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