Wednesday, July 17, 2013

Planck's summer: gauge independence of cosmological surveys

third post in the "Planck's summer"-series by Youness:

Power spectra in linear perturbation theory depend on the gauge chosen. The differences are small on scales well within the horizon, but they grow as we go to very large scales. However, observers apparently measure galaxy positions unambiguously and calculate power spectra from this. How would you interpret their data on the largest scales?


  1. Youness's answer: Obviously, the observers are implicitly picking a particular gauge. It's then important to know which one in order to compare with theoretical predictions. I think the use of the redshift as a distance variable is the key point. This defines a time slicing and thereby a gauge. It's certainly not identical to any of the typical choices made in theory (conformal Newtonian / synchronous gauge) so that gauge transformations are essential to compare theory to observations on scales comparable to the horizon.