  Recent work has suggested that black holes (BHs) could be cosmologically
coupled to the accelerated expansion of the universe, potentially becoming a
candidate for dark energy. This would imply BH mass growth following the
cosmological expansion, with the masses of individual BHs growing as $M_{\rm
BH}\propto (1+z)^3$. In this letter, we discuss the binary systems Gaia BH1 and
Gaia BH2, which contain $\sim 9\,M_{\odot}$ BHs orbited by $\sim 1\,M_{\odot}$
stars in widely-separated orbits. The ages of both systems can be constrained
by the properties of the luminous stars. If BH masses are indeed growing as
$(1+z)^3$, the masses of both BHs at formation would have been significantly
smaller than today. We find a 77% probability that the mass of the BH in Gaia
BH2 would have been below $2.2M_\odot$ at formation. This is below the
classical Tolman-Oppenheimer-Volkov limit, though it is not yet clear if BHs
subject to cosmological coupling should obey this limit. For Gaia BH1, the same
probability is 70%. This analysis is consistent with results from two BHs in
the globular cluster NGC3201, but unlike the NGC3201 BHs, the Gaia BHs have
well-constrained inclinations and thus firm upper mass limits. The discovery of
more BHs in binary systems with Gaia astrometry in the coming years will allow
us to test the cosmological coupling hypothesis decisively.
