The discovery that the expansion of space is accelerating was named by the journal Science as the 1998 “Breakthrough of the Year”. The cause of the unexpected acceleration is one of the most important scientific problems of our time. The implication that three quarters of the mass-energy in the Universe is due to an unknown entity, called “dark energy”, has revolutionized cosmology and may drive a new understanding of physics when this phenomenon is fully understood through observations by WFIRST. The two top-level questions of the field are:
Three different types of surveys will be performed in order to answer these questions. (1) High Latitude Spectroscopic Survey: This survey will measure accurate redshifts and positions of a very large number of galaxies. By measuring the changes in the distribution of galaxies over cosmic time relative to the “standard ruler” that was calibrated with precision by NASA's WMAP mission, the time evolution of the dark energy can be determined. Additionally, the distortions in the distributions of galaxies in redshift space induced by galaxy peculiar velocities provides an approach to measuring the growth of large structure, testing Einstein's General Relativity. (2) Type Ia Supernovae (SNe) Survey: This survey uses type Ia SNe as “standard candles” to measure absolute distances. Patches of the sky are monitored to discover new supernovae and measure their light curves and spectra. Measuring the distance to and redshift of the SNe provides another means of measuring the time evolution of the dark energy, providing a cross-check with the galaxy redshift survey. (3) High Latitude Imaging Survey: This survey will measure the shapes and photometric redshifts of a very large number of galaxies and galaxy clusters. The shapes of very distant galaxies are distorted by the bending of light as it passes more nearby mass concentrations. These distortions are measured and used to infer the three-dimensional mass distribution in the Universe. The redshift measurements allow an alternative method of measuring “standard ruler” distances in galaxy cluster patterns. Thus, this survey will determine both the time evolution of the nature of the dark energy as well as another independent measurement of the growth of large structure.
WFIRST will be designed to perform precision measurements using the above 3 surveys. It will make an order of magnitude step forward in dark energy studies by combining these surveys and tightly controlling systematic uncertainties.
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The first discovery of planetary companions to Sun-like stars was, along with the discovery of dark energy, one of the greatest breakthroughs in modern astronomy. These discoveries have excited the astronomical community and the broader public as well. Since then, the pace of exoplanet discovery has increased each year. There are now nearly 1000 confirmed exoplanets and Kepler has identified thousands of candidates that await confirmation. Nature has surprised astronomers with the enormous and unexpected diversity of exoplanetary systems, containing planets with physical properties and orbital architectures that are radically different from our own Solar System. Since the very first discoveries, we have struggled to understand this diversity of exoplanets, and in particular how our solar system fits into this menagerie.
WFIRST will advance our understanding of exoplanets along two complementary fronts: the statistical approach of determining the demographics of exoplanetary systems over broad regions of parameter space by gravitational microlensing and the detailed approach of characterizing the properties of a sample of nearby exoplanets by means of high-contrast imaging and spectroscopy. These two complementary surveys will provide the most comprehensive view of the formation, evolution, and physical properties of planetary systems. In addition, information and experience gained from both surveys will lay the foundation for, and take the first steps toward, the discovery and characterization of a “pale blue dot ” — a habitable Earth-like planet orbiting a nearby star.
Exoplanets - Microlensing
More than 20 planets have been discovered from the ground using this technique. The WFIRST microlensing survey will detect many more such planets, including smaller mass planets since the planet "spike" will be far more likely to be observed from a space-based platform. This will lead to a statistical census of exoplanets with masses greater than a tenth of the Earth's mass from the outer habitable zone out to free floating planets. The results from the WFIRST microlensing survey will complement the exoplanet statistics from Kepler, and will provide answers to questions about planet formation, evolution, and the prevalence of planets in the galaxy.
