Hebertt Leandro, Valerio Marra, Riccardo Sturani

We investigate a recently proposed method for measuring the Hubble constant from gravitational wave detections of binary black hole coalescences without electromagnetic counterparts. In the absence of a direct redshift measurement, the missing information on the left-hand side of the Hubble-Lema\^itre law is provided by the statistical knowledge on the redshift distribution of sources. We assume that source distribution in redshift depends on just one unknown hyper-parameter, modeling our ignorance of the astrophysical binary black hole distribution. With tens of thousands of these "black sirens" -- a realistic figure for the third generation detectors Einstein Telescope and Cosmic Explorer -- an observational constraint on the value of the Hubble parameter at percent level can be obtained. This method has the advantage of not relying on electromagnetic counterparts, which accompany a very small fraction of gravitational wave detections, nor on often unavailable or incomplete galaxy catalogs.

Nicolas C. Ferree, Emory F. Bunn

We examine the prospects for measurement of the Hubble parameter $H_0$ via observation of the secular parallax of other galaxies due to our own motion relative to the cosmic microwave background rest frame. Peculiar velocities make distance measurements to individual galaxies highly uncertain, but a survey sampling many galaxies can still yield a precise $H_0$ measurement. We use both a Fisher information formalism and simulations to forecast errors in $H_0$ from such surveys, marginalizing over the unknown peculiar velocities. The optimum survey observes $\sim 10^2$ galaxies within a redshift $z_\mathrm{max}=0.05$. The required errors on proper motion are comparable to those that can be achieved by Gaia and future astrometric instruments. A measurement of $H_0$ via parallax has the potential to shed light on the tension between different measurements of $H_0$.

Tim Adamo, Uri Kol

We give two double copy prescriptions which construct asymptotically flat solutions in gravity from asymptotically flat gauge fields. The first prescription applies to radiative fields, which are non-linear vacuum solutions determined by characteristic data at null infinity. For any two such radiative gauge fields (linear or non-linear), the characteristic data of a radiative metric, dilaton and axion is constructed by a simple `squaring' procedure, giving a classical double copy at the level of radiation fields. We demonstrate the procedure with several examples where the characteristic data can be explicitly integrated; for linear fields this also sheds light on the twistorial description of Weyl double copy. Our second prescription applies to all asymptotically flat fields at the level of their asymptotic equations of motion: we give a map between any solution of the asymptotic Maxwell equations and any solution of the asymptotic Einstein equations at null infinity. This also extends to the asymptotic charges and their duals, preserves the soft and hard sectors between gauge theory and gravity, and is related to the usual notion of double copy in scattering amplitudes.