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.
Highlights
1.1 A brief historyThe last century saw an expansion in our view of the world from a static, Galaxy-sized Universe, whose constituents were stars and “nebulae” of unknown but possibly stellar origin, to the view that the observable Universe is in a state of expansion from an initial singularity over ten billion years ago, and contains approximately 100 billion galaxies
A general review of gravitational lensing is given by Wambsganss [233]; here we review the theory necessary for an understanding of the use of lenses in determining the Hubble constant
Optically-measured delays have dominated, due to the fact that only a small optical telescope in a site with good seeing is needed for the photometric monitoring, whereas radio time delays require large amounts of time on long-baseline interferometers which do not exist in large numbers
Summary
A new section (2.1) on megamaser cosmography was added, as this has developed significantly since the first edition. Section 3 on the classical distance ladder has been extensively re-written. It focuses much less on the detailed disagreements based on the metallicity dependence of the P-L relationship of Cepheids, since these have been superseded to a large extent by better calibration. The gravitational-lensing section (2.2) has been extensively updated and much of the detail from pre-2007 papers has been removed in favour of discussion of developments since 2007, the Suyu et al papers and an update on the discussion of mass degeneracies. The section on cosmological measurement (4) has been updated, mainly because of the advent of Planck since the first edition. A few sections (mostly where relatively little progress has been made) have been changed relatively little apart from correction of errors – i.e., the introduction and the sections on S-Z effect and gravitational waves
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