One of the most important and surprising scientific discoveries of the twentieth century is that the expansion of space is not slowing down, but speeding up—contrary to what we expect the gravitational pull of all the matter in the Universe to do. The driver of this accelerating expansion has been labeled "dark energy," but there is much about the phenomenon that researchers don’t understand. We now know that dark energy comprises the bulk of the energy density of the Universe (about 70 percent), but its existence poses major challenges to our basic understanding of fundamental forces at work in the cosmos. On the other hand, the incorporation of dark energy into the prevailing theory of cosmology has been enormously successful. For example, in earlier cosmological models, the Universe appeared to be younger than its oldest stars. When dark energy is included in the model, that problem goes away.

To untangle the complexities of modern observational cosmology, international communities of astronomers, astrophysicists, cosmologists, and experimental and theoretical particle physicists—many of them from KIPAC—have joined together in the effort to design increasingly precise instruments to make detailed measurements of the history of the expansion of space. The data gathered by these probes could then be used to devise and test theoretical explanations for the mechanisms underlying the ceaseless expansion of the Universe.

Dark energy is the dominant component in the Universe, yet there are currently no compelling explanations for its existence or its distribution. However, KIPAC scientists, along with many others in the astrophysics and cosmology community, are generating a suite of techniques and observational tools that will greatly enhance our understanding of this enigmatic phenomenon.

These techniques are based on several different observable phenomena throughout the Universe, including: the distribution of galaxies on very large scales; the density and distribution of galaxy clusters detected through X-rays, gravitational lensing, and distortions in the cosmic microwave background radiation; the apparent luminosity of Type Ia supernovae, which can be used for measuring vast distances across space; and the distortion of images of background galaxies due to the bending of light as it passes through the intervening dark matter. By applying these techniques to existing data sets and conducting computational studies based on cosmological simulations, researchers are gaining a more robust understanding of dark energy.

KIPAC scientists are actively involved in three highly sensitive ground-based optical telescope projects that will be able to more carefully probe the nature of dark energy than previous instruments. The first is the Dark Energy Survey (DES). This collaboration built a 570-megapixel camera mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory and performed a five-year survey from 2013-2018. KIPAC scientists are currently engaged in using these data to perform the most sensitive test of our cosmological model to date. The second is the Dark Energy Spectroscopic Instrument, which will be doing a survey of 35 million objects to make a 3-D map of the Universe. The third is the Vera Rubin Observatory, which is building the 3.2-gigapixel camera LSST camera to mount on a new telescope that will be sited on the El Peñón peak of Cerro Pachón, also in Chile. The camera construction is currently being completed at SLAC and data taking will start in the next few years.