How did scientists determine the Big Bang occurred 13.7 billion years ago?
The speed of a galaxy receding from another was estimated by the
Doppler Effect ( the light emitted by a source is shifted in wave
length by thee motion of the light source ). Edward Powell Hubble (
1889-1953) had discovered in 1929 a linear correlation between the
speed and the distance.
years is 13.77. This is just a problem in unit conversions.
In my opinion, this is perhaps a lower bound for time that had
elapsed, since Big Bang. There are further research developments
that take this up, to a much higher upper bound.