The Search for New Physics Amid the Hubble Constant Tension
Abstract
Improved measurements of the Hubble constant demonstrate that the Universe is expanding at present about 9% faster than expected from the LambdaCDM model calibrated to the early Universe, with a significance approaching 4 sigma. The higher, local value has been confirmed by 5 independent, geometric calibrations of Cepheids, now including Gaia DR2 parallaxes. Cepheid relative distances have been confirmed by those from TRGB and Miras. At the other end, the low expected value of H_0 predicted from the early Universe is corroborated by independent measurements of the CMB or Omega_B with BAO data. This H_0 Tension'', as it is widely known, offers the exciting and best opportunity in decades of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, dark matter-radiation interactions or a small curvature, each producing a different-sized shift. Pinpointing the cause of the tension requires continued focus on precision and accuracy in the local measurements. After Gaia, the small number of SNe Ia calibrated with Cepheids will dominate the final error. A modest tweak in the observing strategy can provide a 50%-70% boost in the volume and sample reachable by Cepheids. We propose a two-in-one program to achieve a dual purpose: to extend the range of Cepheid measurements to increase the sample of Cepheid/SN Ia hosts and the precision of H_0, opening a path to reach 1%, while simultaneously confirming the tension with a simpler route that is independent of SNe Ia, the only part of the distance ladder without independent corroboration. On the cusp of a possible breakthrough, an investment is needed to pinpoint the cause of the tension.
- Publication:
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HST Proposal
- Pub Date:
- November 2018
- Bibcode:
- 2018hst..prop15640R