Living Reviews in Relativity had been recently acquired by Springer and has now resumed publication with two new review articles: “Exploring New Physics Frontiers Through Numerical Relativity” by Vitor Cardoso et al. and “The Hubble Constant” (major update) by Neal Jackson.
Please find the abstracts and further details below.
Vitor Cardoso, Leonardo Gualtieri, Carlos A. R. Herdeiro and Ulrich Sperhake,
“Exploring New Physics Frontiers Through Numerical Relativity”
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The demand to obtain answers to highly complex problems within strong-field gravity has been met with significant progress in the numerical solution of Einstein’s equations – along with some spectacular results – in various setups. We review techniques for solving Einstein’s equations in generic spacetimes, focusing on fully nonlinear evolutions but also on how to benchmark those results with perturbative approaches. The results address problems in high-energy physics, holography, mathematical physics, fundamental physics, astrophysics and cosmology.
“The Hubble Constant” (major update)
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I review the current state of determinations of the Hubble constant, which gives the lengthscale 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 H_0 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.
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