Cosmologists have created a highly successful model of the universe, in which the cosmic energy budget is dominated by a cosmological constant Λ and cold dark matter. Perhaps more remarkable, observations have enabled the fundamental parameters of the ΛCDM model to be determined with extremely high precision. However, crucial gaps remain in our understanding: We have yet to figure out how and when the first stars and galaxies formed, or how they affected subsequent cosmic history1 (see also the article by Tom Abel in PHYSICS TODAY, April 2011, page 51).
In particular, the complex formation history of our own galaxy is unknown. Observations reveal a galaxy full of stars, gas, and intricate substructures. Surrounding it all is a spherical halo largely composed, presumably, of dark matter. Inhabiting the outer parts of the Milky Way are a variety of star streams, remnants of accretions of smaller “dwarf” galaxies. The...
