Projectivity of the moduli of curves

In this expository paper, we show that the Deligne–Mumford moduli space of stable curves is projective over Spec (Ζ). The proof we exposit is due to Kollár. Ampleness of a line bundle is deduced from nefness of a related vector bundle via the Ampleness Lemma, a classifying map construction. The main positivity result concerns the pushforward of relative dualizing sheaves on families of stable curves over a smooth projective curve.