NASA is beginning to design a space telescope that will provide the largest picture of the universe ever seen with the same depth and clarity as the Hubble Space Telescope. Scheduled to launch in the mid-2020s, the Wide Field Infrared Survey Telescope (WFIRST) will function as Hubble's wide-eyed cousin. While just as sensitive as Hubble's cameras, WFIRST's 300-MP wide-field instrument will image a sky area 100× larger. A single WFIRST image will hold the equivalent detail of 100 pictures from Hubble. "A picture from Hubble is a nice poster on the wall, while a WFIRST image will cover the entire wall of your house," said David Spergel, co-chair of the WFIRST science working group and the Charles A. Young Professor of Astronomy at Princeton University in New Jersey. The mission's wide field of view will allow it to generate never-before-seen big pictures of the universe, which will help astronomers explore some of the greatest mysteries of the cosmos, including why the expansion of the universe seems to be accelerating. One possible explanation for this speed-up is dark energy, an unexplained pressure that currently makes up 68 percent of the total content of the cosmos and may have been changing over the history of the universe. Another possibility is that this apparent cosmic acceleration points to the breakdown of Einstein's general theory of relativity across large swaths of the universe. WFIRST will have the power to test both of these ideas. To learn more about dark energy, WFIRST will use its powerful 2.4-m mirror and wide-field instrument to map how matter is structured and distributed throughout the cosmos and measure how the universe has expanded over time. In the process, the mission will study galaxies across cosmic time, from the present back to when the universe was only half a billion years old, or about 4 percent of its current age. "To understand how the universe evolved from a hot, uniform gas into stars, planets and people, we need to study the beginnings of that process by looking at the early days of the universe," said WFIRST project scientist Jeffrey Kruk at NASA's Goddard Space Flight Center. "We've learned much from other wide-area surveys, but WFIRST's will be the most sensitive and give us our farthest look back in time." WFIRST will do this through multiple observational strategies, including surveys of exploding stars called supernovae and galaxy clusters, and mapping out the distribution of galaxies in three dimensions. Measuring the brightness and distances of supernovae provided the first evidence for the presence of dark energy. WFIRST will extend these studies to greater distances to measure how dark energy's influence increased over time. WFIRST will serve as an important tool for the science community through its General Observer and archival data analysis programs. All data will be publicly available immediately after processing and delivery to the archive. Also, by submitting proposals through the competitive program, scientists around the world will be able to use the observatory to study the cosmos in their own way, from the nearest exoplanets out to clusters of distant galaxies. The mission will complement other missions expected to operate in the next decade, notably the James Webb Space Telescope, scheduled to launch in 2019. Webb provides a detailed look at rare and interesting objects, while WFIRST will take a wide look at the universe. WFIRST will also complement new ground-based observatories such as the Large Synoptic Survey Telescope (LSST) currently in development. By combining data from WFIRST and LSST, scientists will be able to view the universe in nine different wavelengths, data that will provide the most detailed wide-angle view of the universe to date. By pioneering an array of innovative technologies, WFIRST will serve as a multipurpose mission, furnishing a big picture of the universe and helping us answer some of the most profound questions in astrophysics, such as how the universe evolved into what we see today, its ultimate fate and whether we are alone. "By building this telescope, we're enabling a wealth of science and the capability to address those kinds of questions," Spergel said. "It's deeply interesting not only to scientists, but anyone who looks up at the sky and wonders."