Question

How long has the universe been expanding?

Answer 1

13.8 billion years, give or take a few million.

Explanation:

The big bang happened and then, it is believed, that at`10^-34` seconds afterwards the universe expanded to `1/2` the size it is today.
"http://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html"