AbstractTaiji is a Chinese space mission to detect gravitational waves in the frequency band 0.1 mHz to 1.0 Hz, which aims at detecting super (intermediate) mass black hole mergers and extreme (intermediate) mass ratio in-spirals. A brief introduction of its mission overview, scientific objectives, and payload design is presented. A roadmap is also given in which the launching time is set to the 2030s.https://doi.org/10.1093/ptep/ptaa083Progress of Theoretical and Experimental Physics（2020）
AbstractThe historic launch of the frst several hundred out of 12,000 planned Starlink satellites heralds the arrival of the era of ultra-large satellite constellations. If it will bring new opportunities or insurmountable challenges to astronomy will probably depend on whether you are conducting your observations in space or from the surface of the Earth.Nature Astronomy (2020)延伸阅读————————————————————————探索宇宙的新时代已经到来——挑战与机遇并存引言人类对于浩瀚宇宙从来没有停止过观测和想象，早在上个世纪，摩托罗拉就提出过非常宏伟的星链计划，后来由于种种原因中断了卫星的发射工作，但这种星链的设想还是被一代又一代的科学家继承了下来。正文近日，国科大杭州高等研究院空间盘古实验室、新加坡南洋理工大学、中科院理论所、澳大利亚昆士兰大学、澳大利亚国立大学研究人员联合在《自然 天文》杂志上发表评论文章，结合“太极一号”卫星的发射和后期在轨测试，对卫星星座天文学的发展和展望进行了综述。2019年8月31号（北京时间）“太极一号”成功发射，取得了一系列宝贵的在轨测试数据，迈出了我国空间引力波探测的第一步。为探测大质量黑洞并合产生的低频引力波，太极计划最后一步将发射三颗卫星，形成边长300万公里的等边三角形编队星座，从而极大的推动仪器和观测天文学的发展。文章指出，使用分布式仪器系统收集天文和天体物理数据并非新的想法。目前已经有几个地面射电天文学仪器和光学望远镜系统组成的全球网络。直到近些年，在空间建立和维持一个庞大的分布式的天文仪器网络，仍然超出了工程技术和财政支持能力。但我们要看到，人工智能和新技术的发展又给这一分布式的计划增添了希望。文章强调，绕地球运行的卫星群如主动通信的立体卫星和商业小型卫星组成的大规模编队星座，又比如Taiji或欧洲对应的LISA，可能会带来新的技术能力，因为这种星座网络具有大面积覆盖特性，也更加具有灵敏度和灵活性。卫星编队星座最大的进步在于，使对遥远天体的探索在财政上变得更容易负担，同时降低了出现失败的风险。文章最后讨论了卫星编队星座对地面观测天文学的潜在威胁，目前正在建造的大规模的卫星群，可以用来支持导航、大地测量、电话和信息服务、宽带互联网、天气和气候监测、遥感等等。但很快，数以万计的这些卫星将照亮夜空，对地面天文学的运行环境构成威胁。当我们处于卫星星群爆炸性增长的同时，天文学家、卫星工程师和政策制定者以及其他可能的利益攸关方应共同努力，为这一潜在的有价值的技术发展找到一条双方都能接受的道路----保护我们已拥有的环境，同时拓展我们在空间上的探索能力。该综述文章作者包括：徐淑岩教授（太极一号霍尔微推系统负责人）、Igor Levchenko教授,吴岳良院士（太极计划首席科学家）、Kateryna Bazaka教授。该文章2020年6月29日上线，原文链接：Nat Astron (2020)https://doi.org/10.1038/s41550-020-1141-0该新闻稿转载自中国科学院大学官网
AbstractTwo recent papers (by Amaro-Seoane and Gourgoulhon and co-workers) revealed that in our Galaxy there are very extreme mass-ratio inspirals composed of brown dwarfs and a supermassive black hole at the centre of the Galaxy. The event rates estimated in these papers are very considerable for future space-borne detectors. In addition, there are plunge events during the formation of inspiralling orbits. In this work, we calculate the gravitational waves from compact objects (brown dwarfs, primordial black holes, etc.) plunging into or being scattered by the central supermassive black hole. We find that for space-borne detectors the signal-to-noise ratios of these bursts are quite high. The event rates are estimated as ∼ 0.01 yr–1 for the Galaxy. If we are lucky, this kind of very extreme mass-ratio burst will offer a unique chance to reveal the nearest supermassive black hole and nuclei dynamics. The event rate could be as large as 4∼8 yr–1 within 10 Mpc, and because the signal is strong enough for observations by space-borne detectors, we have a good chance of being able to probe the nature of neighbouring black holes.DOI: 10.1093/mnrasl/slaa115
AbstractSpace-based gravitational-wave detectors consist of a triangle of three spacecraft, which makesit possible to detect polarization modes of gravitational waves due to the motion of the detectors in space. In this paper we explore the ability of Taiji to detect the polarization modes in the parameterized post-Einsteinian framework. Assuming massive black hole binaries with the total mass of M = 4 × 105 M⊙ at redshift of z = 1, we find that Taiji can measure the dipole and quadruple emission (∆αD/αD and ∆αQ/αQ) with the accuracy of up to ∼ 0.04%, the scalar transverse and longitudinal modes (∆αB and ∆αL) up to ∼ 0.01, and the vector modes (∆αV ) up to ∼ 0.0005.https://arxiv.org/pdf/2006.04413.pdfPhys. Rev. D 102, 124050 (2020)
AbstractWe investigate a new mechanism to create large curvature perturbations on small scales due to parameter resonance in a single-field inflationary model with a small periodic structure upon the potential. After reentering the horizon, the amplified curvature perturbations can lead to observable primordial black holes as well as stochastic gravitational waves. The mass of primordial black holes and frequency of the induced gravitational waves depend on the model parameters. The resulted primordial black hole could constitute all dark matter or a fraction of dark matter in the universe, and corresponding stochastic gravitational waves fall in the frequency band measurable for the pulsar timing array and the space-based gravitational wave detectors.DOI: 10.1088/1475-7516/2020/06/013https://arxiv.org/pdf/1912.10437.pdf
AbstractWe review potential low-frequency gravitational-wave sources, which are expected to be detected by Taiji, a Chinese space-based gravitational-wave detector, estimate the detection rates of these gravitational-wave sources and present the parameter estimation of massive black hole binaries.DOI:10.1142/S0217751X2050075Xhttps://www.worldscientific.com/doi/abs/10.1142/S0217751X2050075XInternational Journal of Modern Physics A（2020）
AbstractInflation in the early universe can generate the nearly conformal invariant fluctuation that leads to the structures we observe at the present. The simple viable Starobinsky R2 inflation has an approximate global scale symmetry. We study the conformal symmetric Weyl R2 gravity and demonstrate its equivalence to Einstein gravity coupled with a scalar and a Weyl gauge field. The scalar field can be responsible for inflation with Starobinsky model as the attractor, potentially distinguishable from the latter by future experiments. The intrinsic Weyl gauge boson becomes massive once the Einstein frame is fixed, and constitutes as a dark matter candidate through gravitational production.https://arxiv.org/pdf/2006.02811v1.pdf
AbstractWe propose a new type of search for a pseudoscalar particle η pair produced via an off-shell Higgs, pp→h∗→ηη. The search is motivated by a composite Higgs model in which the η is extremely narrow and decays almost exclusively into Zγ in the mass range 65 GeV≲mη≲160 GeV. We devise an analysis strategy to observe the novel ZγZγ channel and estimate potential bounds on the Higgs-η coupling. The experimental sensitivity to the signatures depends on the power to identify fake photons and on the ability to predict large photon multiplicities. This search allows us to exclude large values of the compositeness scale f, being thus complementary to other typical processes.https://arxiv.org/pdf/2005.13578.pdf
AbstractThe detection of binary black hole coalescences by LIGO/Virgo has aroused the interest in primordial black holes (PBHs), because they could be both the progenitors of these black holes and a compelling candidate of dark matter (DM). PBHs are formed soon after the enhanced scalar perturbations re-enter horizon during radiation dominated era, which would inevitably induce gravitational waves as well. Searching for such scalar induced gravitational waves (SIGWs) provides an elegant way to probe PBHs. We perform the first direct search for the signals of SIGWs accompanying the formation of PBHs in North American Nanohertz Observatory for Gravitational waves (NANOGrav) 11-year data set. No statistically significant detection has been made, and hence we place a stringent upper limit on the abundance of PBHs at 95% confidence level. In particular, less than one part in a million of the total DM mass could come from PBHs in the mass range of [2∗10-3,7∗10-1]MΘ.DOI: 10.1103/PhysRevLett.124.251101https://arxiv.org/pdf/1910.12239.pdf
AbstractTreating the gravitational force on the same footing as the electroweak and strong forces, we present a quantum field theory of gravity based on spin and scaling gauge symmetries. A biframe spacetime is initiated to describe such a quantum gravity theory. The gravifield sided on both locally flat noncoordinate spacetime and globally flat Minkowski spacetime is an essential ingredient for gauging global spin and scaling symmetries. The locally flat gravifield spacetime spanned by the gravifield is associated with a non-commutative geometry characterized by a gauge-type field strength of the gravifield. A coordinate-independent and gauge-invariant action for the quantum gravity is built in the gravifield basis. In the coordinate basis, we derive equations of motion for all quantum fields including the gravitational effect and obtain basic conservation laws for all symmetries. The equation of motion for gravifield tensor is deduced in connection directly with the total energy-momentum tensor. When the spin and scaling gauge symmetries are broken down to a background structure that possesses the global Lorentz and scaling symmetries, we obtain exact solutions by solving equations of motion for the background fields in a unitary basis. The massless graviton and massive spinon result as physical quantum degrees of freedom. The resulting Lorentzinvariant and conformally flat background gravifield spacetime is characterized by a cosmic vector with a nonzero cosmological mass scale. The evolving Universe is, in general, not isotropic in terms of conformal proper time. The conformal size of the Universe becomes singular at the cosmological horizon and turns out to be inflationary in light of cosmic proper time. A mechanism for quantum scalinon inflation is demonstrated such that it is the quantum effect that causes the breaking of global scaling symmetry and generates the inflation of the early Universe, which is ended when the evolving vacuum expectation value of the scalar potential gets a minimal. Regarding the gravifield as a Goldstone-like field that transmutes the local spin gauge symmetry into the global Lorentz symmetry with a hidden general coordinate invariance, a spacetime gauge field is constructed from the spin gauge field that becomes a hidden gauge field. The bosonic gravitational interactions are described by the Goldstone-like gravimetric field and spacetime gauge field. Two types of gravity equation result; one is as the extension to Einstein’s equation of general relativity, and the other is a new one that characterizes spinon dynamics. The Einstein theory of general relativity is considered to be an effective low-energy theory.DOI:10.1103/PhysRevD.93.024012https://arxiv.org/pdf/1506.01807.pdf
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