Accurate Extra-Galactic Distances and Dark Energy: Anchoring the Distance Scale with Rotational Parallaxes

Abstract

We investigate how the uncertainty on the Hubble constant (H_0) affects the uncertainty in the Equation of State (EOS) of Dark Energy and the total density of the Universe (Omega_tot). We use the approximate relations between the cosmological parameters [Spergel etal (2007)] and use error-propagation to estimate the effects of improving the CMB parameters and H_0 on the EOS of Dark Energy (DE). First we assume that the additional data does not improve significantly, but decrease the error on H_0 by a factor <~10. Second, we allow improved additional data but current H_0 errors (i.e., the DE Task Force case). In the 1st scenario, improvements of the CMB parameters hardly change the accuracy of the EOS and Omega_tot, unless H_0 can be measured with an accuracy of a few %. We find that a combination of moderate improvements for both H_0 and other data significantly constrains the evolution of dark energy, but at a reduced cost. We review several methods (and their strengths and weaknesses) that might yield extra-galactic distances with errors of about 1%. We review: the Velocity Field method, two Maser methods, two Light Echo techniques, the Binary Star method, and the Rotational Parallax (RP) technique. Because these methods substantially rely on geometry rather than astrophysics or cosmology, their results are quite robust. We focus on the advantages of the RP technique which can provide single-step, bias-free distances to nearby spirals. These distances can be used to improve the zero-point for other methods which in turn allow for a much improved H_0 errors. Achieving an accuracy of ~2% in the distances to M31, M33 and the LMC by the RP method requires proper motions from future astrometric missions (SIM, GAIA and OBSS, or the SKA).