Have dark forces been messing with the Cosmos? is the headline of Dennis Overbye’s piece in the New York Times on February 25 last year, describing tension between measurements of the age of the Universe when atoms first appeared and the age of the Universe now. If the standard models of particle physics and of cosmology are fully correct, these two measurements should agree about “little h”, one of the six parameters of the cosmology standard model. A grant will determine whether the discrepancy between h(CMB) and h(ladder) is significant. CMB is the cosmic microwave background, and ladder is the extragalactic distance ladder, that starts with the dimensions of the solar system and works its way out to the distances of far off galaxies.
Our project will reduce the uncertainties in the ladder and evaluate the statistical significance of the discrepancy. Two space telescopes will be used to do this, one of them the Hubble. Our goal is to reduce the ladder uncertainties to the same level as existing CMB measurement.
Our deliverable is a value of the Hubble Constant with an uncertainty between 1 and 2%. In what follows the terms "little h", H0, and the Hubble Constant are used interchangeably.
Little h is the dimensionless Hubble Constant, that is, the Hubble Constant divided by 100 kilometers per second per Megaparsec. For present purposes, they are all the same thing, the rate of expansion of the Universe.
If two the standard models meet and agree, this is a landmark for fundamental physics. If not, hypotheses such as dark photons and additional neutrinos must be explored.
