What is the fine-grained topic of the following text: This universal expansion was predicted from general relativity by Alexander Friedmann in 1922 andGeorges Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker. Only return the topic.
Universal expansion and its role in the Big Bang theory