Question
The universe started as a big bang? true or false
Answer
No one knows because its a theory.
As is UA and Celestial Gravitation and a load of other things in Flat Earth Theory...
But understand what a theory is in science, whatever the root of the word is.. It's not just some vague idea.
It's a model which explains observations and can predict certain things.
The big bang is the best model we have now which explains the observations of red shift and noticing how galaxies are moving apart.
Could it be amended or replaced? Of course. As new observations are taken it tests the model and can potentially mean a new model is required entirely.
So the geocentric model - earth at the centre of everything - was challenged by retrograde motion of planets. Various attempts were made at fixing that and in the end it was realised that a heliocentric model fixed the problems and explained those observations.
Relativity is a theory and observations and experiments keep on giving confidence that the theory is correct. Gravitaitonal lensing has been observed, time dilation (proven by experiment and accounted for in GPS calculations) and recently gravitational waves have been detected.
A sun sinking slowly below the horizon can be explained by a spinning ball (or, to be fair, by a sun going under a plane, but then it would be dark everywhere so that causes other problems). What it can't be explained by is a sun rotating round the centre of a flat plane no matter how much you shout "perspective".
Long shadows at sunset prove that the sun is physically close to the horizon.
Every assumption of space and space travel is a theory from every so called professional out there.
Space travel is not a theory, it is a documented fact. Your only counter argument is to shout "fake" at everything and claim it's all a conspiracy. You can do that about anything.