Q: What was the average density of the universe at the time the light was emitted by the z = 6.56 galaxy? For the question we know the current average density of the universe and red-shift wavelength z = 6.56. It says to calculate the average density of the universe at the moment when light was emitted by the galaxy at red-shifted wavelength 6.56. I have the linear scale factor but I'm not really sure what to do next. I've read equations about density on the internet but haven't found one that uses a wavelength or linear scale factor. Any help is appreciated.