Once upon a time—up until the late 1990s, in fact--we were pretty certain we knew one key thing about the expansion of the Universe: that the gravity of celestial objects was certain to slow the expansion as time went on. Then, around 1998, observations of very distant supernovae from the Hubble Space Telescope (HST), which were intended to be used by scientists to measure the rate of deceleration, instead showed that long ago the Universe was expanding more slowly than it is today. So not only was the expansion of the Universe not slowing down due to gravity, as everyone thought, it actually is accelerating.
After much theoretical study, which included looking at the possibility there was something wrong with Einstein's theory of gravity, astrophysicists decided maybe there was some new, strange kind of energy that filled space and could be blamed for this cosmic acceleration. They gave their solution a name: Dark energy, using the adjective “dark” to describe the invisible nature of the theoretical property that is giving the Universe the energy to expand.
It turns out that most of the Universe is made of stuff that researchers are pretty sure exists but have not been able to see or directly measure. Looking at how mystery ingredients affect the Universe's expansion theorists have calculated that roughly 68% of the Universe is dark energy and 27% is dark matter (clouds of matter we cannot see that distort and magnify light from distant galaxies). That leaves about 5% for normal matter, which we can see.
To find out the nature of the dark stuff scientists want to obtain wide field-of-view images and spectroscopic surveys of the near infrared (NIR) sky. To do so NASA has come up with WFIRST-AFTA (deep breath now: Wide Field Infrared Survey Telescope- Astrophysics Focused Telescope Assets) a project selected by the National Research Council committee as the top priority for the next decade of astronomy. WFIRST-AFTA will address many of the most profound questions in astrophysics and is expected to be a key part of NASA’s mission portfolio for launch by 2024.
The current design of the mission makes use of an existing 2.4m telescope to enhance sensitivity and imaging performance. With the 2.4m telescope, a coronagraph instrument has been added to the payload for direct imaging of exoplanets (planets that orbit a star other than the Sun) and what are called debris disks (disks of dust and other matter in orbit around a star).
WFIRST-AFTA will use three independent techniques to probe dark energy:
Baryon acoustic oscillations (BAO), which are regular, periodic fluctuations in the density of the visible (baryonic) matter of the universe. Baryonic matter is the ordinary stuff made up of protons, neutrons and electronics bundled together into atoms.
Observing distant supernovae, the explosion of star, during which the star's luminosity increases by as much as 20 magnitudes and most of the star's mass is blown away at very high velocity. These explosions can be used as cosmic distance indicators. Basically, the further away an object is located in space, the further back in time it is, allowing researchers to measure cosmic expansion throughout the history of the universe.
Weak gravitational lensing. Strong gravitational lensing (distortion) in the presence of a mass is the large scale bending of the path of light passing near an object. Most lines of sight in the universe, however, are in the weak lensing regime, in which the deflection is impossible to detect for a single background source such as a galaxy. Which is to say we don’t have the necessary alignment between a foreground mass and a background galaxy. However, using statistical measurements to determine the masses of astronomical objects without requiring assumptions about their composition or state, scientists can, we are told, provide a way to map the distribution of dark matter around galaxies and clusters of galaxies (it is estimated that approximately 80% of cluster content is in the form of dark matter).
The Federal Government’s FY 2015 budget provides $56M for WFIRST; the President’s current FY ’16 budget request provides $36 million less than last year's appropriation but most observers expect the final appropriated amount for FY 2016 to end up close to or above last year's appropriation. Behind this optimism is the fact that over the past two years significant WFIRST-AFTA funding was added to the NASA budget by Congress for FY14 and FY15 for a total of $106.5M.