Measuring the Hubble Constant Near and Far in the Era of ELT's
  Many of the fundamental physical constants in Physics, as a discipline, are
measured to exquisite levels of precision. The fundamental constants that
define Cosmology, however, are largely determined via a handful of independent
techniques that are applied to even fewer datasets. The history of the
measurement of the Hubble Constant (H0), which serves to anchor the expansion
history of the Universe to its current value, is an exemplar of the
difficulties of cosmological measurement; indeed, as we approach the centennial
of its first measurement, the quest for H0 still consumes a great number of
resources. In this white paper, we demonstrate how the approaching era of
Extremely Large Telescopes (ELTs) will transform the astrophysical measure of
H0 from the limited and few into a fundamentally new regime where (i) multiple,
independent techniques are employed with modest use of large aperture
facilities and (ii) 1% or better precision is readily attainable. This quantum
leap in how we approach H0 is due to the unparalleled sensitivity and spatial
resolution of ELT's and the ability to use integral field observations for
simultaneous spectroscopy and photometry, which together permit both familiar
and new techniques to effectively by-pass the conventional 'ladder' framework
to minimize total uncertainty. Three independent techniques are discussed --
(i) standard candles via a two-step distance ladder applied to metal, poor
stellar populations, (ii) standard clocks via gravitational lens cosmography,
and (iii) standard sirens via gravitational wave sources -- each of which can
reach 1% with relatively modest investment from 30-m class facilities.
