Spitzer Space Telescope - General Observer Proposal #60010


The Hubble Constant

Principal Investigator: Wendy L. Freedman
           Institution: Carnegie Institution of Washington

     Technical Contact: Jane R. Rigby, Carnegie Institution of Washington

Co-Investigators: 
Barry Madore, OCIW                                                    
Jane Rigby, OCIW                                                      
Eric Persson, OCIW                                                    
Violet Mager, OCIW                                                    
Laura Sturch, OCIW                                                    

Science Category: cosmology 
 Observing Modes: IracPostCryoMap 
  Hours Approved: 705.0 

Abstract:
We present a plan to measure a value of the Hubble constant having a final
systematic uncertainty of only 3% by taking advantage of Spitzer's unique
mid-infrared capabilities. This involves using IRAC to undertake a fundamental
recalibration of the Cepheid distance scale and progressively moving it out to
pure Hubble flow by an application of a revised mid-IR Tully-Fisher relation.
The calibration and application, in one coherent and self-consistent program,
will go continuously from distances of parsecs to several hundred megaparsecs.
It will provide a first-ever mid-IR calibration of Cepheids in the Milky Way,
LMC and Key Project spiral galaxies and a first-ever measurement and calibration
of the TF relation at mid- infrared wavelengths, and finally a calibration of
Type Ia SNe. Most importantly this program will be undertaken with a single
instrument, on a single telescope, working exclusively at mid-infrared
wavelengths that are far removed from the obscuring effects of dust extinction.
Using Spitzer in this focused way will effectively eliminate all of the major
systematics in the Cepheid and TF distance scales that have been the limiting
factors in all previous applications, including the HST Key Project. By
executing this program, based exclusively on Spitzer data, we will deliver a
value of the Hubble constant, having a statistical precision better than ±1%,
with all currently known systematics quantified and constrained to a level of
less than 3%. A value of Ho determined to this level of systematic accuracy is
required for up-coming cosmology experiments, including Planck. A more accurate
value of the Hubble constant will directly result in other contingently measured
cosmological parameters (e.g., Omega_m, Omega_L, & w) having their covariant
uncertainties reduced significantly now. Any further improvements using this
route will have to await JWST, for which this study is designed to provide a
lasting and solid foundation, and ultimately a value of Ho accurate to 2%.