2016 Fall Seminar List

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Fall 2016 LeCosPA Cosmology and Particle Astrophysics Seminars

Location : Room 812, Astro-Math Building, NTU
Time : 11:00-12:00, Monday
Time for ASIAA/LeCosPA Joint Seminar : 14:20-15:20 on specific Thursdays
Organizers : Dr. Jakub Ripa & Dr. Dong-han Yeom

Date Name Title Affiliation File
Semester starts
Dr. Yen Chin Ong
When is holography consistent?
Holographic duality relates two radically different kinds of theory: one with gravity, one without. The very existence of such an equivalence imposes strong consistency conditions which are, in the nature of the case, hard to satisfy. Recently a particularly deep condition of this kind, relating the minimum of a probe brane action to a gravitational bulk action (in a Euclidean formulation), has been recognized; and the question arises as to the circumstances under which it, and its Lorentzian counterpart, are satisfied. We discuss the fact that there are physically interesting situations in which one or both versions might, in principle, not be satisfied. These arise in two distinct circumstances: first, when the bulk is not an Einstein manifold, and, second, in the presence of angular momentum. Focusing on the application of holography to the quark-gluon plasma (of the various forms arising in the early universe and in heavy-ion collisions), we find that these potential violations never actually occur. This suggests that the consistency condition is a "law of physics" expressing a particular aspect of holography.
Shanghai Jiao Tong University
Prof. Jinn-Ouk Gong
Features in the primordial spectra
In this presentation, I discuss how features in the primordial curvature perturbation may arise. Subsequently I present how we can compute the correlation functions of the primordial perturbation systematically, and argue the possible observational signatures and prospects.
Asia Pacific Center for Theoretical Physics
Dr. Shoichi Kawamoto
Long strings as small black holes
Highly excited fundamental strings are massive states and the semiclassical length is proportional to its mass; namely they are long strings. Due to self-interaction, such long strings eventually collapse to form a dense, compact object---precursor of black holes. Long time ago, Susskind conjectured that the entropy of black holes may be explained by a large entropy of long string states. Motivated by this observation, in this talk, I will discuss relations between long strings and black holes. First, I argue that the massless emission from long strings has a black body spectrum with small corrections, greybody factors. By use of Green-Schwarz superstring formulation, I calculate the greybody factors for open and closed strings and discuss physical interpretation. Secondly, I consider a gravitational collapse of long strings, by use of a method of polymer physics. A free long string configuration is given by a Gaussian random walk and occupies a large volume. By turning on interaction, the size starts to decrease, and I show that it exhibits scaling behavior with respect to the strength of interaction. This talk is based on a joint work with T. Matsuo, Phys.Rev.D87 and PTEP2015 123B02.
Chung Yuan Christian University
National holiday
Dr. Fang Ye
Axion-mixing-induced large field inflation
We explore the possibility of large field inflation driven by axions in the general multi-axion systems. We find that axion mixings from a non-diagonal kinetic metric on the moduli space and from the so-called Stueckelberg coupling to a U(1) gauge field can induce an effectively super-Planckian decay constant without the need of "alignment" in the individual axion decay constants as required by many natural inflation models. The gauge consistency conditions (e.g. vanishing gauge anomalies, etc) of such multi-axion systems are also studied. We show that our scenarios apply not only to the field theory context but also to stringy models. As an explicit example, a D-brane model presenting all the characteristics of our ideas is given.
National Taiwan University
Prof. Yi Chou
Variability of X-ray binaries
An X-ray binary is an accreting binary system with either a neutron star or a stellar size black hole as accretor. The time scales of variability of X-ray binaries ranging from less than a millisecond to thousands of days can be periodic, quasi-periodic or even aperiodic. Investigating on the variability with various time scales of X-ray binaries allow us to probe different parts of systems, including the binary orbits, the accretion disks and the compact objects. In this talk, I will present our achievements in studying the variability of several X-ray binaries recent years. Using the observed-minus-calculated method, we do not only trace the evolutions of orbital and neutron star spin parameters for the accreting millisecond X-ray pulsar XTE J1807-294 and the partial eclipsing X-ray binary X 1822-371, but also reveal some interesting phenomena for further understanding the natures of these systems. For the non-stationary periodic variations, we adopt Hilbert-Huang transfer, a newly developed time-frequency analysis tool which was rarely applied in astronomy before, to resolve the superorbital modulation of SMC X-1 and the ~4 Hz quasi-periodic oscillation of black hole binary XTE J1550-564.
National Central University
Prof. Xiao-Gang He
Higgs portal dark matter in light of the recent
PandaX-II and Lux data
Direct searches for dark matter (DM) by the LUX and PandaX-II Collaborations employing xenon- based detectors have recently come up with the most stringent limits to date on the elastic scattering of DM off nucleons. For Higgs-portal scalar DM models, the new results have precluded any possibility of accommodating low-mass DM as suggested by the DAMA and CDMS II Si experiments utilizing other target materials, even after invoking isospin-violating DM interactions with nucleons. In the simplest model, SM+D, which is the standard model plus a real scalar singlet named darkon acting as the DM candidate, the LUX and PandaX-II limits rule out DM masses from 5 GeV to about 330 GeV, except a small range around the resonant point at half of the Higgs mass where the interaction cross- section is near the neutrino-background floor. I will also discuss how some of the excluded regions can be recovered by going beyond the minimal darken model, for example, the two Higgs doublet model with darken.
