Gravitational Radiation and Relativistic Astrophysics

激光干涉引力波天文台（LIGO）探测到了引力波（GW）-空间和时间的涟漪-由距离地球数十亿光年的两个黑洞碰撞发出。 随着LIGO的升级，这些探测的速度和准确性将得到提高。通过将探测到的波与波的详细预测进行比较，科学家可以测量黑洞的属性（位置，质量，自旋），并了解它们如何形成以及它们如何在碰撞时扭曲空间和时间。 这些预测是使用爱因斯坦在1915年写下的广义相对论方程做出的，但直到2005年左右，随着先进方法和强大的超级计算机的发展，这些方程才得以解决（对于碰撞的黑洞）。该项目支持旨在支持和提高LIGO从观测到的GWs中提取波携带的丰富信息的能力的理论工作。 这包括改进和使用光谱爱因斯坦代码（SpEC），目前最准确的计算机代码解决爱因斯坦方程的黑洞二进制。SpEC将用于进行黑洞碰撞的数值解，以分析LIGO数据。 此外，还将开发新的计算机代码SpECTRE;这是SpEC的下一代升级，旨在用于新的即将到来的计算机架构和超越当前代码的准确性。SpECTRE将用于预测两颗中子星碰撞和黑洞与中子星星碰撞的引力波形。 LIGO有望观测到来自中子星碰撞的引力波，并可以告诉科学家物质在超高密度下的行为。 该计划还将作为年轻物理学家和天体物理学家的培训基地。新代码SpECTRE及其输出将公开发布。 小组成员将通过讲座、互动网页和YouTube视频与公众接触。通过将分析技术和数值模拟与SpEC相结合：（一）将产生黑洞双星的引力波信号，用于LIGO数据分析，将用于校准分析模型，并将用于产生数值替代模型，该模型可以在毫秒内评估单个波形，同时保持完整数值模拟的准确性;及（ii）将透过解析、微扰及数值方法探讨高度弯曲时空的动力学行为。下一代开放源代码SpECTRE将用于物质和辐射的数值相对论模拟。SpECTRE使用不连续Galerkin有限元方法。它是专为高精度和高可扩展性的当前和未来的超级计算机通过以下一个新的基于任务的并行化范式。SpECTRE的初始版本已经实现了广义相对论磁流体力学。它将升级以处理中子星-中子星星和黑洞-中子星星合并的动力学时空，包括基于核理论的热状态方程和中微子。SpECTRE将用于BH和吸积盘形成的研究，以了解动力学和GW发射。它将用于高精度超长中子星星螺旋模拟，以预测GW信号并帮助LIGO约束核状态方程。

The Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected gravitational waves (GWs) --ripples in space and time-- that were emitted by the collision of two black holes billions of light-years from Earth. The rate and accuracy of these detections will improve as LIGO is upgraded. By comparing the detected waves with detailed predictions of the waves, scientists can measure properties (locations, masses, spins) of the black holes and learn about how they might have formed and how they warp space and time as they collide. The predictions are made using the equations of General Relativity, written down by Einstein in 1915 but unsolvable (for colliding black holes) until about 2005 with the development of advanced methods and powerful supercomputers. This project supports theoretical work designed to underpin and improve LIGO's ability to extract from observed GWs the rich information that the waves carry. This includes the improvement and use of the Spectral Einstein Code (SpEC), currently the most accurate computer code for solving Einstein's equations for black hole binaries. SpEC will be used to carry out numerical solutions of black-hole collisions for the purpose of analyzing LIGO data. In addition, a new computer code SpECTRE will be developed; this is a next-generation upgrade of SpEC designed for new upcoming computer architectures and accuracies beyond those of current codes. SpECTRE will be used to predict gravitational waveforms from two colliding neutron stars and from a black hole colliding with a neutron star. Gravitational waves from neutron-star collisions are expected to be observed by LIGO, and can teach scientists about how matter behaves at ultra-high densities. This program will also serve as a training ground for young physicists and astrophysicists. The new code SpECTRE and its output will be publicly released. Group members will reach out to the general public through lectures, interactive web pages, and YouTube videos.By combining analytical techniques and numerical simulations with SpEC: (i) Gravitational-wave signals for black-hole binaries will be generated for use in LIGO data analysis, will be used to calibrate analytic models, and will be used to produce numerical surrogate models that can evaluate a single waveform in milliseconds while retaining the accuracy of full numerical simulations; and (ii) the dynamical behavior of highly curved spacetime will be explored via analytic, perturbative, and numerical approaches. The next-generation open-source code SpECTRE will be developed for numerical relativity simulations with matter and radiation. SpECTRE uses Discontinuous Galerkin finite element methods. It is designed for high accuracy and high scalability on current and future supercomputers by following a novel task-based parallelization paradigm. SpECTRE's initial version already implements general-relativistic magnetohydrodynamics. It will be upgraded to handle the dynamical space times of neutron star-neutron star and black hole-neutron star mergers, including nuclear-theory based hot equations of state and neutrinos. SpECTRE will be applied to studies of BH and accretion disk formation to understand dynamics and GW emission. It will be used for high-accuracy ultra-long neutron star inspiral simulations to predict GW signals and help LIGO constrain the nuclear equation of state.