In general, the term “Big Bang” is associated with all the theories that describe our universe as from a rapid expansion in the beginning to current state of the universe. Also it describes the very early stage of the universe as it extreme dense and hot in 13.8 billion years ago.
Ever since 1929 astronomers have known the universe is expanding “Edwin Hubble” observe that all galaxies except the closest ones to us which are those in our local group
galaxies are moving away. In fact the farther away galaxy is the faster it is proceeding.
This expansion isn’t motion through space.
It’s the expansion and space itself. So if space is expanding the universe must be slightly bigger cooler and less dense than it was yesterday. If we extrapolate backwards than earlier days the universe must’ve been smaller hotter and denser been observed today. So the universe must began in a very hot in very compact state.
It’s been expanding for the past 13.7 billion years. For a time after the beginning event, the universe was so hot atoms couldn’t form.
The cosmic microwave background called the CMB for short also provides evidence to back up the Big Bang Theory.
The CMB is an amazingly uniform microwave glow ,that’s it every direction in the sky. What could have caused a microwave background.
As the universe expands light is stretched and red shifted. The CMB light has been traveling through space since electrons and protons first combined into atoms some 380,000 years after the Big Bang. That light has stretched to microwave wavelengths by now.
The CMB shows that a long time ago the universe must have been smaller and denser than it is now. Typically when we show illustrations of the history of the universe we show images like this one.
Contrary to popular belief however Big Ben wasn’t any kind of explosion in space. The name Big Bang implies something
along the lines that chemical explosion think fireworks. But the universe is beginning was really an unfolding our creation
of matter, energy, time and space itself. It was not an explosion which to brief one out in the pre-existing space.
The term Big Bang actually refers to the moment when the universe started expanding. In fact another common confusion is that today we can point to a spot with the Big Bang occurred. The truth is that everything in the universe every point every galaxy originated from
the same tiny infinitely dense point 13.7 billion years ago. So really the Big Bang happened everywhere.
Interestingly enough an astronomer discussed now competing thoeary coined the term Big Bang in the late 1940s
Fred Hoyle was on a radio show to talk about the steady-state model and refer to the expanding universe
model as the Big Bang idea. The term stock and its cuz plenty of confusion since.
Instead of the term Big Bang Theory a better term for the model and our universe would probably be be “Expanding universe Theoary”