National Taiwan University
Dr. Sichun Sun
Classical gravitational waves on CMB from first-order phase transition
Particle phase transitions in the early universe including electroweak and grand unification ones are well-studied subjects. We point out that there are new possible particle phase transitions around inflation. Those new inflationary particle phase transitions, if of the first order, may yield low-frequency gravitational waves (GWs) due to bubble dynamics, leaving imprints on the cosmic microwave background (CMB). In contrast to the nearly scale-invariant primordial GWs caused by vacuum fluctuation, these bubble-generated GWs are classical and have scale dependent B-mode spectra. If decoupled from inflaton, the electroweak phase transition during inflation may serve as a mirror image of the one after reheating where the baryon asymmetry could be generated via electroweak baryogenesis (EWBG). The second new electroweak phase transition may also be the source for EWBG.
National Taiwan University
Dr. Yong Tian
The insufficient dark matter in high-surface-brightness
The "missing mass" problem is a long standing issue in astronomy. Since 2003, the "dearth" of dark matter was claimed in three ordinary elliptical galaxies by using planetary nebulae (PNe). With accumulating data, we investigate all the elliptical galaxies with PNe. We found all of them (seven cases) have either insufficient dark matter or insufficient baryon within five effective radius. Moreover, the same result was found in 57 elliptical lenses of Einstein rings from gravitational lensing. The common in these elliptical galaxies is all belong to High-Surface-Brightness galaxies which is the prediction of small mass discrepancy in modified Newtonian dynamics.
National Central University
Dr. Kyung Kiu Kim
Holographic entanglement entropy of anisotropic minimal surfaces in LLM geometries
We calculate the holographic entanglement entropy (HEE) of the Z_k orbifold of Lin-Lunin-Maldacena (LLM) geometries which are dual to the vacua of the mass-deformed ABJM theory with Chern-Simons level k. By solving the partial differential equations analytically, we obtain the HEEs for all LLM solutions with arbitrary M2 charge and k up to \mu^2_0 -order where \mu_0 is the mass parameter. The renormalized entanglement entropies are all monotonically decreasing near the UV fixed point in accordance with the F -theorem. Except the multiplication factor and to all orders in \mu_0, they are independent of the overall scaling of Young diagrams which characterize LLM geometries. Therefore we can classify the HEEs of LLM geometries with Z_k orbifold in terms of the shape of Young diagrams modulo overall size. HEE of each family is a pure number independent of the 't Hooft coupling constant except the overall multiplication factor. We extend our analysis to obtain HEE analytically to \mu^4_0-order for the symmetric droplet case.
Yonsei University
Prof. Yi Yang
The measurements of antiproton flux and antiproton-proton flux ratio at AMS
Alpha Magnetic Spectrometer (AMS-02), led by the Nobel Prize winner Professor Samuel C. C. Ting, is an international collaborative research project in experimental high energy physics. AMS-02 is a general purpose particle detector mounted on the International Space Station (ISS) since May 19 2011. After 5 years data-taking, AMS-02 has collected more than 80 billions cosmic rays events. Today, I am going to present the latest AMS-02 results on the antiproton flux and antiproton-proton flux ratio.
National Cheng Kung University
Prof. Antonino Marciano
Cosmology with fermions
Moving from the consideration that matter fields must be treated in terms of their fundamental quantum counterparts, we analyze models for early cosmology that involve fermion fields. We comment on the phenomenological consequences of this choice, and show that cosmological quantum perturbations can actually be accounted for in terms of Dirac fields. We recover a class of states of the Hilbert space that implement the semiclassical limit of the theory, and correspond to non Bunch Davies group coherent states. We then discuss quantum features of these non Bunch Davies states, and derive signatures for CMB observables also deploying path-integral methods.
Fudan University
NTU LeCosPA Mini Workshop:
New perspectives of Cosmology and Quantum Gravity
Dr. Sheng-Lan Ko
Gravitational positive energy theorem and relative entropy for torsion gravity
We apply the covariant Noether charge formalism of Wald to derive the explicit expression of quasi-local energy for the Einstein-Cartan-fermion system. The quasi-local energy is then evaluated with a perturbative solution. The positivity of the energy turns out to impose exactly the same condition on the parameters as the positivity of the relative entropy does, for which sufficiently massive fermion is preferred. This serves as an explicit nontrivial example of the proof of the equivalence between quasi-local energy and relative entropy outlined in the work of Lashkari et al. We also discuss the consequences of this equivalence.
