Baryon bias and structure formation in an accelerating universe

TL;DR

The paper tackles the problem that structure formation stalls during cosmic acceleration in standard models. It studies a dark-energy scalar field coupled to dark matter, yielding a stationary accelerated regime with nonzero $\Omega_c$ and constant $w_e$, which preserves perturbation growth. The authors derive background and perturbation equations, identify a stationary attractor where parameters $\mu$ and $\beta$ are fixed by observations, and predict a scale-independent baryon bias $b<1$ as a distinctive signature. The work demonstrates that a DM-DE coupling can produce observable growth and a measurable baryon bias, offering a potential test of stationary dark-energy scenarios and a means to address the cosmic coincidence problem.

Abstract

In most models of dark energy the structure formation stops when the accelerated expansion begins. In contrast, we show that the coupling of dark energy to dark matter may induce the growth of perturbations even in the accelerated regime. In particular, we show that this occurs in the models proposed to solve the cosmic coincidence problem, in which the ratio of dark energy to dark matter is constant. Depending on the parameters, the growth may be much faster than in a standard matter-dominated era. Moreover, if the dark energy couples only to dark matter and not to baryons, as requested by the constraints imposed by local gravity measurements, the baryon fluctuations develop a constant, scale-independent, large-scale bias which is in principle directly observable. We find that a lower limit to the baryon bias b>0.5 requires the total effective parameter of state w_e=1+p/rho to be larger than 0.6 while a limit b>0.73 would rule out the model.

Figures (2)

• Figure 1: Numerical evolution of the density contrast for a 100 Mpc/$h$ perturbation of dark matter (continuous lines), baryons (dashed lines) and scalar field (dotted lines). Thick lines: $\beta ,\mu =1.5,3$ (or $\Omega _{\phi }=0.55,w_{e}=0.67$), resulting in a bias $b\approx 0.3$. Thin lines: $\beta ,\mu =0.25,3$ (or $\Omega _{\phi }=0.5,w_{e}=0.92$): here the dark matter and baryon curves are almost indistinguishable since $b\approx 1$.
• Figure 2: Constraints on the stationary model: below the horizontal line the expansion is accelerated; in the light grey region the bias is between 0.5 and 1; between the vertical lines $\Omega _{\phi }$ is within the observed range. The dark grey region is the surviving parameter space.