@drewdevault When the light wave is stretched we get lower wavelengths and thus lower energy light particles. If we where to go along a ray of light and collect it we see a lower watt reading, but we will be able to collect the light during a longer time/distance. The energy has not disappeared just been spread out.

Disclaimer: This is only how I on the spot made sense of this. My master in physics is a few years old at this point and I have not worked with physics since my studies.