structures formed: groups and clusters of galaxies, and the filaments that connect these clusters to one another in the vast cosmic web.
Thanks to major surveys of the last decade, we now have a precision map of the cosmic cartography of the present-day local universe that is the result of this process of merging. Over the next decade it will be a high priority to extend such precision mapping over cosmic time: to have, in effect, a 13-billion-year-long movie that traces the buildup of structure since the universe first became transparent to light. This can be done by using radio telescopes to provide more detailed maps of the cosmic microwave background and to detect the atomic hydrogen gas all the way back into the dark ages; large spectroscopic surveys in the visible and near-infrared to trace the distribution of galaxies; gravitational lensing to trace the distribution of the dark matter halos; ultraviolet spectroscopic surveys to map out the warm tenuous gas lying in the vast cosmic filaments; and radio Sunyaev-Zel’dovich effect and X-ray surveys that reveal the distribution of the hot gas found in groups and clusters of galaxies.
Most stars with masses smaller than that of the Sun will live even longer than the current age of the universe. This means that low-mass stars that formed at any time over the history of the universe are still present in galaxies today. Thus, detailed studies of the populations of stars within a galaxy provide a fossil record that traces the history of star formation over the whole course of the galaxy’s evolution. Such studies also trace the buildup of the heavy elements in the galaxy as successive generations of stars formed, converted their light elements into heavier ones, and then exploded, contributing their newly formed heavier elements to their surroundings. This observational approach is currently practical only in the Milky Way and its nearest neighbors. Future generations of optical telescopes in space and large ground-based telescopes will enable us to extend this technique farther afield and study the histories of the full range of galaxies by imaging their stellar populations.
In the past decade we have discovered two remarkable things about black holes. The first is that supermassive black holes—objects with masses of a million to billions of times the mass of the Sun—are found in the centers of all galaxies at least as massive as our Milky Way. This means that the formation of black holes is strongly related to the formation of galaxies. The second is that supermassive black holes were already present, and growing rapidly, at a time less than a billion years after the big bang, when the first galaxies were being assembled. This strains our understanding of the early universe: How could such dense and massive objects have formed so rapidly? Which formed first, the black hole or the galaxy around it? Radio observations of star-forming molecular gas in some of the most distant