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The recent observations of the global 21cm signal by EDGES and gravitationalwaves by LIGO/VIGO have revived interest in PBHs. Different from previousworks, we investigate the influence of PBHs on the evolution of the IGM for themass range $6\times 10^{13} {\rm g} \lesssim M_{\rm PBH}\lesssim 3\times10^{14} \rm g$. Since the lifetime of these PBHs is smaller than the presentage of the Universe, they have evaporated by the present day. Due to Hawkingradiation, the heating effects of PBHs on the IGM can suppress the absorptionamplitude of the global 21cm signal. In this work, by requiring that thedifferential brightness temperature of the global 21cm signals in the redshiftrange of $10\lesssim z \lesssim 30$, e.g., $\delta T_{b} \lesssim -100~\rm mK$,we obtain upper limits on the initial mass fraction of PBHs. We find that thestrongest upper limit is $\beta_{\rm PBH} \sim 2\times 10^{-30}$. Since theformation of PBHs is related to primordial curvature perturbations, by usingthe constraints on the initial mass fraction of PBHs we obtain the upper limitson the power spectrum of primordial curvature perturbations for the scale range$8.0\times 10^{15}\lesssim k \lesssim 1.8\times 10^{16}~\rm Mpc^{-1}$,corresponding to the mass range considered here. We find that the strongestupper limit is $\mathcal P_{\mathcal R}(k) \sim 0.0046$. By comparing withprevious works, we find that for the mass range (or the scale range)investigated in this work the global 21cm signals or the 21cm power spectrumshould give the strongest upper limits on the initial mass fraction of PBHs andon the power spectrum of primordial curvature perturbations.

Constraints on primordial black holes and curvature perturbations from the global 21cm signal