Institute for Fundamental Study
Dr. Yen-Ting Lin
Observational tests of massive galaxy assembly
Although the standard model of cosmology — cold dark matter with a cosmological constant (LCDM) — is extremely successful in describing observations of the large scale structure, many aspects of galaxy formation remain to be understood. In this talk I will address two important issues in the formation and assembly of massive galaxies, namely the stellar mass growth history of the most massive galaxies in the Universe — the brightest cluster galaxies (BCGs), and the assembly bias phenomenon of typical, massive galaxies, for which LCDM has made clear predictions. I shall compare these predictions with observations as fairly as possible. Although discrepancies were found, I point out improvements in observational techniques and data quality that may help resolve the inconsistency. I will finish with prospects of extending these analyses to different halo mass scales, as well as to higher redshifts.
Academia Sinica Insititute of Astronomy and Astrophysics
Prof. Yuji Urata
GRBs as probe of early universe
Gamma-Ray Bursts (GRBs) are currently being exploited as probes of the first generation of stars. In fact, the highest-z events at the re-ionization epoch (z ∼8) have already been observed, and their discovery at z > 10 is highly possible. Therefore, ongoing research aimed at understanding GRB progenitor stars represents one of the most pressing inquiries in modern astrophysics. In this talk, I will talk about two topics; (1) physical correlations studies related with prompt gamma-ray spectral energy peak that are expected to apply cosmological application, and (2) emissions from reverse shock that are powerful tool to explore early universe. In final, I will briefly talk about a planned GRB satellite mission.
National Central University
National holiday
Dr. Young-Hwan Hyun
De Sitter entropy and area law with topological soliton
Global topological solitons of the hedgehog ansatz are added to de Sitter space-time in arbitrary dimensions larger than three, and thermodynamic law is checked at the cosmological horizon without additional horizon. All geometric and thermodynamic quantities are varied in the presence of this interacting matter distribution including pressure, however the area law is satisfied in exact form.
Sungkyunkwan University
Prof. Wei-Tou Ni
A brief history of gravitational wave research and its century outlook
Einstein in 1916 predicted gravitational waves (GWs) and obtained the quadrupole formula for GW radiation in general relativity (GR). Joseph Weber started GW detection experiment more than fifty years ago. In 1966, his bar for GW detection reached a strain sensitivity of a few times 10^(−16). His announcement of coincident signals (now considered spu- rious) stimulated many experimental efforts from room temperature resonant masses to cryogenic detectors and laser-interferometers. Now there are km-sized interferometric detectors (LIGO Hanford, LIGO Livingston, Virgo and KAGRA). Advanced LIGO first reached a strain sensitivity of the order of 10^(−22). During their first 130 days of observation (O1 run), with the aid of templates generated by numerical relativity, they did make the first detections: two 5-σ GW events and one likely event. Besides earth-based GW detectors, the drag-free sensitivity of the LISA Pathfinder has already reached to the LISA goal level, paving the road for space GW detectors. Over the whole GW spectrum (from aHz to THz) there are efforts for detection, notably the very-low-frequency band (pulsar timing array [PTA], 300 pHz –100 nHz) and the extremely-low (Hubble)-frequency (cosmic microwave background [CMB] experiment, 1 aHz –10 fHz). We summarize the various projects, their sensitivities and potential sources with a century outlook into the future.
National Tsing Hua University
Prof. Dimitrios Giataganas
Gauge/Gravity duality with anisotropies
The anisotropic theories have attracted considerable attention in the context of gauge/gravity duality. We derive such theories that have AdS asymptotics in UV, while in the IR flow to a Lifshitz-like theory with arbitrary scaling z. We then extend the derivation to hyperscaling violation theories in the IR. Finally, we discuss transport coefficients and heavy quark observables to find their dependence on the scaling z and the anisotropy, where we also demonstrate the violation of the known universal relations that have been found to hold for the isotropic gauge/gravity dualities.
National Center for Theoretical Sciences
Prof. Tomotsugu Goto
A new distance measure using the correlation between CO luminosity and its line width
Does the dark energy vary over cosmic time? To answer this question, we need to map the expansion of the Universe over a large span of the cosmic time. Type Ia supernovae have been used to measure distances to z~1.7, but beyond this, no reliable distance measure has been established. We propose a new distance measure using sub-millimeter (submm) galaxies to determine distances out to z~6. Using a large sample of submm galaxies compiled from the literature, we show there exists a significant correlation between the CO luminosity (L'CO) and the CO line width (FWHM) of submm galaxies. We use this correlation to measure intrinsic luminosity of submm galaxies, based on the observed FWHM. Through comparison with their observed brightness, we measure their luminosity distance, and construct the Hubble diagram to z~6. Submm galaxies are detected all across the history of the Universe, including some at z>6. With the advent of ALMA, it is expected that large numbers of distant submm galaxies will be discovered in the near future. This method is suitable for such an era, providing a new opportunity to constrain the earliest cosmic expansion.
National Tsing Hua University
National holiday / Semester ends