Recent development in string theory has led to the extension of General Relativity, i.e. Stringy Gravity. It postulates the entire closed string massless sector to be geometric and thus gravitational. I will first introduce the mathematical foundation and then discuss the solution to dark matter/energy problems. In terms of R/(MG), i.e. the dimensionless radial variable normalized by mass, Stringy Gravity agrees with General Relativity near infinity, but modifies it at short distance. At far short distance, gravitational force can be even repulsive. These may solve the dark matter and energy problems, as they essentially arise from small R/(MG) observations: far distance divided by much heavier mass.
온도를 낮추고자 하는 이유는 여러 가지가 있다. 미생물의 번식을 막기 위해서, 음식의 장기 보존을 위해서, 식품 가공을 위해서, 식물의 생장을 억제하기 위해서, 부피를 줄여서 보관하는 공간을 절약하기 위해서, 뻘과 같은 질척한 토질에서 굴착을 위한 토목공사를 용이하게 하기 위하여, 피부의 탄성을 좋게 하기 위하여, 물질의 강도를 높이기 위하여, 물질의 전기저항을 줄이기 위하여, 온도를 낮춤으로 나타나는 상변이, 양자현상 현상들을 보기 위하여, 전자기기 등의 잡음을 줄이기 위하여, 물질의 미세 진동을 줄이기 위하여 등등... 이러한 저온을 얻기 위한 기본적 방법은 열전달을 억제하기 위한 대류, 전도, 복사와 같은 기본 조건들을 제어함으로 얻어질 수 있다. 여기에는 물질의 열적 성질에 대한 지식과 온도를 낮춤으로 일어나는 물질의 수축, 그리고 구조적 변형 등 온도변화에 따른 제반 상태에 대한 이해가 필요하다.
국내에서는 초전도 기술의 발전과 응용에 의해 극저온 기술의 발전이 크게 진전하였다. 2000 년대에 들어서면서 대형 초전도자석이 핵심장비인 KSTAR 와 같은 대형 구조물의 극저온 냉각, MRI, 가속기용 초전도 자석 냉각, 고온초전도 선재를 이용한 전력 송전용 케이블의 냉각 등의 개발을 들 수 있으며, 이외에도 반도체 라인에서의 clean vacuum 을 위한 냉각기술, 식품가공 등을 위한 냉동기술, LNG 의 대량 수송을 위한 액화기술 등을 들 수 있다. 중력파 탐사 등 미세한 신호를 검출하기 위한 실험에서는 검출기 자체의 검출한계를 높이기 위한 노력과 더불어 검출기 및 실험장치에서의 잡음 감소와 검출 한도를 높이기 위하여, 초기에 상온에서의 검출 한계를 극복하기 위해 저온으로 냉각하기 시작하였으며, 극한적으로 양자잡음을 줄이기 위해 ~mK 영역에까지 도달하고자 노력하고 있다. 이와 관련한 냉각기술에 대하여 논의하고자 한다.
I will discuss the central dark matter distribution of dwarf galaxies in the local Universe within 10 Mpc derived using their resolved kinematics from Very Large Array (VLA) HI galaxy surveys in tandem with Spitzer 3.6 micron and optical observations. The degree of the central dark matter concentration of the galaxies quantified by measuring their inner density slopes indicates a mass distribution with a sizeable constant density-core towards the centres of the galaxies. This is in contrast to classical dark-matter-only cosmological simulations where cusp-like dark matter distribution is predicted in the halos. Instead, our results are more in line with shallower slopes found in Lambda CDM simulations of dwarf galaxies in which the effect of baryonic feedback processes is included. I will the talk with the discussion of science cases that I plan to carry out at KASI in order to timely prepare for the upcoming SKA era as well as best exploit the observing facilities currently accessible at KASI.
Very rapid (<1 minute), high amplitude (>100) variability at >1e40 erg/s is nearly unprecedented in our Universe. We have recently discovered a new class of X-ray point sources showing such variability in two nearby galaxies while analyzing archival Chandra and XMM-Newton data. One source is located within a suspected globular cluster of the host galaxy and flared one time, while the other source is located in either a globular cluster of the host galaxy or the core of a stripped dwarf companion galaxy that flared on five occasions over a seven year time span. When not flaring, the sources appear as normal accreting neutron star or black hole X-ray binaries, indicating that the flare event does not significantly disrupt the system. We speculate on the nature of these explosive, yet non-destructive objects.
