Does the Light Horizon include cosmic microwave background radiation?
Answer The light horizon is identical to the ‘surface’ defined by the microwave background radiation. When the Universe was first forming, it was so dense that nothing, including light, could move from one place to another. As it expanded and became less dense, it became less opaque. Eventually, it became sufficiently transparent so that light moving through space was able to just keep going, without much chance of running into anything. When we look outward, we can see through the nearly empty space created by the expansion of the Universe only as far as the space is empty, and not opaque. Once we’ve looked so far that we’re seeing the Universe as it was when it was opaque, we can’t see any farther. That’s the light horizon. But that’s also where the light of the microwave background is coming from, so the two terms, although describing different concepts, refer to the same distance/place relative to us.
