John F. Donoghue, Gabriel Menezes

In the philosophy of science, the origin of an arrow of time is viewed as problematic. We describe here how this arrow really follows from the causal structure of quantum physics. This connection is not really new -- it is just overlooked in most discussions of the arrow of time.

Roman. A. Konoplya, Antonina F. Zinhailo

The $(3+1)$-dimensional Einstein-Gauss-Bonnet theory of gravity bypasses the Lovelock's theorem and avoids Ostrogradsky instability. Here we calculate grey-body factor for Dirac, electromagnetic and gravitational fields and estimate the intensity of Hawking radiation for asymptotically flat black holes in this theory. Positive coupling constant leads to smaller evaporation rate and longer life-time of a black hole, while the negative one enhances Hawking radiation. The axial and polar channels are not iso-spectral anymore and have different intensities of radiation. The grey-body factors for electromagnetic and Dirac fields are smaller for larger $\alpha$, while for the gravitational field the dependence is the opposite.

Richard Howl, Vlatko Vedral, Marios Christodoulou, Carlo Rovelli, Devang Naik, Aditya Iyer

Until recently, table-top tests of quantum gravity (QG) were thought to be practically impossible. However, due to a radical new approach to testing QG that uses principles of quantum information theory (QIT) and quantum technology, such tests now seem, remarkably, within sight. In particular, a promising test has been proposed where the generation of entanglement between two massive quantum systems, both in a superposition of two locations, would provide evidence of QG. In QIT, quantum information can be encoded in discrete variables, such as qubits, or continuous variables. The latter approach, called continuous-variable QIT (CVQIT), is extremely powerful as it has been very effective in applying QIT to quantum field theory. Here we apply CVQIT to QG, and show that another signature of QG would be the creation of non-Gaussianity, a continuous-variable resource that is necessary for universal quantum computation. In contrast to entanglement, non-Gaussianity can be applied to a single rather than multi-partite quantum system, and does not rely on local interactions. We use these attributes to describe a table-top test of QG that is based on just a single quantum system in a single location.

Zhi-Qiang You, Xing-Jiang Zhu, Gregory Ashton, Eric Thrane, Zong-Hong Zhu

Gravitational-wave astronomy provides a unique new way to study the expansion history of the Universe. In this work, we investigate the impact future gravitational-wave observatories will have on cosmology. Third-generation observatories like the Einstein Telescope and Cosmic Explorer will be sensitive to essentially all of the binary black hole coalescence events in the Universe. Recent work by \cite{farr2019future} points out that features in the stellar-mass black hole population break the mass-redshift degeneracy, facilitating precise determination of the Hubble parameter without electromagnetic counterparts or host galaxy catalogues. Using a hierarchical Bayesian inference model, we show that with one year of observation by the Einstein Telescope, the Hubble constant will be measured to $\lesssim0.5\%$. We show that this method can be used to perform Bayesian model selection between cosmological models. As an illustrative example, we show that a decisive statement can be made comparing the $\Lambda$CDM and RHCT cosmic models using just one week of data from the Einstein Telescope.