I shall introduce three new techniques of magnetic field tracing. The first two use Doppler-shifted emission lines and employs the gradients of velocity in order to trace magnetic fields in the diffuse interstellar media as well as to trace regions of star formation associated with the gravitational collapse. The differences between these techniques is that they use different observationally available measures, i.e. the first one uses the velocity centroids and the other uses velocity channel maps. I shall provide the theoretical justification of the use of these measures, its numerical testing as well as the comparison of the directions obtained with the velocity centroid gradients using GALFA HI data and those of magnetic field as traced by Planck as well as 13CO data and far infrared polarimetry. I shall also discuss the third technique which employs the synchrotron intensity gradients that also trace magnetic field and, unlike synchrotron polarization, are insensitive to Faraday rotation. I shall also show its correspondence with the magnetic field tracing by Planck and discuss the synergy of using this technique with synchrotron polarization studies. I shall discuss the big promise of the new techniques both for the star formation and CMB foreground studies.
Detection of gravitational waves (GWs) from binary black holes (BHs) by Advanced LIGO has opened a new window of astronomical observation. Many conceivable sources such as intermediate-mass BH binaries and white dwarf binaries, as well as stellar-mass BH inspiral, would emit GWs below 10 Hz. It is highly desirable to open a new window in the infrasound frequency band below 10 Hz. A low-frequency tensor detector could be constructed by combining six magnetically levitated superconducting test masses. Seismic noise and Newtonian gravity noise are serious obstacles in constructing terrestrial GW detectors at low frequencies. A tensor detector can reject the near-field Newtonian gravity noise more efficiently. Such a detector is equally sensitive to GWs coming from anywhere in the sky, and is capable of resolving the source direction and wave polarization. I will present a design concept of a new low-frequency detector, named SOGRO, which could reach a strain sensitivity of 10-19-10-21 Hz-1/2 at 0.1-10 Hz. I will discuss ways to mitigate the seismic and Newtonian noise, as well as foreseeable technical challenges and limitations in developing such a detector.
Galaxy evolution is the grandest of all environmental sciences. Just how a galaxy forms and evolves in a given environment is one of the most pressing unanswered questions in astrophysics. This talk will describe plans to address this question through the construction of a new and unrivalled multi-object integral field unit (IFU) spectrograph for the 3.9m Anglo-Australian Telescope (AAT) called HECTOR. This instrument will make it possible to obtain IFU spectros of unprecedented quality for many tens of thousands of galaxies, that will make it possible to fully understand the physical basis for the diversity of galaxy properties. This will build on the heritage of the very successful but much smaller SAMI survey of ~3,000 galaxies currently being conducted on the AAT. As well as describe the science drivers and hence design requirements for HECTOR, I will also discuss the opportunity for Korea to partner with Australia in the construction of HECTOR and the realisation of its science, something the Australian astronomy community would warmly welcome.
It is widely believed that magnetic fields play a crucial role for the dynamics of molecular clouds and for the star formation processes. One of the most informative techniques of studying magnetic fields in molecular clouds and protostellar disks is based on the use of polarized emission arising from magnetically aligned dust. In this talk, I will talk about the Chandrasekhar-Fermi (CF) method, which is a simple and powerful technique to measure magnetic field strength from FIR/sub-mm polarization observations. I will demonstrate that the conventional CF method tends to overestimate the strength of the magnetic field and describe how to correct the tendency. When time permits, I’ll briefly talk about polarized emission from T Tauri disks.
Interstellar nanoparticles, including polycyclic aromatic hydrocarbons (PAHs), are believed to play an important role in modern astrophysics. Mid-infrared emission from PAHs is widely used as a tracer of star formation activity. PAHs is also thought to be a leading carrier behind the long-standing mystery of Diffuse Interstellar Bands (DIBs). In this talk, I will discuss new insights into the crucial importance of interstellar nanoparticles. I will start with a review on anomalous microwave emission (AME) by rapidly spinning nanoparticles, which is a critical challenge for early universe study via cosmic microwave background (CMB) radiation. Then, I will present a new way to tracing magnetic fields via polarized mid-IR emission from PAHs. Finally, I will discuss future perspectives to study interstellar nanoparticles through multiwavelength observations with Square Kilometer Array and ALMA Band 1.
