  Local and distant measurements of the Hubble constant are in significant
tension: local measurements of the Hubble constant appear to show a Universe
that is significantly contracted when compared to distant measurements. From
the point of view of an observer, a passing gravitational wave could cause the
Universe to appear locally contracted and expanded in a quadrupolar pattern.
The inspiral of a pair of stupendously large black holes (SLABs) with a chirp
mass of $\sim10^{21}~\rm{M}_\odot$ may produce gravitational radiation with
sufficiently large amplitude and wavelength to increase $H_0$ in one direction,
and decrease it by the same amount in the other. These tensions would then
oscillate with a period corresponding to half the orbital period of the binary.
If such a gravitational wave were aligned with the plane of the Milky Way, most
readily visible galaxies would appear closer than they actually are, thereby
causing the apparent Hubble tension. Due to the long binary period, we would be
in the same phase of the gravitational wave for the complete history of
astronomical observation. The opposite tension would be visible in the
orthogonal directions, thus giving the opportunity to falsify the existence of
inspiraling SLAB binaries. As a corollary, the Hubble tension may place an
upper limit on the maximum mass of inspiraling black holes in the Universe.
