Einstein's legacy in galaxy surveys
  Non-Gaussianity in the primordial fluctuations that seeded structure formation produces a signal in the galaxy power spectrum on very large scales. This signal contains vital information about the primordial Universe, but it is very challenging to extract, because of cosmic variance and large-scale systematics - especially after the Planck experiment has already ruled out a large amplitude for the signal. Cosmic variance and experimental systematics can be alleviated by the multi-tracer method. Here we address another systematic - introduced by not using the correct relativistic analysis of the power spectrum on very large scales. In order to reduce the errors on fNL, we need to include measurements on the largest possible scales. Failure to include the relativistic effects on these scales can introduce significant bias in the best-fit value of fNL from future galaxy surveys. 
Stefano Camera, Roy Maartens and Mario G. Santos
astro-ph.CO gr-qc