The substorm is a dissipation process of the energy stored in the magnetotail, causing active auroras in the nightside polar regions. What processes in the magnetotail trigger the substorm is a major issue in magnetospheric research and has been extensively debated for decades. To understand the substorm triggering mechanism, I statistically studied substorm-associated evolution of the near-Earth magnetotail, using more than ten years of plasma, and magnetic and electric field data mainly from the Geotail spacecraft. I also analyzed data from the recent five THEMIS spacecraft. My results revealed the overall morphological picture of substorm-associated magnetotail evolution as well as energy release and transport and clarified that magnetic reconnection in the near-Earth magnetotail plays an essential role in substorm triggering.
While morphology has been the dominant property to describe and classify galaxies, spin is emerging newly as an alternative and perhaps more fundamental property of a galaxy. Integral field unit spectros revealed that the majority of elliptical galaxies do have a substantial rotational component unlike previous understanding. I present a new result from cosmological-volume hydrodynamic simulations and discuss the origin of spin of galaxies.
The Hyper-Kamiokande (Hyper-K) is a next generation experiment based in Japan succeeding the Super-Kamiokande (Super-K) experiment which achieved the 2015 Nobel prize. It will consists of two identical 260 kton water Cherenkov detectors, 20 times bigger than Super-K, to cover particle physics to astronomy.
The main goals are to solve important problems remaining in neutrino physics using J-PARC neutrino beam, and to detect Super Nova burst/relic neutrinos as well as to search for proton decay and dark matter. Hyper-K is indeed a multi-purpose experiment and telescope from precise measurements to new discoveries.
If one of the detectors is located in Korea instead of the two in Japan, then the physics sensitivities will improve. World-class discoveries are expected and Korea will play a critical role by co-leading the experiment. There are several good candidate sites in Korea to host a Hyper-K detector thanks to higher mountains to reduce spallation background and better quality of rocks to excavate than Japan.
In this talk I will focus on the Hyper-K as a neutrino telescope which will run for 30 years or more.
Initially stimulated by the predictions of high resolution LCDM simulations (e.g. Millennium and Aquarius) and then dramatically advanced by scrutinizing large digital imaging data sets such as SDSS, Pan-STARRS, and the Dark Energy Survey, our picture of the Milky Way and its satellite population has dramatically changed in the past decade. After an overview of our historical understanding of the Milky Way and its dwarf galaxies I will reflect over the tremendous progress in the emerging field of ultra-faint stellar systems, present the most fascinating results we were able to obtain and discuss the new mysteries waiting for us to be solved in future research.
This colloquium is partly supported by the BK21 Plus of Chungnam National University.
2016년 12월 31일 기준으로 인터스텔라의 누적 관객수는 10,304,503명으로 천만 관객 이상 영화로는 역대 15위입니다. 인터스텔라 흥행의 성공 요인은 천체물리학이나 양자역학 등 방대한 과학 주제들을 3시간에 걸쳐 뛰어난 visual effect로 보여 주었다는 것입니다. 영화 인터스텔라의 제작에서 킵손 박사와 영국의 Double Negative라는 시각효과 회사가 협업을 한 것처럼, 국내의 척박한 과학 콘텐츠 현실에서도 어려운 천체물리학을 computer graphics를 이용한 표현을 통해서, 대중에게 소개하는데 새로운 기회를 마련할 수 있을 것으로 예상해 봅니다. 웜홀, 중력렌즈, 빅뱅 등 이해하기 힘든 천체물리학의 주제들을 시각적으로 표현하는 것은 그 자체로도 의미가 있고, 이러한 노력은 학문의 분야에만 제한되지 않고 학계가 대중에게 천체물리학을 소개하는 과정에서 관심을 두어야 하는 부분입니다. 이번 강연에서는 천체물리학을 전공한 연구자가 Double Negative와 협업으로 성공적인 과학 콘테츠를 제작할 수 있었던 것처럼, LG엔시스가 virtual reality 천문우주 과학 콘테츠 제작에 대해서 가지고 있는 계획 등을 소개하도록 하겠습니다. LG엔시스에서는 가상현실 몰입감을 극대화 하고 대중화를 위한 방안으로 4D 모션체어 기반 HMD(Head-Mounted Display), 반구형 돔(Dome)을 제안하고 있습니다. 4D 모션체어 기반 HMD, 반구형 돔이 어떻게 천문우주 과학 콘텐츠 확산하는데 대안이 될 수 있는지 알아보도록 하겠습니다.