Testing the wavelength dependence of cosmological redshift down to $\Delta z \sim 10^{-6}$

Understand

At the core of the standard cosmological model lies the assumption that the redshift of distant galaxies is independent of photon wavelength.

• This invariance of cosmological redshift with wavelength is routinely found in all galaxy spectra with a precision of $\Delta$z~10$^{-4}$.
• The combined use of approximately half a million high-quality galaxy spectra from the Sloan Digital Sky Survey (SDSS) allows us to explore this invariance down to a nominal precision in redshift of one part per million (statistical).
• Our analysis is performed over the redshift interval 0.02<z<0.25.