The Square Kilometre Array (SKA) is a planned large radio interferometer designed to operate over a wide range of frequencies, and with an order of magnitude greater sensitivity and survey speed than any current radio telescope. The SKA will address many important topics in astronomy, ranging from planet formation to distant galaxies. However, in this work, we consider the perspective of the SKA as a facility for studying physics. We review four areas in which the SKA is expected to make major contributions to our understanding of fundamental physics: cosmic dawn and reionisation; gravity and gravitational radiation; cosmology and dark energy; and dark matter and astroparticle physics. These discussions demonstrate that the SKA will be a spectacular physics machine, which will provide many new breakthroughs and novel insights on matter, energy, and spacetime.

Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the

Context.The hot plasma in a galaxy cluster is expected to be heated to high temperatures through shocks and adiabatic compression. The thermodynamical properties of the gas encode information on the processes leading to the thermalization of the gas in the cluster’s potential well and on non-gravitational processes such as gas cooling, AGN feedback, shocks, turbulence, bulk motions, cosmic rays and magnetic field.Aims.In this work we present the radial profiles of the thermodynamic properties of the intracluster medium (ICM) out to the virial radius for a sample of 12 galaxy clusters selected from thePlanckall-sky survey. We determine the universal profiles of gas density, temperature, pressure, and entropy over more than two decades in radius, from 0.01R₅₀₀to 2R₅₀₀.Methods.We exploited X-ray information fromXMM-Newtonand Sunyaev-Zel’dovich constraints fromPlanckto recover thermodynamic properties out to 2R₅₀₀. We provide average functional forms for the radial dependence of the main quantities and quantify the slope and intrinsic scatter of the population as a function of radius.Results.We find that gas density and pressure profiles steepen steadily with radius, in excellent agreement with previous observational results. Entropy profiles beyondR₅₀₀closely follow the predictions for the gravitational collapse of structures. The scatter in all thermodynamical quantities reaches a minimum in the range [0.2 − 0.8]R₅₀₀and increases outward. Somewhat surprisingly, we find that pressure is substantially more scattered than temperature and density.Conclusions.Our results indicate that once accreting substructures are properly excised, the properties of the ICM beyond the cooling region (R > 0.3R₅₀₀) follow remarkably well the predictions of simple gravitational collapse and require few non-gravitational corrections.

Galaxy clusters are the endpoints of structure formation and are continuously growing through the merging and accretion of smaller structures. Numerical simulations predict that a fraction of their energy content is not yet thermalized, mainly in the form of kinetic motions (turbulence, bulk motions). Measuring the level of non-thermal pressure support is necessary to understand the processes leading to the virialization of the gas within the potential well of the main halo and to calibrate the biases in hydrostatic mass estimates. We present high-quality measurements of hydrostatic masses and intracluster gas fraction out to the virial radius for a sample of 13 nearby clusters with availableXMM-NewtonandPlanckdata. We compare our hydrostatic gas fractions with the expected universal gas fraction to constrain the level of non-thermal pressure support. We find that hydrostatic masses require little correction and infer a median non-thermal pressure fraction of ∼6% and ∼10% atR₅₀₀andR₂₀₀, respectively. Our values are lower than the expectations of hydrodynamical simulations, possibly implying a faster thermalization of the gas. If instead we use the mass calibration adopted by thePlanckteam, we find that the gas fraction of massive local systems implies a mass bias 1 − b = 0.85 ± 0.05 for Sunyaev–Zeldovich-derived masses, with some evidence for a mass-dependent bias. Conversely, the high bias required to matchPlanckcosmic microwave background and cluster count cosmology is excluded by the data at high significance, unless the most massive halos are missing a substantial fraction of their baryons.

We have implemented an adaptive mesh refinement criterion explicitly designed to increase spatial resolution around discontinuities in the velocity field in ENZO cosmological simulations. With this technique, shocks and turbulent eddies developed during the hierarchical assembly of galaxy clusters are followed with unprecedented spatial resolution, even at large distances from the clusters center. By measuring the spectral properties of the gas velocity field, its time evolution and the properties of shocks for a reference galaxy cluster, we investigate the connection between accretion processes and the onset of chaotic motions in the simulated inter-galactic medium over a wide range of scales.

A radio ridge between two galaxy clusters: Galaxy clusters contain dozens or hundreds of galaxies, vast quantities of hot gas, and large amounts of dark matter. The gas can emit at radio wavelengths if it contains electrons at relativistic speeds, which can be injected by active galaxies or accelerated during a merger between two clusters. Govoni et al. used the Low-Frequency Array (LOFAR) radio telescope to observe a ridge of radio-emitting plasma extending between two galaxy clusters that are approaching a merger. The results imply that intergalactic magnetic fields connect the two clusters and challenge theories of particle acceleration in the intergalactic medium. Science , this issue p. 981