The history, geometry, and ultimate fate of the Universe are determined by its composition. Our current understanding is that the Universe is 68% Dark Energy, 27% Dark Matter, and almost 5% Hydrogen and Helium. Everything else (including us!) accounts for less than 1%.

Revolutionary observations in the past two decades have shown that the expansion of the Universe is accelerating. Dark Energy is the proposed explanation for this acceleration. The nature of dark energy, however, remains a mystery. Alternatively, the acceleration may be evidence of a breakdown in Einstein's theory of gravity on cosmic scales. In either case, the cause of the observed cosmic acceleration is one of the most fundamental questions in physics today.

Roman Space Telescope's Primary Dark Energy Science Objective is to determine the expansion history of the Universe and the growth history of its large-scale structure in order to test possible explanations of its apparent accelerating expansion including Dark Energy and modifications to Einstein's gravity (see the final report of the WFIRST-AFTA Science Definition Team). More accurately, the Roman Space Telescope will measure the equation of state of dark energy and its time evolution, helping determine whether it is a cosmological constant. This goal will be achieved through a combination of the dedicated wide-area, multicolor, high latitude survey, which will provide four band (Y, J, H, F184) imaging and low-resolution (grism) spectroscopy (1.00 - 1.93 μm) over 2000 square degrees, measuring millions of redshifts for galaxies between z=1.1 and 2.8, along with a three-tiered High Latitude Time Domain Survey aimed at detecting Type Ia supernovae. Using these data Roman Space Telescope will measure the expansion history of the Universe and the growth of large-scale structure (the clustering of galaxies and their associated halos of dark matter in the Universe) by measuring precise distances and matter clustering using three complementary techniques:

• Supernova Ia (SNe): use the peak luminosity of Type Ia supernovae as "standardizable candles" to measure the cosmic expansion history.
• Weak Gravitational Lensing (WL): determine the expansion history and the growth of structure simultaneously, by measuring the distribution of dark matter structures through their effect on the light from distant galaxies.
• Baryon Acoustic Oscillations (BAO): use the imprint of primordial sound waves on the clustering of galaxies as a "standard ruler" to measure the expansion history. The same data used to examine BAO will tell us further information about the growth of structure via measurements of the local velocities of galaxies, a phenomenon referred to as Redshift Space Distortions (RSD).

Roman is expected to provide the most powerful measurements of supernovae, weak lensing and large-scale maps of structure, especially at the redshift range between 1 and 2, per unit time, in the foreseeable future.