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Central to the Galaxies across Cosmic Time panel are the following questions: How do galaxies
form and result in the rich variety of phenomena that they exhibit How does intergalactic gas flow into
galaxies and eventually form stars Does feedback from the massive black holes in the centers of
galaxies (Figure 5-4) quench galaxy formation, and how is this process related to the high-velocity
outflows of gas that are seen around galaxies, and the vast bubbles of hot gas found in clusters of
galaxies What explains the number of stars in a galaxy What explains the sizes and chemical
compositions of galaxies, and whether they form their stars early or late in the history of the universe As
discussed below, analytic theory and computational modeling will take a central role in addressing these
questions.
Supernovae are the most energetic explosions in the universe since the Big Bang and the furnaces
in which most of the chemical elements from which we are made are forged. These spectacular cosmic
events are visible from halfway across the universe and provide some of the strongest evidence that the
universe is accelerating. As pointed out by the committee’s Stars and Stellar Evolution panel,
understanding why and how stars explode as supernovae demands three-dimensional computations
similar to those used to study fuel efficiency in cars and the design of new rockets but in far more exotic
and challenging conditions (Figure 5-5).
Finally, understanding planet formation, an issue central to the committee’s Planet and Star
Formation panel, is one of the most challenging tasks in astrophysics. A comprehensive theory of planet
formation requires following the growth of dust grains in the protoplanetary disk into small rocky bodies,
the growth of these bodies into planets, and the subsequent acquisition of oceans and atmospheres---a
study spanning some 42 orders of magnitude in mass and a vast array of processes ranging from the
sticking properties of dust grains, through the dynamics of bodies in shearing gas flows, to gravitational
stability of planetary orbits on billion-year timescales.
FIGURE 5‐3 Two views of dark matter distribution. (Credit: Edmund Bertschinger, MIT.)
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