We propose to carry out a project which will produce a decisive step towards improving the accuracy of the Hubble constant as determined from the Cepheid-SN Ia method to 1%, by using 28 extremely rare eclipsing binary systems in the LMC which offer the potential to determine their distances to 1%. To achieve this accuracy we will reduce the main error in the binary method by interferometric angular diameter measurements of a sample of red clump stars which resemble the stars in our binary systems. We will check on our calibration with similar binary systems close enough to determine their orbits from interferometry. We already showed the feasibility of our method which yielded the best-ever distance determination to the LMC of 2.2% from 8 such binary systems. With 28 systems and the improved angular diameter calibration we will push the LMC distance uncertainty down to 1% which will allow to set the zero point of the Cepheid PL relation with the same accuracy using the large available LMC Cepheid sample. We will determine the metallicity effect on Cepheid luminosities by a) determining a 2% distance to the more metal-poor SMC with our binary method, and by b) measuring the distances to LMC and SMC with an improved Baade-Wesselink (BW) method. We will achieve this improvement by analyzing 9 unique Cepheids in eclipsing binaries in the LMC our group has discovered which allow factor- of-ten improvements in the determination of all basic physical parameters of Cepheids. These studies will also increase our confidence in the Cepheid-based H0 determination. Our project bears strong synergy to the Gaia mission by providing the best checks on possible systematic uncertainties on Gaia parallaxes with 200 binary systems whose distances we will measure to 1-2%. We will provide two unique tools for 1-3 % distance determinations to individual objects in a volume of 1 Mpc, being competitive to Gaia already at a distance of 1 kpc from the Sun.
After detection of the accelerated expansion of the Universe (Nobel prize 2011) and the existence of an enigmatic “dark energy” component of the matter-energy content of the Universe the physical explanation of the nature of dark energy has become a major challenge for astronomers and physicists in recent years. The recent empirical determinations of H0 complicated even more our understanding of the Universe since they differ by about 4σ from the value obtained from Planck data and the ΛCDM model, which suggest that new physics might be required in the models. We will calibrate two geometrical methods which will yield 1% distances a thousand times further out than Gaia parallaxes. The great advantage of our approach is the full control on all potential errors affecting the distance determinations, including systematic errors elusive in most other methods. In addition, we will provide mutual crosschecks at the sub-percent accuracy level with three completely independent geometrical methods. This will allow for the first time to verify the accuracy (and not only precision) at this unprecedented level of precision. Applying these methods we will calibrate the extragalactic distance scale with an unprecedented precision and accuracy. This will allow for a 1% H0 determination with Cepheids and SN Ia. Novel reverberation studies of AGN continua will allow us to determine H0 completely independently, and provide direct insight into the larger redshift Universe, including the H(z) dependence which will constrain other cosmological parameters. Our results will have strong impact on many fields of modern astrophysics. In particular they will definitively answer the question if new physics beyond the standard cosmological model is required. They will also be central to understand the physical nature of dark energy which constitutes about 72% of the matter-energy of the Universe.