For the Frobenius automorphism

, Grothendieck proved that the zeta function

is equivalent to

where the polynomial

on the L-adic cohomology group

. In his

1974 paper, Deligne proved that all zeros of

lie on the critical line of complex numbers

with real part

, a geometric analogue of the Riemann hypothesis.

My question is that if Deligne proved the Riemann hypothesis using étale cohomology theory, then how come the Riemann hypothesis is still an open problem?