Currently we observe that the universe is expanding. And this expansion even goes faster and faster.
Why do we know (or presume) that this expansion started slowly at a singularity (i.e. big bang) and is getting faster and faster?
Did a serious scientist ever presumed that the universe might be oscillating, i.e. like a breathing bubble? Currently the universe is expanding but who knows, perhaps in 1 Million year we will observe a shrinking universe and 3 Million year later again expanding - provided the humans will still exist at this time which I doubt.
Of course, this would open many new questions like: How and when did it start? What is the frequency and amplitude? But let's keep this question off side.