The foundations of observing dark energy dynamics with the Wilkinson Microwave Anisotropy Probe

TL;DR

This study constrains dynamical dark energy by jointly analyzing CMB, SN-Ia, and large-scale structure data within a physically motivated 10-parameter framework for a time-varying equation of state $w(a)$. Using an MCMC approach and a specially adapted Boltzmann code to track rapid transitions in $w$, the authors find that standard cosmological parameters remain robust and that $w_{0}$ is constrained to $w_{0}<-0.80$ (2$\sigma$), while $w_{m}$ and transition parameters are weakly constrained. The best-fit model favors late-time evolution with a rapid transition around $z\sim1$, but overall the data are still well fitted by $\Lambda$CDM, and there is no decisive evidence for dark energy dynamics. The work highlights the importance of careful numerical treatment for rapidly varying $w(a)$ and suggests that additional probes (e.g., ISW correlations, reionisation measurements, cluster polarisation) and reduced cosmic variance will be key to detecting true dynamics if it exists.

Abstract

Detecting dark energy dynamics is the main quest of current dark energy research. Addressing the issue demands a fully consistent analysis of CMB, large scale structure and SN-Ia data with multi-parameter freedom valid for all redshifts. Here we undertake a ten parameter analysis of general dark energy confronted with the first year WMAP, 2dF galaxy survey and latest SN-Ia data. Despite the huge freedom in dark energy dynamics there are no new degeneracies with standard cosmic parameters apart from a mild degeneracy between reionisation and the redshift of acceleration, both of which effectively suppress small scale power. Breaking this degeneracy will help significantly in detecting dynamics, if it exists. Our best-fit model to the data has significant late-time evolution at z<1.5. Phantom models are also considered and we find that the best-fit crosses w=-1 which, if confirmed, would be a clear signal for radically new physics. Treatment of such rapidly varying models requires careful integration of the dark energy density usually not implemented in standard codes, leading to crucial errors of up to 5%. Nevertheless cosmic variance means that standard LCDM models are still a very good fit to the data and evidence for dynamics is currently very weak. Independent tests of reionisation or the epoch of acceleration (e.g. ISW-LSS correlations) or reduction of cosmic variance at large scales (e.g. cluster polarisation at high redshift) may prove key in the hunt for dynamics.

Figures (14)

• Figure 1: Schematic plot of the equation of state paratrisation Eq. (\ref{['bcc']}).
• Figure 2: CMB power spectrum for the QCDM (red solid line) and $\Lambda$CDM (blue dashed line) best fit models.
• Figure 3: Marginalized likelihoods for the various cosmological parameters in the $\Lambda$CDM scenario (blue solid curve) and including the QCDM models (yellow shaded region). Also shown are QCDM models with a prior on the baryon energy density $\Omega_b h^2 = 0.0216 \pm 0.002$ (red dashed line). The results agree very well in all cases.
• Figure 4: The degeneracy between the scalar spectral index, $n_S$, and the physical baryon density $\Omega_b h^2$. The filled contour are the 1 and 2 $\sigma$ limits of the quintessence models, while the black solid contours are those of the $\Lambda$CDM models.
• Figure 5: The degeneracy between the scalar spectral index, $n_S$, and the optical depth $\tau$. The filled contour are the 1 and 2 $\sigma$ limits of the quintessence models. The black lines show the corresponding limits for the $\Lambda$CDM case.