I was reading an article about the recent type Ia supernova in the big dipper when the question popped into my head that I've forgotten about until now. Since we use the Ia supernova as our candles to measure the expansion of the universe, how do we know that everything is accelerating? It's easy to look at objects that are further away and see them moving away from us faster because that's how universal expansion works. What I'm wondering about is from seeing the universe expanding ever faster as we look at further objects, how do we know that it is ACCELERATING rather than expanding at a constant rate? What I'm thinking in my head is the universe might be expanding at a constant rate, but objects further away may just appear to be receding faster just because of the expansion process (With all the red shifting and such). Not sure how we're extrapolating an acceleration because we're already seeing something similar. I'm hoping I'm just misconstruing things here and that I don't have any misconceptions of how the universe works.