Is the universe always expanding 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.
It will always expand (probably).

The mass and energy in the universe curves space, and it seems that there is exactly the right amount of mass and energy to keep the universe expanding forever. 

Had you asked this question 20 years ago the answer would have been "we don't know". Recent discoveries of "Dark Energy" suggest that the expansion of the universe is not slowing down, but speeding up. This would mean that the universe is destined to expand forever, at a faster and faster rate.

The notion that there will be a "Big Crunch" followed by another "Big Bang" was seriously considered by multiple scientists: among them Willem de Sitter and George Gamow. There are problems with this hypothesis: It is not well supported by observations of the rate of expansion. Moreover, there would need to be way to reduce the entropy of the big crush, as entropy can only increase. And moreover any "pre-big bang state" may be hidden behind the singularity at the start of the (current) universe and so fundamentally hidden from us.