  X-ray spectroscopy is key to address the theme of "The Hot Universe", the
still poorly understood astrophysical processes driving the cosmological
evolution of the baryonic hot gas traceable through its electromagnetic
radiation. Two future X-ray observatories: the JAXA-led XRISM (due to launch in
the early 2020s), and the ESA Cosmic Vision L-class mission Athena (early
2030s) will provide breakthroughs in our understanding of how and when
large-scale hot gas structures formed in the Universe, and in tracking their
evolution from the formation epoch to the present day.
