按日存檔:2017-05-20

中國科學家提出利用宇宙大尺度結構的形態學特徵來檢驗重力理論

由2微米全天巡天的擴散源目錄的星系和點源
中國科學技術大學天文學系特任研究員方文娟與合作者在利用天文數據檢驗重力理論的研究中取得新成果,提出了利用宇宙大尺度結構的形態學特徵來檢驗重力理論的新方法,並通過大型數值模擬驗證了該方法的有效性。

宇宙在加速膨脹這個現象自1998年發現以來,給基礎物理學帶來了巨大的挑戰。 2011年,該發現授予諾貝爾物理學獎。但宇宙加速膨脹的物理機制至今仍然是個謎。在廣義相對論的框架下,解釋宇宙加速膨脹通常需引入一種具有負壓強的能量形式(暗能量)作為其動力機制,然而基本粒子物理目前並不能給出令人滿意的暗能量模型。與此同時,鑑於廣義相對論迄今只在太陽系、並合黑洞、脈衝雙星等天體物理尺度上得到驗證,如果在宇宙學等更大尺度上該理論需要修正,宇宙的加速膨脹也可以得到合理解釋,而無需引入神秘的暗能量。近十幾年來,這類修正廣義相對論的重力理論發展迅速,而如何利用宇宙學的觀測數據來檢驗和甄別不同的重力理論也相應成為宇宙學新的研究方向。

宇宙的大尺度結構蘊涵著關於宇宙演化的豐富信息,是檢驗重力理論的理想場所。現有的檢驗方法以利用其兩點關聯函數等傳統觀測量為主,但N點關聯函數只能探測其N階統計性質,並且在N>2時,測量複雜度急劇增加。為此,方文娟與英國杜倫大學教授李寶九及中國科學院國家天文台研究員趙公博提出了利用大尺度結構的形態學特徵來檢驗重力理論的新方法。在數學上,一個三維幾何體的形態學特徵可由0至3階閔可夫斯基泛函完全描述。在大尺度結構的研究中,將由某一密度閾值指定的高密度區域取為該幾何體,即可進行相應測量。與N點關聯函數相比,閔可夫斯基泛函可以同時探測各階統計性質,不僅測量簡單易行,還具有受系統誤差影響較小等優點。

借助最新的大型電腦數值模擬,該團隊首次揭示並詳細分析了在修正重力和廣義相對論框架下大尺度結構形態學特徵的差異,並發現上述方法對修正重力理論具有顯著的限制能力:對於一個巡天體積約為0.1(h-1Gpc)3、星系密度約為1/(h-1Mpc)3的理想星系巡天觀測而言,單個測量值即可將不同引力理論以5σ以上置信度區分開來;而對於目前正在運行或已提上日程的實際星系巡天觀測而言,雖然單一測量值的區分度減弱,但通過聯合所有的測量值,該方法的強大區分功能應仍可奏效。這類檢驗手段的逐步完善不但將前所未有地在宇觀尺度上驗證廣義相對論,還有望揭示出宇宙加速膨脹的真正起源。

【圖:互聯網;文:節錄自中國科學技術大學網頁;新聞訊息由林景明提供】研究全文刊登在已經出版的《物理評論快報》

教科書編成《NSS Physics Insight》


物理系學生會議:教科書編成《NSS Physics Insight - A Short Introduction to Special Relativity》

Talk:- NSS Physics Insight - A Short Introduction to Special Relativity

We have developed a textbook (NSS Physics Insight – A short introduction to Special Relativity) teaching Secondary Senior Form students Special Relativity. There are 6 major chapters about Special Relativity included, together with 4 supplementary chapters providing required mathematics skills, like limits and differentiation, integration, vectors and matrices, for students to tackle Special Relativity problems. Time dilation, length contraction, Lorentz transformation and the application of Spacetime diagrams are included in the textbook. We adopted the example-based approach in the textbook, that is, we provided examples for students to follow and some follow-up challenges for them to apply what they have learnt. To arouse interest of the readers, we also made use of comics and dialogues to exemplify the physics concepts and theories. Problem sets are available at the end of each chapter, in which students can test their understanding. It is notable that most of the problems in the textbook were originally designed, with extra features like problems are implemented based on novels, increasing difficulty of problems of similar situations. A full solution manual is also available for students to check their answers and steps. Supplementary notes and explanation are also provided in the manual. 
 
