The compelling science promise outlined in this report offers opportunities for making discoveries—both anticipated and unanticipated—for which the next decade will be remembered. The ingenuity and means are at hand to address the most promising and urgent scientific questions raised by the SFPs and summarized in Chapter 2, albeit on various timescales. The committee concluded that the way to optimize and consolidate the science return with the resources available is to focus on three broad science objectives for the decade—targets that capture the current excitement and scientific readiness of the field, and are motivated by the technical readiness of the instruments and telescopes required to pursue the science. These targets—Cosmic Dawn: Searching for the First Stars, Galaxies, and Black Holes; New Worlds: Seeking Nearby, Habitable Planets; and the Physics of the Universe: Understanding Scientific Principles—are the drivers of the priority rankings of new activities and programs identified below. However, they form only part of the much broader scientific agenda that is required for a healthy program.

Cosmic Dawn: Searching for the First Stars, Galaxies, and Black Holes

Astronomers are on the threshold of finding the root of our cosmic origins by revealing the very first objects to form in the history of the universe. This step will conclude a quest that is akin to that of an anthropologist in search of our most ancient human ancestors. The foundations for this breakthrough are already in place with the current construction of ALMA, which will detect the cold gas and the tiny grains of dust associated with the first large bursts of star formation, and JWST, which will provide unparalleled sensitivity to light emitted by the first galaxies and pinpoint the formation sites of the first stars. This powerful synergy between JWST and ALMA applies not only to these first objects in the universe, but also to the generations of stars that followed them. The emergence of the universe from its “dark ages,” before the first stars ignited, and the buildup of galaxies like our own from the first primordial seeds will be recorded. A staged development program is proposed beginning with the Hydrogen Epoch of Reionization Array-I (HERA-1) telescopes that are already under construction. The reionization of the primordial hydrogen by these first stars will be constrained by detections of cool gas from the dark ages with the first generation of HERA experiments. Much of what has already been learned has been informed by the results of theoretical investigations and sophisticated numerical simulations, and these are likely to play an increasingly important role in planning and interpreting future observations.

However, completing the record of galaxy formation, and understanding the composition and nature of these faint distant early galaxies, will require a new generation of large ground-based telescopes. A number of activities proposed to

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