The Wide-Field Infrared Survey Telescope (WFIRST) is a NASA mission responding to the 2010 National Research Council (NRC) New Worlds, New Horizons (NWNH) Astronomy and Astrophysics Decadal Survey top priority recommendation in the large space mission category. WFIRST includes science objectives in exoplanet exploration, dark energy research and galactic and extragalactic surveys.
The NWNH report noted that, "WFIRST will settle fundamental questions about the nature of dark energy, the discovery of which was one of the greatest achievements of U.S. telescopes in recent years. It will employ three distinct techniques - measurements of weak gravitational lensing, supernova distances, and baryon acoustic oscillations - to determine the effect of dark energy on the evolution of the Universe. An equally important outcome will be to open up a new frontier of exoplanet studies by monitoring a large sample of stars in the central bulge of the Milky Way for changes in brightness due to microlensing by intervening solar systems. This census, combined with that made by the Kepler mission, will determine how common Earth-like planets are over a wide range of orbital parameters. It will also, in guest observer mode, survey our galaxy and other nearby galaxies to answer key questions about their formation and structure, and the data it obtains will provide fundamental constraints on how galaxies grow ... WFIRST addresses fundamental and pressing scientific questions and will contribute to a broad range of astrophysics. It complements the committee's proposed ground-based program in two key science areas: dark energy science and the study of exoplanets. It is a part of coordinated and synergistic programs in fields in which the United States has pioneered the progress to date. It presents opportunities for interagency and perhaps international collaboration that would tap complementary experience and skills. It also presents relatively low technical and cost risk, making its completion feasible within the decade, even in a constrained budgetary environment. For all these reasons it is the committee's top priority recommendation for a space mission."
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The discovery of over 500 exoplanets in systems that are mostly quite different from our own planetary system is inspiring observers and theoreticians to understand how these exoplanets were formed, how they evolve, and ultimately whether there are any Earth-like planets on which we might someday search for signs of life. The first exoplanets to be discovered were gas giants, but today it is becoming clear that there are probably many more "small" planets, in the Earth to Super-Earth range, than there are giants. Discovering the statistics of these planets is crucial for understanding how they formed and how common Earth-like planets may be. Gravitational microlensing is an observational effect that was predicted in 1936 by Einstein using his General Theory of Relativity. When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the gravitational "attraction" of the foreground star. This star is then a virtual magnifying glass, amplifying the brightness of the background source star, so we refer to the foreground star as the lens star. If the lens star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source. Thus we discover the presence of each exoplanet, and measure its mass and separation from its star. Planets the size of Mars or smaller can be detected. This technique will tell us how common Earth- like planets are, and will guide the design of future exoplanet imaging missions.
More than 10 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 at distances at 0.5 AU and beyond. 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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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 objective of the WFIRST dark energy survey is to determine the nature of dark energy in the Universe. Does the nature of the dark energy change with time? Does it require a modification to Einstein's laws of gravity (the General Theory of Relativity)? WFIRST seeks to answer these questions by measuring the expansion history and the growth rate of large-scale structure (the pattern of galaxies and galaxy clusters in the Universe). Three different types of measurements will be used. (1) Baryon Acoustic Oscillations (BAO): This technique is based on a "standard ruler" that was calibrated with precision by NASA's WMAP mission. By measuring the changes in the spatial distribution of galaxies over cosmic time relative to the standard ruler, the time evolution of the dark energy can be determined. This requires a spectroscopic survey to measure accurate redshifts and positions of a very large number of galaxies. (2) Type Ia Supernovae (SNe): This technique 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. (3) Weak Gravitational Lensing: This technique exploits the fact that images 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-dimension mass distribution in the Universe. The evolution of the mass distribution is then used to determine the time evolution of the nature of the dark energy. Additionally, both Redshift Space Distortions caused by from the galaxies measured in the BAO survey, and the distortions seen in the Weak Gravitational Lens survey put Einstein's General Relativity to the test.
WFIRST will be designed to perform precision measurements using the above 3 techniques. It will make an order of magnitude step forward in dark energy studies by combining these techniques and tightly controlling systematic uncertainties.
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WFIRST will conduct large-area infrared imaging and spectroscopic surveys over multiple epochs to enable scientific investigations that touch upon virtually every class of astronomical object, environment and distance. These investigations will exploit archival data acquired by the dark energy and microlensing surveys, and dedicated surveys proposed by the community through the WFIRST Guest Observer program.
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.
Predecessor infrared space missions such as NASA's Spitzer Space Telescope and WISE have discovered thousands of stars in nearby star formation regions that exhibit infrared emission in excess of what is expected from their photospheres, indicating the presence of surrounding structures of gas and dust. WFIRST imaging will determine the sizes and morphology of these structures that may evolve into planetary systems. WFIRST imaging will elucidate the structure of the Milky Way by identifying distant red clump giant stars on the far side of the Galactic disk that trace the location of spiral arms, and will simultaneously reveal the amount of gas and dust along lines of sight towards most stars in the Galaxy.
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.
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WFIRST will offer a Guest Observer program that supports both community-based observing programs as well as archival studies to address a broad range of astrophysical research questions. While the baseline mission emphasizes the planet census and dark-energy measurements, the additional surveys carried out via the guest observer program can exploit WFIRST's unique capabilities to substantially broaden the science return of the mission.
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