The Stacks project

Proof. The morphism $X \times _{\mathop{\mathrm{Spec}}(k)} Y \to \mathop{\mathrm{Spec}}(k)$ is of finite type and separated because it is the composition of the morphisms $X \times _{\mathop{\mathrm{Spec}}(k)} Y \to Y \to \mathop{\mathrm{Spec}}(k)$ which are separated and of finite type, see Morphisms, Lemmas 29.15.4 and 29.15.3 and Schemes, Lemma 26.21.12. To finish the proof it suffices to show that $X \times _{\mathop{\mathrm{Spec}}(k)} Y$ is integral. Let $X = \bigcup _{i = 1, \ldots , n} U_ i$, $Y = \bigcup _{j = 1, \ldots , m} V_ j$ be finite affine open coverings. If we can show that each $U_ i \times _{\mathop{\mathrm{Spec}}(k)} V_ j$ is integral, then we are done by Properties, Lemmas 28.3.2, 28.3.3, and 28.3.4. This reduces us to the affine case.

The affine case translates into the following algebra statement: Suppose that $A$, $B$ are integral domains and finitely generated $k$-algebras. Then $A \otimes _ k B$ is an integral domain. To get a contradiction suppose that

in $A \otimes _ k B$ with both factors nonzero in $A \otimes _ k B$. We may assume that $b_1, \ldots , b_ n$ are $k$-linearly independent in $B$, and that $d_1, \ldots , d_ m$ are $k$-linearly independent in $B$. Of course we may also assume that $a_1$ and $c_1$ are nonzero in $A$. Hence $D(a_1c_1) \subset \mathop{\mathrm{Spec}}(A)$ is nonempty. By the Hilbert Nullstellensatz (Algebra, Theorem 10.34.1) we can find a maximal ideal $\mathfrak m \subset A$ contained in $D(a_1c_1)$ and $A/\mathfrak m = k$ as $k$ is algebraically closed. Denote $\overline{a}_ i, \overline{c}_ j$ the residue classes of $a_ i, c_ j$ in $A/\mathfrak m = k$. The equation above becomes

which is a contradiction with $\mathfrak m \in D(a_1c_1)$, the linear independence of $b_1, \ldots , b_ n$ and $d_1, \ldots , d_ m$, and the fact that $B$ is a domain. $\square$