 We've come a long way from our start triangulating and directly measuring sizes in my backyard in our segment on the earth. In this segment, we split redshift, our final rung of the cosmic distance ladder, into two parts. The original was based on the Doppler effect. The second is cosmological redshift based on the expansion of the universe. It is important to remember that this kind of redshift can only provide distance information if we have a cosmological model for the expansion. And we do. It's called the Lambda Cold Dark Matter Benchmark Model. It is covered in depth in the How Old Is It? video book.