時間:2017.05.20(六) 16:00 ~ 16:15
地點:香港中文大學 科學館LT4
講者:Li Ka Yue Alvin 及 Pang Yiu Yung
語言:英語

#131

此活動由香港中文大學物理系主辦。

具有限化學勢的大質量中微子於宇宙擴張史及大尺度結構的影響


物理系學生會議:具有限化學勢的大質量中微子於宇宙擴張史及大尺度結構的影響

Talk:- Influence of Massive Neutrinos with a Finite Chemical Potential on the Cosmic Expansion History and Large Scale Structure

Due to their high abundance, massive cosmic neutrinos would have a significant influence on cosmology, especially in terms of large scale structure. Ever since the discovery of neutrino oscillation, which confirmed that neutrinos have finite masses, many efforts have been made to constrain neutrino masses from cosmology. However, none of them has considered the possibility of a finite chemical potential, if the neutrinos are not majorana fermions. The work would be the first one to analyse the combined effect of masses and chemical potentials of neutrinos on matter power spectrum of large scale structure, through two physical processes, their influence on the cosmic expansion history, and the neutrino free-streaming. The result would serve as a correction to the constraints of neutrino masses from large scale structure. 
 
時間:2017.05.20(六) 16:15
地點:香港中文大學 科學館LT4
講者:
  ●Zhichao Zeng
  ●Shek Yeung
  ●Ming Chung Chu
語言:英語

#131

此活動由香港中文大學物理系主辦。

磁場方向在絲狀雲中恆星形成率的影響


物理系學生會議:磁場方向在絲狀雲中恆星形成率的影響

Talk:- The Effect of Magnetic Field Orientation on Star Formation Rate in Filament Clouds

With the rapid growth of computational power, numerical simulation has become more and more important in astrophysical researches in the last few decades. In this presentation, we will briefly introduce the simulation we are adopting in our group, as well as one of the currently undergoing simulation projects, aiming to reveal the effect of magnetic field orientation and other effects on star formation rate in filament clouds. The importance of magnetic field orientation in star formation has been gradually realised in the recent decades. However, the effect of different orientations on star formation rate in particular types of filaments is still yet to be understood, which will be the central topic of the project. 
 
時間:2017.05.20(六) 15:45 ~ 16:00
地點:香港中文大學 科學館LT4
講者:GUO Zitan
語言:英語

#131

此活動由香港中文大學物理系主辦。

探測中等質量黑洞的可能途徑


物理系學生會議:探測中等質量黑洞的可能途徑 - 經重力透鏡效應的重力波

Talk:- A Possible Way to Probe Intermediate Mass Black Hole – Gravitational Lensing of Gravitational Wave

Existence of intermediate mass black hole is a mystery in astronomy. Theoretically it should exist, but it hasn’t been confirmed by observation yet. Since intermediate mass black hole is a possible candidate of gravitational lens, its lensing effect on gravitational wave in diffraction limit could be a direct observational evidence. Basic analysis shows the lensing effect is observable within LIGO detectability. Serious Bayesian analysis is ongoing to investigate under what circumstances we can confirm that the signal indicates an intermediate mass black hole. 
 
時間:2017.05.20(六) 15:15 ~ 15:30
地點:香港中文大學 科學館LT4
講者:Lai Kwun Hang 及 Otto Hannuksela
語言:英語

#131

此活動由香港中文大學物理系主辦。

磁場方向與恆星形成率之間的關聯


物理系學生會議:磁場方向與恆星形成率之間的關聯

Talk:- The Link between Magnetic-Field Orientations and Star Formation Rates

Star formation rates (SFR) is a key component in modern star-formation models, which mainly involve gravity, turbulence and, in some cases, magnetic fields (B-fields). But the connection between B-fields and SFR has never been observed. A comparison between the survey of SFR and a study of cloud-field alignment – which revealed a bimodal (parallel or perpendicular) alignment – shows that clouds with lower SFR per solar mass are tend to perpendicular to the B-fields. This might be an evidence of B-fields being a primary regulator of SFR. We used B-fields data from starlight method and 353 GHz dust thermal emission, which comes from by Planck mission’s all sky survey. 
 
時間:2017.05.20(六) 15:30 ~ 15:45
地點:香港中文大學 科學館LT4
講者:GU Qilao
語言:英語

#131

此活動由香港中文大學物理系主辦。

磁場作為恆星形成的指揮


物理系學生會議:磁場作為恆星形成的指揮 - 一個觀測角度

Talk:- Magnetic field as a Conductor in Star Formation - An Observational Perspective

The role of the magnetic field, among other mechanisms (gravity & turbulence) in star formation remain uncertain. However, within the last decade, multi-scales observational studies have started to show that magnetic field has an important role in regulating star formation environment from Cloud through the filament to core scale and consequence star forming properties. In this presentation, we briefly review observational signatures of the dynamically important magnetic field at large/small scales. We will also briefly present multi-scale observational evidence. 
 
時間:2017.05.20(六) 14:45 ~ 15:00
地點:香港中文大學 科學館LT4
講者:
  ●Law Chi Yan
  ●Li Hua Bai
  ●Leung Po Kin
  ●Gu Qilao
  ●Zhang Yapeng
語言:英語

#131

此活動由香港中文大學物理系主辦。

從併合中的高質量雙黑洞尋找重力波


物理系學生會議:從併合中的高質量雙黑洞尋找重力波

Talk:- Searching for Gravitational Waves from the Coalescence of High Mass Black Hole Binaries

GstLAL is a search pipeline used by LIGO to search for gravitational waves from the coalescence of compact binaries (neutron stars and/or black holes), which can be operated in on-line and/or off-line configuration. Before the second observing run (O2), the parameter space covered by GstLAL consisted of binaries with component masses m1,m2 ≥ 1M⊙ (M⊙ is the solar mass) and total mass 2M⊙ ≤ M1+M2 ≤ 100M⊙. In this project, we aim to increase the upper bound of the total mass to 600M⊙ to search for gravitational waves from intermediate mass black holes (IMBH). Simulated signal injections with real noises were used to evaluate the sensitivity of GstLAL when decreasing the lower frequency cut-off to observe signals from IMBH since they merge at a lower frequency. We experimented various techniques such as different bank splitting and h(t) gating schemes to optimize the performance and sensitivity after the inclusion of IMBH signals in a search. This would enable us to detect more gravitational wave signals from the coalescence of black holes with higher masses in real time gravitational wave detection. 
 
時間:2017.05.20(六) 14:30 ~ 14:45
地點:香港中文大學 科學館LT4
講者:Ka Lok Lo
語言:英語

#131

此活動由香港中文大學物理系主辦。

通過緻密雙星的重力波研究其潮汐變形能力


物理系學生會議:通過緻密雙星的重力波研究其潮汐變形能力 - 包含合併前階段的效應

Talk:- Studying the Tidal Deformability of Compact Stars Through Gravitational Wave from Binary Mergers: The Effect of Including Post-Merger Phase

The recent discoveries of binary black holes by gravitational-wave detection hint at the opportunity of detecting gravitational-wave signals from binaries consisting of compact stars. Binaries typically undergo three stages during coalescence: inspiral, merger and post-merger. Information on the compact stars’ matter is encoded throughout the gravitational-wave signal, detections of the signal can constrain the equation of state of ultradense matter. The feasibility of constraining the equation of state through gravitational-wave detections by second and future generation detectors have been studied using the inspiral and post-merger stages separately. We present the effect and the possible improvement in constraining the equation of state by using a toy model of binary mergers, which consists of inspiral and post-merger. 
 
時間:2017.05.20(六) 14:15 ~ 14:30
地點:香港中文大學 科學館LT4
講者:Peter, Tsun Ho Pang
語言:英語

#131

此活動由香港中文大學物理系主辦。

速旋中子星循雙曲軌道接近大質量黑洞之動力學


物理系學生會議:速旋中子星循雙曲軌道接近大質量黑洞之動力學

Talk:- Dynamics of Fast Spinning Neutron Stars in a Hyperbolic Orbit near a Massive Black Hole

The trajectory of a non-spinning test particle is given by the geodesics equations. However, when the test particle is spinning, it’s spin interacts with the orbit and the background curvature, giving rise to interesting dynamical behaviors, which in turn can probe the background space-time structure. In this research, we study a fast spinning pulsar orbiting in a hyperbolic orbit around a massive black hole. Millisecond pulsars are very accurate “clock” in the universe. Their abundant existence is predicted by various studies, which however, are not verified, due to the limit of pulsar timing techniques and the complicated pulse profiles of a accelerating pulsar. In this research, the spin problem in relativity and the dynamical features of such system were reviewed and studied. An orbital shift of ~100km and a velocity shift of ~1 km/s are observed, and the former will correspond to a shift of arrival time of pulse ~100μs, while the latter is related to the gravitational wave memory effect. The result of this study will provide useful insights for both the gravitational wave detector LISA, and the pulsar timing array SKA. 
 
時間:2017.05.20(六) 13:45 ~ 14:00
地點:香港中文大學 科學館LT4
講者:Jiale Kaye Li
語言:英語

#131

此活動由香港中文大學物理系主辦。