The small protogalactic fragments containing the first stars were embedded in halos of dark matter, which formed first and provided most of the total mass. Through their mutual gravitational attraction, these small fragments of gas and dark matter would have fallen slowly toward other such objects, collided, and then merged into larger objects. This process continued over the entire history of the universe: in the densest regions, small objects merged to form medium-size objects that later merged to form large objects (Figure 2.7). Over time even larger
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.
The Origin of Black Holes
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
But we cannot answer these questions definitively yet, because we do not have a robust theory for how supermassive black holes form. In the coming decade we expect a major breakthrough in our understanding. A space-based observatory to detect gravitational radiation will allow us to measure the rate at which mergers between less-massive black holes contributed to the formation process. Are the supermassive black holes we can now detect only the tip of the iceberg (the biggest members of a vast unseen population)? Deep imaging surveys in the near-infrared and X-ray, with follow-up spectroscopy with JWST and ground-based extremely large telescopes, will detect and study the growth of the less massive objects through the capture of gas and accompanying emission of electromagnetic radiation. These surveys will also allow us to search for such black holes at even earlier eras: back to the end of the dark ages.