Research at the Cosmic Frontier has revealed a universe far different from what was previously thought even as little as 15 years ago. Studies of Type 1a supernovae reveal that rather than slowing down due to the far-reaching effects of gravity, the universe is expanding at a faster rate. What’s driving this acceleration is something physicists have called dark energy, dark because what it precisely is remains unknown. Is it Einstein’s cosmological constant? Or is it something else? Then there’s dark matter. Strong empirical data from the rotational spectra of galaxies indicate that ordinary matter—the matter that we thought comprised the entire universe—is not the only matter in the universe. Dark matter is now thought to be approximately five times as abundant as ordinary matter. What dark matter is remains unknown and various types of particles have been proposed. If Copernicus dislodged us from the center of the universe, then recent results from the Cosmic Frontier indicate that we must once again step aside.
Unlike the Energy and Intensity Frontier research, the Cosmic Frontier gathers its data and reaches its conclusions from a variety of experiments and experimental techniques, none of which uses an accelerator or a reactor as a particle source. Cosmic Frontier detectors can be found deep underground in former gold mines, on mountain tops, and even on board the International Space Station. This wide array of experimental techniques suggests a wide array of physics topics. As such, the Cosmic Frontier takes the lead on the search for dark matter and dark energy, sources of the highest energy cosmic rays, and high-energy gamma rays, and neutrinoless double beta decay.
Physicists at work on the Cosmic Frontier ask themselves such questions as:
The search for answers to these questions involves a variety of experiments and detectors. These detectors may be cryogenic in nature, such as liquid xenon detectors used to search for direct signs of dark matter and for a rare (if it exists) double beta decay process. Observation of the latter may tell us much about what happened to antimatter that existed in the early Universe. Large arrays that cover many square kilometers on mountainsides are used to detect cosmic ray showers. These showers are initiated by energetic and extraterrestrial particles high in the earth’s atmosphere that collide with an air molecule, often nitrogen because it is most abundant. These showers are studied to understand what particle first set up the cascade, how much energy it initially had, and (hopefully) identify where it came from. Other detectors study the light from distant galaxies and stars, or to understand the large-scale structure of the universe. Knowledge of these will increase our understanding of how the rate of acceleration has changed over time.
We live at a defining moment in the history of physics. Thanks due in large part to accelerator-based experiments, physicists have produced an astoundingly successful model known as the Standard Model of Particle Physics. With the discovery of the Higgs boson, announced in 2012, one of the last missing pieces of the model, the piece that helps explain how fundamental particles gain mass, was added to the roster. But as HEP’s accelerator-based Energy Frontier research program explored the world on the terascale, the Cosmic Frontier opened up whole new areas of research on the cosmological scale. These two frontiers must come full circle, just like Newton and his apple. When that apple hit him on the head-an apocryphal story-- he came full circle and realized that that same force that caused the apple to fall on earth was the same force that kept the Moon in its orbit. The physics that drives the universe on the cosmological scale must be the same physics on all scales.
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