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Exoplanets - Direct Imaging
Our understanding of the internal structure, atmospheres, and evolution of planets was originally developed through models that were tuned to explain the detailed properties of the planets in our own solar system. Surveys of exoplanetary systems have led to the realization that there exists a diversity of worlds with very different properties and environments than those in our solar system. Subsequently, these models have had to be expanded and generalized to explain the properties of these new worlds, often including new and uncertain physics. Our understanding of these new worlds therefore remains primitive. The best hope of understanding the physical properties of this diversity of worlds is through comparative planetology: detailed measurements of, and comparisons among, the properties of individual planets and their atmospheres. These measurements provide the primary empirical constraints on our models. Understanding the structure, atmospheres, and evolution of a diverse set of exoplanets is also an important step in the larger goal of assessing the habitability of Earth-like planets discovered in the habitable zones of nearby stars. It is unlikely that any such planets will have exactly the same size, mass, or atmosphere as our own Earth. A large sample of characterized systems with a range of properties will be necessary to understand which properties permit habitability and to properly interpret these discoveries.
High-contrast, high-angular-resolution direct imaging provides the critical approach to studying the detailed properties of exoplanets. Images and spectra of directly imaged planets provide some of the most powerful diagnostic information about the structure, composition, and physics of planetary atmospheres, which in turn can provide constraints on the origin and evolution of these systems. The direct imaging technique is also naturally applicable to the nearest and brightest, and thus best-characterized, solar systems. High contrast imaging is also ideally suited to studying the diversity and properties of debris disks around the nearest stars; these disks serve as both fossil records of planet formation, and signposts of extant planets through their dynamical influences.
Advancing the technology for direct imaging of exoplanets was the top priority medium-scale space investment recommended by NWNH. Developing a coronagraph with active wavefront control for AFTA-WFIRST accomplishes this objective and, thanks to the 2.4-m telescope, achieves far more real science than would be possible on a technology demonstration mission with a much smaller aperture. Coronagraphy on AFTA-WFIRST will be a major step towards the long-term goal of a mission that can image habitable Earth-mass planets around nearby stars and measure their spectra for signs of life.
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WFIRST will conduct large-area near infrared imaging and spectroscopic surveys over multiple epochs to enable scientific investigations that touch upon virtually every class of astronomical object, environment and distance. The Guest Investigator program supports archival studies to address a broad range of astrophysical research questions using data acquired by the dark energy and exoplanet surveys.
Sensitive near-infrared imaging and spectroscopy over large areas by WFIRST will probe the star formation history of the Universe and the evolution of active galactic nuclei, and will trace the large scale structure and clustering properties of galaxies at z>1. Deep, Wide Field imaging with WFIRST will also determine whether the fluctuations in the infrared background surface brightness that have been observed by Spitzer arise from faint galaxies at z~2 or if they are produced by first-light galaxies containing Population III stars near the epoch of reionization.
WFIRST will answer fundamental questions about the efficiency and mechanisms of formation of low-mass stars, brown dwarfs, and planets in the Milky Way. These most common constituents of the Universe will be identified by their unique infrared colors, by their proper motion acquired through multi-epoch WFIRST surveys and comparison with earlier surveys such as WISE and UKIDSS, and by their spectral signatures from the dominant absorption bands of H2O and CH4.
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WFIRST will offer a Guest Observer program that supports community-based observing programs. While the baseline mission emphasizes the dark-energy and exoplanet measurements, the additional surveys carried out via the guest observer program will exploit WFIRST's unique capabilities to substantially broaden the science return of the mission. The Guest Observer program will provide broad support to many fields of astrophysics in the tradition of HST, no doubt with the same astonishing results of new, creative, field-changing science. In an extended mission, the Guest Observer program would likely become the dominant part of the WFIRST mission. HST has demonstrated clearly that the combination of a powerful facility and peer-reviewed proposals has the greatest impact in advancing the extraordinarily broad field of astrophysics research.
Appendix A of the WFIRST SDT (2013) report contains ~ 50 potential GO or Guest Investigator science programs, each described in one page, contributed by members of the broader astronomical community. Here we simply list their titles to illustrate the remarkable range of ideas for scientific investigations enabled by WFIRST: