Leung Center for Cosmology and Particle Astrophysics,
National Taiwan University
DEPARTMENT OF PHYSICS AND LEUNG CENTER FOR COSMOLOGY
AND PARTICLE ASTROPHYSICS, NATIONAL TAIWAN UNIVERSITY
Fig. 1: Chee-Chun Leung Cosmology Hall, the permanent home of LeCosPA, on the NTU main campus, is due to be completed in mid 2017.
During the past two decades there have been dramatic breakthroughs in cosmology, which have revolutionized our understanding of the Universe. After the tremendous progress in physics in the last century, we now recognize that we only understand no more than 5% of the substance in the Universe. The remaining 95% of it is believed to be made of "dark matter" and "dark energy", whose respective natures are still to be determined. In the July 1, 2005 Special Issue to celebrate Science Magazine's 125th anniversary, the journal listed 125 of the most compelling puzzles and questions facing scientists today. The number one question was: "What Is the Universe Made Of?" The importance of cosmology in the 21st century requires little persuasion, and in recent years many leading universities around the world have established institutions dedicated to research in this area.
A BRIEF HISTORY
In order to promote the advancement of this field in Taiwan, Mr. Chee-Chun Leung donated NT$205 million (US $7M) to National Taiwan University in November 2007 to found the Leung Center for Cosmology and Particle Astrophysics (LeCosPA). Mr. Leung, who received his Bachelor of Science in Physics from NTU, is the co-founder and vice president of Taiwan-based Quanta Computers Inc. Quanta is one of the world's largest producers of laptop computers and is, according to Forbes Magazine, among the world's top 500 companies. Leung's philanthropy was partially triggered by his college classmate, Pisin Chen of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University, who joined the faculty of NTU in 2007. Chen has since been appointed as the inaugural director of LeCosPA and the NTU C.C. Leung Chair Professor of Cosmology.
Established on November 13, 2007, LeCosPA aspires to become an international center for cosmology and particle astrophysics through domestic and international collaborations and exchanges. Its fellows are recruited from leading institutions in Taiwan and abroad. Its merit-based Distinguished Junior Fellows Program, which provides a competitive salary, eagerly invites international applications. In 2012, Mr. Leung pledged an additional US $25M (thus making his total donation equivalent to US $32M) for LeCosPA to become a permanent institution at NTU, which involved the construction of the 10,000m2 Chee-Chun Leung Cosmology Hall to become LeCosPA's permanent home (see Fig. 1). After three years of preparation, the construction of the building began in June 2015. It is due to complete in mid 2017, in time to celebrate LeCosPA's 10th anniversary.
TWO NEW CLOUDS ON THE HORIZON
At the turn of the last century in 1900, Lord Kelvin famously proclaimed that physics was over, except for two small clouds on the horizon. Namely, the blackbody spectrum could not be explained by Maxwell's theory of electro-magnetism, and the aether that was supposed to support the propagation of light was not found. We all know that these two small clouds eventually developed into two huge storms in the history of physics: quantum theory and relativity.
History seems to be repeating itself. At the turn of the 21th century, there emerged two new dark clouds. One is the drastic discrepancy between quantum vacuum energy, which is a natural microscopic explanation for Einstein's cosmological constant, and the observed value of dark energy deduced from the dramatic discovery in 1998 of the accelerating expansion of the universe. Another figurative cloud is the seeming loss of information as a result of black hole Hawking evaporation. These two new clouds have one thing in common. They both point to the conflict between the two fundamental pillars of modern physics: quantum field theory and general relativity. What could be the resolution? Would that require a new stormy revolution?
Arising from this historical perspective, the Theory Group of LeCosPA attempts to shed some light on these large problems. It is a priori unclear whether the final solutions to these challenges would require a true revolution where the concept of space and time would be based on an entirely different new paradigm or if they would require only minor fixes within the existing framework. Such is the nature of pioneering research, and the LeCosPA members strive to explore all possible paths.
This, however, does not mean that the LeCosPA Theory Group concentrates only on these two topics. There is no lack of important issues in cosmology, gravitation, and particle astrophysics, such as the initial (big bang) and future singularities, inflation, dark matter, cosmic neutrinos, to name just a few, and these areas have been actively pursued by LeCosPA fellows and students.
