Forgive me if these questions are asked here a lot, but I couldn't find any posts answering them in the first few pages... I'm just starting to learn a bit about cosmology...because I'm currently learning general relativity. I'm just curious, what does it mean to say that the universe has a certain age and a certain size? With respect to what are these measured? Does every observer in the universe agree on the value of these measurements? I ask because in special relativity, different observers don't agree on things like the rate of the passage of time. If every observer agrees on the age and size of the universe, can't these values be used to define an absolute time and an absolute length from which all others can be measured?