ΛCDM Graphical History

Concluding Remarks

The graphical histories presented here provide a visual summary of the significant progress made over the last two decades toward consensus values for cosmological parameters within the framework of ΛCDM. In general, parameter determinations from the WMAP 9-year (Hinshaw et al. 2013) and most recent Planck (Planck XIII 2015, Planck Int. XLVI 2016) analyses have the smallest quoted uncertainties. It is important to remember that these points represent a combined result from several datasets, in which the WMAP and Planck teams combined their own data with external datasets to provide the tightest possible constraints. Since some of these external datasets are shared in common, these determinations cannot be considered completely independent of each other. Wherever possible, it is useful to check for consistency with more independent determinations.

When parameters are compared individually as shown here, we see in general that there is good agreement between recent determinations within the quoted uncertainties, assuming Gaussian statistics. However, this can be somewhat misleading, as the full parameter covariance is not illustrated, nor has cosmic variance consistently been accounted for (Larson et al. 2015). It is thus best to consider these plots as an indication of trends rather than the complete picture. Taken in this light, we still see that there are some parameters in particular which merit further scrutiny and a need for future progress. These include the optical depth τ, matter and cold dark matter content, and the fluctuation amplitude σ8.

A number of future and ongoing experiments are working toward furthering our understanding. This includes major efforts† toward detection of the B-mode polarization of the CMB, which is a predicted signature from inflation, and should assist in further constraining τ. A search for BAO in the large scale structure observed via 21 cm hydrogen emission is also underway (Canadian Hydrogen Intensity Mapping Experiment, CHIME Newburgh et al. 2014).

†For current B-mode results, please see http://lambda.gsfc.nasa.gov/graphics/.

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