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Cosmology is in its golden age and is expected to be a growing field of science in the next couple of decades, considering the variety of surveys currently active and the long list of surveys funded and planned for observation of the universe at different wavelengths, scales, and depths in the near and far future. The fact that two Nobel prizes in physics (in 2006 and 2011) were awarded for work in cosmology in less than a decade probably reflects the current importance of cosmology.

While there have been many developments in cosmology, many puzzles remain to be solved. The nature of dark energy and dark matter, which together constitute more than 95% of the energy budget of the universe, are totally unknown. The initial conditions of the universe are also unknown, and we are not yet confident of the mechanisms and theories that explain how the universe began. To study the universe and more closely examine these mysterious components of nature, there have been extensive efforts to build bigger and more advanced telescopes functioning at different wavelengths to probe the universe. Soon we will have the fourth generation of astrophysical surveys performing from different corners of the Earth Dark Energy Spectroscopic Instrument (DESI), Giant Magellan Telescope (GMT), Thirty-Meter Telescope, Large Synoptic Survey Telescope (LSST) and from space [(Euclid, Wide-Field Infrared Survey Telescope (WFIRST), James Webb Space Telescope (JWST)] to probe deeper into the cosmos and shed light on these mysterious unknowns.

Korea Astronomy and Space Science Institute (KASI), considering its vibrant environment, strong infrastructure, and rich human resources, as well as its modern technical and observational facilities and instrumentation, would certainly allow us to participate as a pioneer in this international effort to understand the universe. KASI clearly has the potential to be involved in most major international collaborations (as it is already; for example, KASI is very much involved with GMT and DESI) and is also active on all fronts, from instrumentation design and fabrication to observation and also data analysis and high-level theoretical work. Korea is certainly moving in this direction, but there is still some work to do to catch up with other countries, because the field does not stand still, huge efforts and appropriate planning are necessary to achieve excellence in science and realize great results. Our strategic goal is to participate in this quest to obtain for Korea a larger share in global science production and major discoveries in astronomy and astrophysics. KASI is playing a crucial role in this direction, and we hope we can help it to perform its weighty and important duties. In our proposal, we aim to become a leading center in cosmology at the international level, to grow in various directions, and to prepare the environment for Korea to become a pioneer country worldwide in the field of cosmology.

The cosmology group at KASI (CosKASI) has joined DESI. The primary science goal of the DESI experiment is to clarify the nature of dark energy and/or gravity through the baryon acoustic oscillation and redshift-space distortion effects. Starting in 2018, it will obtain optical spectra for tens of millions of galaxies and quasars, constructing a three-dimensional map spanning the nearby universe out to 10 billion light years. DESI observations will be conducted on the Mayall 4-m telescope at Kitt Peak National Observatory. The observatory is supported by the Department of Energy’s Office of Science to perform this Stage IV dark energy measurement. Two members of CosKASI have participated in DESI since 2014 and collaborate on galaxy clustering and time streaming physics. CosKASI plans to sign a memorandum of agreement with the LSST in 2015. The LSST is a wide-field survey reflecting telescope that will photograph the entire available sky every few nights. It is currently in the design and mirror development phases. Site construction began in 2014, with engineering first light expected in 2019, science first light in 2021, and full operation for a 10-year survey commencing in January 2022. CosKASI members have joined the dark energy working group. DESI is the premier spectroscopic galaxy survey, and LSST is the premier imaging galaxy survey. The cross-correlation of both maps will resolve the systematics in each survey to enhance cosmological constraints and provide a unique window to test Einstein's gravity at the cosmological scale. In addition, both will guide the followup observations by the GMT as pathfinders to suitable targets.

With the facilities available at CosKASI, we will investigate key issues of the standard model of the Universe and to test fundamental physics at cosmological scales.

Initial conditions of the universe

beyond slow-roll single-field inflation : Although the assumed scale-invariant initial power spectra may be a generic prediction of the simplest scenarios of the generation of perturbations during inflation, initial spectra with radical deviations are known to arise from very reasonable extensions or refinements of the simplest scenarios. Consequently, cosmological parameter estimation from the cosmic microwave background (CMB) anisotropy and the matter power spectrum obtained from redshift surveys, weak gravitational lensing, and Ly-α absorption depends sensitively on the dimensionality, nature, and freedom in parameter space of the initial conditions. Understanding the initial conditions of the universe and the form of the primordial spectrum is a key issue in modern cosmology that can pave the way to understanding how our universe began. Measuring non-Gaussianity in the CMB fluctuations can add another dimension to studies of the early universe, as we can break the degeneracies between single field inflationary models and scenarios considering more than a single field.

Dark matter : Beyond the invisibility of darkness

Since Zwicky speculated that the Coma Cluster might contain a large amount of dark matter in the 1930s, the puzzle of its nature has become one of the foremost unresolved questions of particle physics and cosmology. The existence of dark matter can simply explain various phenomena involved in astrophysical and cosmological problems at different scales. As more dark matter properties have been discovered, it is no longer dark. CMB experiments yielded the precise relic density of dark matter, and the Bullet Cluster constrained the size of its self-interaction. The warmness of dark matter is further examined by the formation of small-scale structure such as the Ly-α forest. Complementary experiments involving direct and indirect detection of dark matter and collider experiments are also used to determine the visibility of dark matter.

Dark energy versus modified gravity : Beyond Einstein’s gravity

The discovery of cosmic acceleration decades ago~\cite{cosmic_acceleration} has forced cosmologists to modify their simple picture of the universe, i.e., a universe dominated only by matter and described solely by general relativity. Over the last 10 years, physicists have suggested two main avenues for explaining the late-time acceleration of the universe. The first is the introduction of dark energy, with an effective negative pressure, which dominates the late-time energy density of the universe, thus causing the acceleration. An alternative explanation is a modification of the law of gravity on large scales, which alters the predicted expansion history of the universe to be in line with the observations. With the level of precision available in future surveys, we can use the high-resolution maps yielded by next-generation surveys to test the foundations of gravity and particle physics. The premier spectroscopic survey, DESI, will provide information about the dynamic potential, and the premier imaging survey, LSST, will provide information about the lensing potential. CosKASI has a privileged position for testing the fundamental knowledge of gravity using both of these precise maps in the near future.

Instruments

  • Spectroscopic wide deep survey: Dark Energy Spectroscopy Instrument (DESI)
  • Photometric wide deep surveys: Large Synoptic Survey Telescope (LSST)
  • Giant Magellan Telescope (GMT)
  • Space-based wide deep surveys: SPHEREx, Euclid, Wide-Field Infrared Survey Telescope (WFIRST), James Webb Space Telescope (JWST)