Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration.
WFIRST will contribute to our understanding of the nature of dark energy by addressing two questions:
WFIRST will conduct 3 different types of surveys in order to answer these questions. WFIRST will make an order of magnitude step forward in dark energy studies by combining these surveys and minimizing uncertainties in measurement techniques.
The High Latitude Spectroscopic Survey will measure accurate distances and positions of a very large number of galaxies. By measuring the changes in the distribution of galaxies, the evolution of dark energy over time can be determined. The High Latitude Survey will measure the growth of large structure of the universe, testing theory of Einstein's General Relativity.
Type Ia Supernovae (SNe) Survey 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. Measuring the distance to and redshift of the SNe provides another means of measuring the evolution of dark energy over time, providing a cross-check with the high latitude surveys.
High Latitude Imaging Survey will measure the shapes and distances of a very large number of galaxies and galaxy clusters. The shapes 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-dimensional mass distribution in the Universe. This survey will determine both the evolution of dark energy over time as well as provide another independent measurement of the growth of large structure of the universe.