The Carnegie-Chicago Hubble Program. I. An Independent Approach to the
  Extragalactic Distance Scale Using only Population II Distance Indicators
  We present an overview of the Carnegie-Chicago Hubble Program, an ongoing
program to obtain a 3 per cent measurement of the Hubble constant using
alternative methods to the traditional Cepheid distance scale. We aim to
establish a completely independent route to the Hubble constant using RR Lyrae
variables, the tip of the red giant branch (TRGB), and Type Ia supernovae (SNe
Ia). This alternative distance ladder can be applied to galaxies of any Hubble
Type, of any inclination, and, utilizing old stars in low density environments,
is robust to the degenerate effects of metallicity and interstellar extinction.
Given the relatively small number of SNe Ia host galaxies with independently
measured distances, these properties provide a great systematic advantage in
the measurement of the Hubble constant via the distance ladder. Initially, the
accuracy of our value of the Hubble constant will be set by the five Galactic
RR Lyrae calibrators with Hubble Space Telescope Fine-Guidance Sensor
parallaxes. With Gaia, both the RR Lyrae zero point and TRGB method will be
independently calibrated, the former with at least an order of magnitude more
calibrators and the latter directly through parallax measurement of tip red
giants. As the first end-to-end "distance ladder" completely independent of
both Cepheid variables and the Large Magellanic Cloud, this path to the Hubble
constant will allow for the high precision comparison at each rung of the
traditional distance ladder that is necessary to understand tensions between
this and other routes to the Hubble constant.
