Dark energy and cosmic microwave background bispectrum

TL;DR

The paper addresses the degeneracy between the matter density $\Omega_0$ and the dark energy equation of state $w_Q$ in CMB analyses by focusing on the cross-correlation bispectrum produced by primordial, lensing, and Rees-Sciama signals. It develops a formalism to compute the reduced bispectrum via the coupling ${\cal Q}(\ell)$, with ${\cal Q}(\ell)$ computed using nonlinear power-spectrum mappings and a generalized growth factor for $w_Q\neq -1$. Forecasts for MAP+ACT and Planck data indicate that joint measurements of $\Omega_0$ and $w_Q$ with about 10% and 30% accuracy at the 90% confidence level are feasible when the bispectrum is included. The work also notes caveats, such as possible time variation in $w_Q$, the assumption of Gaussian initial conditions, and the need to subtract SZ, dust, and point-source contributions. Overall, it demonstrates a viable approach to breaking CMB parameter degeneracies and improving dark-energy constraints through small-scale bispectrum information.

Abstract

We compute the cosmic microwave background bispectrum arising from the cross-correlation of primordial, lensing and Rees-Sciama signals. The amplitude of the bispectrum signal is sensitive to the matter density parameter, Omega_0, and the equation of state of the dark energy, which we parameterize by w_Q. We conclude that the dataset of the Atacama Cosmology Telescope, combined with MAP 2-year data or the Planck data set alone will allow us to break the degeneracy between Omega_0 and w_Q that arises from the analysis of CMB power spectrum. In particular a joint measurement of Omega_0 and w_Q with 10% and 30% error on the two parameters respectively, at the 90% confidence level can realistically be achieved.

Figures (1)

• Figure 1: Absolute value of ${\cal Q}(\ell)$ for two different cosmologies: $\Omega_0=0.3, w=-1.0$ (thick line) and $\Omega_0=0.2, w=-0.2$ (thin line). The solid line indicates ${\cal Q}>0$ while dashed line indicates ${\cal Q}<0$. If linear theory was a good approximation for the evolution of the power spectrum, ${\cal Q} \ge 0$: non-linear effects can be important at $\ell \sim 200$.