The Hubble Constant

Abstract

I review the current state of determinations of the Hubble constant, which gives the length scale of the Universe by relating the expansion velocity of objects to their distance. There are two broad categories of measurements. The first uses individual astrophysical objects which have some property that allows their intrinsic luminosity or size to be determined, or allows the determination of their distance by geometric means. The second category comprises the use of all-sky cosmic microwave background, or correlations between large samples of galaxies, to determine information about the geometry of the Universe and hence the Hubble constant, typically in a combination with other cosmological parameters. Many, but not all, object-based measurements give H0 values of around 72–74 km s−1 Mpc−1, with typical errors of 2–3 km s−1 Mpc−1. This is in mild discrepancy with CMB-based measurements, in particular those from the Planck satellite, which give values of 67–68 km s−1 Mpc−1 and typical errors of 1–2 km s−1 Mpc−1. The size of the remaining systematics indicate that accuracy rather than precision is the remaining problem in a good determination of the Hubble constant. Whether a discrepancy exists, and whether new physics is needed to resolve it, depends on details of the systematics of the object-based methods, and also on the assumptions about other cosmological parameters and which datasets are combined in the case of the all-sky methods.

Strong gravitational lensing of explosive transients

“…Quasars are suited for this application, because they are known to change their brightness, presumably due to the variation of the gas inflow and accretion disk instabilities. Indeed the time delay for the first gravitationally lensed quasar Q0957+561 is measured to 417 days [45], and subsequently time delays have been measured for more than 20 quasar lens systems [46][47][48]. In combination with detailed modeling of mass distributions of lensing galaxies, now quasar lens time delays constrain H 0 at better than 3% precision [49][50][51][52].…”

Can accelerated expansion of the universe be due to spacetime vorticity?

“…This value of the Hubbles's parameter however excludes the contribution of magneto-vorticity term 4πB 2 0 in the acceleration equation (10). For a better estimate we therefore use energy equation (3) [3][4][5]). Also, a relevant parameter to the expansion is the acceleration magnitude.…”

Gravitational wave observations, distance measurement uncertainties, and cosmology

“…where π(β ) represents the prior probability on β . Using (9), the likelihood can be factorized as In the exponential we have neglected the terms η|h − h that scale with ρ 0 and are thus subdominant with respect to the scalar product h − h |h − h . For simplicity we drop the term η|η as it is independent of the intrinsic parameters and acts as a normalization constant.…”

“…This type of study is particularly relevant for BNS mergers with electromagnetic counterparts for which the redshift (together with an accurate sky localization) are known. It should be noted that the value of H 0 is currently under debate [9]: Hubble Space Telescope measurements using supernovae (and Cepheids) measure H 0 = 73.24 ± 1.74 km s −1 Mpc −1 [10], whereas cosmic microwave background measurements give H 0 = 67.8 ± 0.9 km s −1 Mpc −1 [11].…”