The Experiment Group of LeCosPA is currently pursuing three directions in particle astrophysics, cosmology, and laboratory astrophysics using intense lasers. In particle astrophysics, their main attention has been on the search for the highest energy cosmogenic neutrinos and cosmic rays. These projects are carried out through the balloon-borne ANITA project in Antarctica, the ground-based ARA Observatory at the South Pole, and the TAROGE project in Taiwan and Antarctica. These projects are the backbone of LeCosPA's experimental activities since the founding of the center. In the area of cosmology, most recently LeCosPA is engaging in a new CMB B-mode initiative in Ali, Tibet. As for laboratory astrophysics, the major area of focus is on a new laser-plasma analog black hole experiment to investigate the information loss paradox.
Cosmic neutrinos can be subdivided into three categories according to their origin and energy, which are the cosmic neutrino background (CNB, or CnB), stellar neutrinos, and cosmogenic neutrinos. The cosmogenic neutrinos are the neutrinos produced by the collisions between ultra-high energy cosmic rays (UHECR) and CMB photons, also referred to as Greisen-Zatsepin-Kuzmin (GZK) neutrinos. The fact that UHECRs (mostly protons) with energies up to 1020 eV and CMB have been observed on Earth implies that GZK neutrinos with energies around 1017-1019 eV must exist with a sufficient flux based on known, standard model particle physics. However, so far these neutrinos have not been observed. LeCosPA has been an active member of both ANITA and ARA projects.
The ANITA experiment is based on the Askaryan effect. That is, it searches for the radiowaves emitted from the cosmic neutrino-induced showers in ice in the frequency range of 200–1,000MHz. Using NASA's long-duration balloons, ANITA flies at 35-40km altitude above the Antarctic ice, carried by the circumpolar winds that orbit around the pole once every 10-15 days, so that the entire field of ice within ANITA's horizon can be a vast target for neutrino detection from all azimuth directions, which provides > 1 million km2 of an instant field of view (see Fig. 2). The average flight time is about 1 month using NASA Long-duration balloons, but can be extended up to 3 months with ultra-long-duration balloons in the future.
Fig. 2: Schematic diagram for the concept of ANITA.
Since 2006, ANITA has launched four missions with successive and sizable upgrades in sensitivity each time, where the latest, the ANITA-VI, has just successfully completed its 27-day mission in December 2016. The first three flights have already generated significant results, including the discovery from ANITA-I data of ultra-high energy cosmic rays through their geomagnetic field induced synchrotron radiation by the electron-positron pairs in the air shower. This discovery demonstrated the powerfulness and cost-effectiveness of the radiowave detection approach to the study of ultra-high energy cosmic neutrinos and cosmic rays.
In addition to have set the best upper bound for the GZK neutrino flux, ANITA Collaboration has found 4 upward moving shower events through the combined analysis of data from ANITA-I, -II, and –III (see Fig. 3). While there is no strong evidence that these are earth-skimming neutrino events, the result is very encouraging. In both cases the LeCosPA Experiment Group Leader Jiwoo Nam has made key contributions as the first person in the collaboration to identify these events. With significant improvements in sensitivity, ANITA-VI is poised to discover the long sought-after GZK neutrinos.
Fig. 3: ANITA-III before launching in December 2015.
While the prospects for ANITA are good, there are also inherent drawbacks in the ANITA experiment for neutrino detection. The drawbacks include the limited observation time due to the duration of the balloon flight and the signal weakening during RF propagation. The signal weakening is caused by power loss through refraction on the ice-air boundary and the considerable decrement of power density by 1/R2 for long distances to the payload, which causes ANITA to be insensitive to neutrino energies below 1019eV, where the highest GZK-neutrino fluxes are expected.
One solution to the drawbacks of ANITA is to place radiowave antennas on the ground or shallowly underground. In 2009, LeCosPA Director Pisin Chen co-initiated the establishment of a ground-based neutrino observatory to be located at the South Pole. This collaboration now includes institutions from Israel, Japan, Taiwan, the UK, and the USA. Its grand vision looks to the eventual installation of 37 antenna stations, each of which consisting of 12 antennas buried 200m in ice, that would cover an area of 200km2 (see Fig.4). Up to now, three stations have been deployed. Two more stations will be deployed in 2017.
Supported by the Vanguard Program of MOST (Ministry of Science and Technology) in Taiwan, LeCosPA is responsible for the essential hardware of ARA-1 to ARA-8 antenna stations including Low Noise Amplifiers, DAQ boxes, and cables. These are produced or assembled in the LeCosPA laboratory. The final full-system integration and calibration for ARA-2 and ARA-3 were carried out in the anechoic chamber and the extreme-low temperature freezers in the LeCosPA laboratory as well (Fig. 5-6).
