  Primordial black holes (PBHs) in the asteroid-mass range
$(10^{-17}$--$10^{-12})~M_{\odot}$ almost entirely escape constraints as dark
matter candidates. In the early universe, however, the Schwarzschild metric is
no longer applicable and we must carefully consider PBH metrics with a proper
cosmological embedding. In particular, the Thakurta solution stands out as
perhaps the most adequate such solution. Notably, the Thakurta metric contains
a time-dependent apparent horizon, proportional to the cosmological
scale-factor. We show that this implies the PBHs are hotter than in the
standard Schwarzschild case, and so evaporate significantly faster via Hawking
radiation. By matter-radiation equality, PBHs up to masses of $\sim
10^{-12}~M_{\odot}$ have totally evaporated, and so cannot be a viable dark
matter candidate.
