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Recent observations of globular clusters (GCs) provide evidence that thestellar initial mass function (IMF) may not be universal, suggestingspecifically that the IMF grows increasingly top-heavy with decreasingmetallicity and increasing gas density. Non-canonical IMFs can greatly affectthe evolution of GCs, mainly because the high end determines how many blackholes (BHs) form. Here we compute a new set of GC models, varying the IMFwithin observational uncertainties. We find that GCs with top-heavy IMFs losemost of their mass within a few Gyr through stellar winds and tidal stripping.Heating of the cluster through BH mass segregation greatly enhances thisprocess. We show that, as they approach complete dissolution, GCs withtop-heavy IMFs can evolve into 'dark clusters' consisting of mostly BHs bymass. In addition to producing more BHs, GCs with top-heavy IMFs also producemany more binary BH (BBH) mergers. Even though these clusters are short-lived,mergers of ejected BBHs continue at a rate comparable to, or greater than, whatis found for long-lived GCs with canonical IMFs. Therefore these clusters,although they are no longer visible today, could still contribute significantlyto the local BBH merger rate detectable by LIGO/Virgo, especially for sourceswith higher component masses well into the BH mass gap. We also report that oneof our GC models with a top-heavy IMF produces dozens of intermediate-massblack holes (IMBHs) with masses $M>100\,{\rm M_\odot}$, including one with$M>500\,{\rm M_\odot}$. Ultimately, additional gravitational wave observationswill provide strong constraints on the stellar IMF in old GCs and the formationof IMBHs at high redshift.

Black Hole Mergers from Star Clusters with Top-Heavy Initial Mass Functions