The writer seems to be describing the trajectory of progress over time as being similar to a curve that ascends for a while but then descends as conditions deteriorate.[...]
This is a book about our age. The arc is down: from the heights of hope and possibility, with the revolutions of 1989, into the confusion, devastation, and loss of 9/11, the Iraq war, the deepening crisis in the Middle East, and—as Tony saw it—the self-defeating decline of the American republic. [...].
Thank you very much. That's what I had thought, in fact, but I felt I needed the "seal of approval" of a native speaker. I am surprised that an idiom nowhere to be found either in the standard dictionaries (Oxford, Merriam Webster etc.) or anywhere else on the Internet made it into a book published by Vintage, to be honest.The writer seems to be describing the trajectory of progress over time as being similar to a curve that ascends for a while but then descends as conditions deteriorate.
Yes, "downward". "Down" is a location (as opposed to "up"). "Downward" is a direction. So I agree with your choice of words.I had to think about it - but partly because of the use of "down" to mean "downwards". I think she could have phrased it better.