 = 70$ km/sec/Mpc and $\Omega_{0m} = 0.28$ (dotted blue). In the Braneworld model the cosmological constant is screened in the past as a result of which the expansion rate drops below that in $\Lambda$CDM at high $z$. This feature permits the Braneworld to better account for the low value of $H(z=2.34)$ discovered in bao_2014. Note that $H_{\rm Brane} \simeq H_0\sqrt{\Omega_{0m} (1+z)^3}$ at $z \simeq 2.4$. The associated pole in $w(z)$ at $z \simeq 2.4$ is shown in the right panel. The parameters for the Braneworld model are $\Omega_{0m} = 0.28$ and $\Omega_\ell = 0.025$ in (\ref{['eq:hubble_brane']}).