The Accuracy of the Hubble Constant Measurement Verified through Cepheid
  Amplitudes
  The accuracy of the Hubble constant measured with extragalactic Cepheids
depends on robust photometry and background estimation in the presence of
stellar crowding. The conventional approach accounts for crowding by sampling
backgrounds near Cepheids and assuming they match those at their positions. We
show a direct consequence of crowding by unresolved sources at Cepheid sites is
a reduction in the fractional amplitudes of their light curves. We use a simple
analytical expression to infer crowding directly from the light curve
amplitudes of >200 Cepheids in 3 SNe~Ia hosts and NGC 4258 as observed by HST
-- the first near-infrared amplitudes measured beyond the Magellanic Clouds.
Where local crowding is minimal, we find near-infrared amplitudes match Milky
Way Cepheids at the same periods. At greater stellar densities we find that the
empirically measured amplitudes match the values predicted (with no free
parameters) from crowding assessed in the conventional way from local regions,
confirming their accuracy for estimating the background at the Cepheid
locations. Extragalactic Cepheid amplitudes would need to be ~20% smaller than
measured to indicate additional, unrecognized crowding as a primary source of
the present discrepancy in H_0. Rather we find the amplitude data constrains a
systematic mis-estimate of Cepheid backgrounds to be 0.029 +/- 0.037 mag, more
than 5x smaller than the size of the present ~0.2 mag tension in H_0. We
conclude that systematic errors in Cepheid backgrounds do not provide a
plausible resolution to the Hubble tension.
