Thread: If the observable universe?

If the observable universe is 84 billion light years wide, and yet is only 13.8 billion years old. How is it known to be 84 billion light years wide seeing as the light hasn't been around that long?

Because it is expanding.

this sounds like a question for SpeedFreak!

Simply put, we measure the expansion of the universe using the cosmological redshift of distant galaxies, which indicates how much the light from a distant galaxy has been "stretched" by the expansion of the universe during its journey, and allows us to therefore work out how much the universe has scaled up since that light was emitted.