How has the Big Bang Theory changed since it was first conceived?

The big bang theory is that from a singularity the universe was formed in a billionth of a second during the first expansion. That has remained constant. What has evolved is the mechanics of the big bang itself, how much did the universe initially expand and in what amount of time, what were the first particles, etc.

Look at the "big bang" as a starting point and nothing more. Call it zero on the time spectrum. What happened in zero plus one billionth second, then the second billionth second, is what is still evolving. And for that matter, everything for the first several hundred million years afterwards is still all theoretical and in some flux.

The Big Bang Theory has changed a lot since it was first proposed. Newer versions did not include information about cosmic inflation, dark matter, or dark energy—all of which are now essential to our comprehension of the universe's evolution. Moreover, improvements in theoretical models and observational methods have improved our comprehension of the conditions of the early universe and its evolution.