system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did scientists measure the age of the universe if spacetime is relative?
gpt
Measuring the age of the universe doesn't really involve relativity because we're not comparing reference frames from different observers. If have 2 observers in 2 different reference frames, they might not agree on the age of the universe, but that's not really useful information for us.

The main way we've measured the age of the universe is by measuring it's rate of expansion and working backwards. The further away an object is, the faster it's accelerating away from us. From this, we were able to make a model of the universe's expansion, and work backwards to when it was first expanding.