Fig. 4: The configuration of the ARA observatory with 37 stations, which will cover 200km2 at the South Pole.
Fig. 5: NTU LeCosPA members participated in the deployment of ARA antenna stations at the South Pole.
Fig. 6: ARA is Taiwan's first major science project at the South Pole.
In 2014, LeCosPA initiated a new earth-skimming cosmic neutrino and UHECR observatory called TAROGE (Taiwan Astroparticle Radiowave Observatory based on Geo-synchrotron Emissions) (see Fig. 7). It stands on top of a steep coastal mountain ridge overlooking the Pacific to search for the signals. In 2016, another new site was developed on the Ross Bay Shelf in Antarctica looking to Mt. Discovery for the same physics (see Fig.8).
Fig. 7: The conception of the TAROGE experiment on the East Coast of Taiwan.
Fig. 8: The ARIANNA-HCR prototype antenna successfully calibrated itself through the measurement of the ascending HiCal Pulser balloon trajectory in Dec. 2016.
TABLETOP ANALOG BLACK HOLES
Taking advantage of the dramatic advancements in laser technology, laboratory astrophysics is an interdisciplinary field of research that aims at cross-fertilization between astrophysics, plasma physics, and particle physics, to investigate astrophysical phenomena under extreme conditions. Fig. 9 shows the relationship of laboratory astrophysics and several closely related fields in physics. LeCosPA particularly seeks to use state-of-the-art ultra intense lasers to investigate Hawking evaporation and the information loss paradox.
Fig. 9: A three-ring diagram indicating the relationship between laboratory astrophysics and astrophysics, particle physics and plasma physics.
The idea that information could be lost through black hole Hawking evaporation has created a paradox in our current understanding of basic physics. The debate over whether information is really lost has persisted in the 40 years since Stephen Hawking discovered the problem. So far, investigations have been mostly theoretical, because of the difficulty of observing black holes in their later stages, when this potential contradiction is most acute (see Fig. 10). Recently Pisin Chen and Gerard Mourou conceived of a laboratory black hole to simulate this evaporation. Using state-of-the-art laser and nanofabrication technologies, "tabletop" black holes can be created to address whether information is truly lost when black holes evaporate (see Fig. 11).
Fig. 10: The worldline of an accelerating relativistic plasma mirror and its relation with vacuum fluctuations around the horizon.
An international collaboration was recently formed, which includes LeCosPA at NTU, IZEST at Ecole Polytechnique, Shanghai Jiao Tong University, and the Kansai Advanced Photon Research Center in Kyoto, to advance such an experiment.
Fig. 11: A schematic diagram for the proposed tabletop analog black hole experiment.
In addition to the theoretical and experimental projects highlighted above, the NTU LeCosPA Center works to foster an active and inspiring environment for academic excellence. Seminars, joint colloquiums, and tea times are held on a weekly basis. Mini-workshops on specific topics are organized from time to time, while large-scale LeCosPA symposium series are organized once every two to three years (see Fig. 12). Aside from these academic activities, there is no lack of social events. One warmly anticipated social activity is the annual LeCosPA Table Tennis Tournament between the Theory Group and Experiment Group (see Fig.13). These activities facilitate in the formation of strong bonds among LeCosPA members as well as international colleagues.
Fig. 12: A Santa Quartet (left to right): Pisin Chen, John Ellis, Bill Unruh, and Don Page, singing Christmas carols during the 2nd LeCosPA Symposium in December 2015.
Fig. 13: Annual LeCosPA Table Tennis Tournament between the Theory Group and the Experiment Group.
TEN YEARS TO SHARPEN A SWORD
By November this year, LeCosPA will celebrate its 10th anniversary. There is a famous saying, derived from a Tang Dynasty poem: "It takes ten years to sharpen a sword". In its first 10 years, LeCosPA strived to establish itself as an active international center for cosmology and particle astrophysics. We believe that our "sword" has by now been sharpened and we eagerly look forward to make bigger contributions to science in the next 10 years.
Chen, Pisin received his PhD in theoretical particle physics from the University of California at Los Angeles in 1983. He then worked at SLAC National Accelerator Laboratory. In 2000, he initiated the establishment of the now Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford. He joined the faculty of National Taiwan University Department of Physics, his alma mater, in 2007 and founded the Leung Center for Cosmology and Particle Astrophysics, where he has served as the director since its inauguration. He is known for having made contributions in plasma physics, beam physics, particle astrophysics, cosmology, and gravitation.