The end of unified dark matter?

TL;DR

This paper assesses whether dark matter and dark energy can be a single unified component by examining the generalized Chaplygin gas with $p = -A \rho^{-\alpha}$. It shows that nonzero sound speed in such models generates oscillations or instabilities in the matter power spectrum on subhorizon scales, constraining the parameter $\alpha$ to $-0.8\times10^{-6} < \alpha < 7.9\times10^{-6}$ and leaving essentially only the $\alpha=0$ (coupled to $\Lambda$CDM) limit. The analysis, anchored in linear perturbation theory and compared to the 2dF Galaxy Redshift Survey, demonstrates that unified dark matter scenarios with density-determined pressure are strongly disfavored, reinforcing the view that dark energy is either a cosmological constant or a separate component from dark matter. The result narrows the viable parameter space for UDM models and highlights the power of precision large-scale structure data in discriminating dark-sector theories, while suggesting that any salvage of UDM would require pressure dynamics beyond a simple $p(\rho)$ relation.

Abstract

Despite the interest in dark matter and dark energy, it has never been shown that they are in fact two separate substances. We provide the first strong evidence that they are separate by ruling out a broad class of so-called unified dark matter models that have attracted much recent interest. We find that they produce oscillations or exponential blowup of the matter power spectrum inconsistent with observation. For the particular case of generalized Chaplygin gas models, 99.999% of the previously allowed parameter space is excluded, leaving essentially only the standard Lambda-CDM limit allowed.

Figures (3)

• Figure 1: UDM solution for perturbations as function of wavenumber, $k$. From top to bottom, the curves are GCG models with $\alpha=-10^{-4}$, $-10^{-5}$, $0$ ($\Lambda$CDM), $10^{-5}$ and $10^{-4}$, respectively. The data points are the power spectrum of the 2df galaxy redshift survey.
• Figure 2: The likelihood function $e^{-\Delta\chi/2}$ as a function of the GCG parameter $\alpha$. It is sharply peaked around $\alpha=0$ which is equivalent to the $\Lambda$CDM model. From top to bottom, the horizontal dashed lines correspond to $\Delta\chi^2=1$ and 4, respectively.
• Figure 3: The graph is showing constraints from previous work by Makler et al.. Our new constraints from first order perturbation theory are superimposed on the plot as shown