Qi Xiong

Weighing a cloud of hot gas the size of a million galaxies is no easy task. Astronomers observe how this gas bends light passing by, and from that bending they calculate its mass. But the gas in galaxy clusters is not static: it churns like a boiling pot, and some of the pressure goes into turbulence rather than holding the gas against collapse.

In the vast cosmic ocean, dark matter gathers into clumps and filaments, like plankton in currents. But if gravity doesn't work exactly as Einstein said, these patterns change — barely noticeably, like ripples from an invisible wind. You can spot such ripples by comparing sky maps with computer simulations.