1 00:00:01,070 --> 00:00:05,070 [Music throughout] What makes up most of the cosmos? 2 00:00:05,070 --> 00:00:09,070 Not stars or planets, or even atoms. It's something scientists call 3 00:00:09,070 --> 00:00:13,070 dark energy. And so far, no one has a good handle on what it actually is. 4 00:00:13,070 --> 00:00:17,070 Dark energy, first discovered in 1998, 5 00:00:17,070 --> 00:00:21,070 is an enigmatic pressure pushing the universe apart at an ever-faster clip. 6 00:00:21,070 --> 00:00:25,070 Scientists suspect it began flexing its muscles around five billion 7 00:00:25,070 --> 00:00:29,070 years ago -- beyond that, we know very little. 8 00:00:29,070 --> 00:00:33,070 Learning more about dark energy is one of the primary reasons NASA is 9 00:00:33,070 --> 00:00:37,070 building WFIRST, a new space telescope whose measurements will help us home 10 00:00:37,070 --> 00:00:41,070 in on this mysterious cosmic component. Without a better understanding of 11 00:00:41,070 --> 00:00:45,070 dark energy, our knowledge of the past and future evolution of the universe is 12 00:00:45,070 --> 00:00:49,070 incomplete. WFIRST will tackle the dark energy problem using 13 00:00:49,070 --> 00:00:53,070 different yet complementary wide-field surveys. A key aspect of them 14 00:00:53,070 --> 00:00:57,070 is a measurement called "redshift." Because space itself is 15 00:00:57,070 --> 00:01:01,070 expanding, the farther we look, the faster galaxies are moving away 16 00:01:01,070 --> 00:01:05,070 from us. This results in a measurable shift in an object's light toward 17 00:01:05,070 --> 00:01:09,070 redder colors. This redshift indicates how fast the expanding 18 00:01:09,070 --> 00:01:13,070 universe is carrying galaxies away from us. If we can also figure out a 19 00:01:13,070 --> 00:01:17,070 galaxy's distance by other methods, we can use both pieces of information 20 00:01:17,070 --> 00:01:21,070 to measure how the universe expanded while the galaxy's light was traveling to us. 21 00:01:21,070 --> 00:01:25,070 WFIRST will map out the positions and distances of 22 00:01:25,070 --> 00:01:29,070 millions of galaxies. This will allow astronomers to see how the distribution of 23 00:01:29,070 --> 00:01:33,070 galaxies has changed, revealing how dark energy has evolved over cosmic time. 24 00:01:33,070 --> 00:01:37,070 An alternative way to measure dark energy is by 25 00:01:37,070 --> 00:01:41,070 using exploding stars called type Ia supernovas. These blasts 26 00:01:41,070 --> 00:01:45,000 are caused by the total destruction of a white dwarf star and each one 27 00:01:45,000 --> 00:01:49,070 emits similar amounts of light. But the farther away they are, the fainter the explosions 28 00:01:49,070 --> 00:01:53,070 look. By measuring how bright type Ia supernovas appear 29 00:01:53,070 --> 00:01:57,000 to be, we have a way to measure their distances. 30 00:01:57,000 --> 00:02:01,070 It was comparing supernovae redshifts to their apparent 31 00:02:01,070 --> 00:02:05,070 brightness that astronomers discovered dark energy. These studies showed that 32 00:02:05,070 --> 00:02:09,070 explosions at greater redshifts were dimmer than they should be in any model 33 00:02:09,070 --> 00:02:13,070 where the expansion of the universe was not speeding up. 34 00:02:13,070 --> 00:02:17,070 WFIRST will study thousands of explosions reaching to even greater distances 35 00:02:17,070 --> 00:02:21,070 to measure dark energy's influence over time. 36 00:02:21,070 --> 00:02:25,070 A quirk of the early universe provides another way to pin down 37 00:02:25,070 --> 00:02:29,070 dark energy. In its first half-million years, the universe consisted of a 38 00:02:29,070 --> 00:02:33,070 hot, dense expanding fluid. Small density changes in the fluid 39 00:02:33,070 --> 00:02:37,070 excited sound waves that traveled throughout it. Although the waves, called 40 00:02:37,070 --> 00:02:41,070 baryonic acoustic oscillations, eventually ceased, astronomers 41 00:02:41,070 --> 00:02:45,070 have observed their faint imprint in the way that galaxies cluster together. This 42 00:02:45,070 --> 00:02:49,070 provides another way to measure galaxy distances. WFIRST will measure 43 00:02:49,070 --> 00:02:53,070 how this imprint changes through cosmic history, allowing astronomers to map 44 00:02:53,070 --> 00:02:57,070 the expansion of the universe in more detail and probe dark energy's effects over 45 00:02:57,070 --> 00:03:01,070 time. With each technique cross-checking the other, 46 00:03:01,070 --> 00:03:05,070 WFIRST's surveys will peer deeply into dark energy, providing important data 47 00:03:05,070 --> 00:03:09,070 to help scientists figure out what, exactly, it is, and how it 48 00:03:09,070 --> 00:03:13,070 will determine the ultimate fate of the universe. 49 00:03:13,070 --> 00:03:19,210 [Explore: Solar System & Beyond] 50 00:03:19,210 --> 00:03:19,211 [NASA]