Searching for Long-Lasting Gravitational-Wave Signals with Second-Generation Gravitational-Wave Detectors

爱因斯坦的广义相对论预言，近一个世纪以来，引力波一直躲过直接探测，因为它们的振幅非常小。然而，随着第二代引力波探测器的到来，如高级LIGO和高级处女座，预计将在未来十年内首次探测到天体物理和宇宙过程中产生的引力波。这个项目的重点是在长时间尺度上发射的引力波，例如由许多独立来源的信号非相干叠加产生的平稳随机引力波背景(SGWB)。该项目还瞄准了几分钟、几小时或更长时间尺度上的瞬时引力波信号，这些信号是由各种天体物理过程产生的，例如动态形成的黑洞双星或黑洞周围吸积盘中的碎片。互相关技术将应用于第二代探测器要获取的第一个数据，以寻找这些持续时间较长的信号。与最新公布的结果相比，这些搜索的灵敏度提高了1000多倍，这些搜索将探测和限制一些拟议的SGWB模型。此外，将对SGWB模型进行系统研究，以准确地确定这些模型中第二代探测器可能追求的开放科学问题。这些悬而未决的问题包括：中子星中的状态方程是什么？双星中子星的聚结率是多少？第二代探测器可以访问SGWB的哪些宇宙学模型？该项目有望对宇宙学和天体物理学产生重大影响。例如，SGWB预计将在大爆炸后不久产生，因此可能包含关于宇宙演化的最初时刻和关于最高能量尺度的物理的独特信息，否则这些信息在实验室是无法获得的。类似地，SGWB可能源于天体物理：例如，将宇宙中所有双星中子星、双星黑洞、磁星或超新星的贡献汇总在一起，就会产生一个随机引力波“前景”，其中包含关于宇宙中这些最猛烈的物体和事件的信息。持续时间长的瞬变引力波信号还将提供有关天体物理物体和产生它们的过程的信息。例如，对动态形成的黑洞双星产生的引力波信号的直接探测将揭示潜伏在附近星系内部区域的黑洞数量，揭示它们的密度和速度分布。

Predicted by Einstein's Theory of General Relativity, gravitational waves have evaded direct detection for nearly a century due to their exquisitely small amplitudes. However, with the arrival of the second-generation gravitational-wave detectors, such as Advanced LIGO and Advanced Virgo, first detections of gravitational waves produced in astrophysical and cosmological processes are expected to be made over the next decade. This project focuses on gravitational waves emitted on long time scales, such as the stationary stochastic gravitational-wave background (SGWB) generated by an incoherent superposition of signals from many independent sources. The project also targets transient gravitational-wave signals on the time-scales of minutes, hours or longer, generated by a variety of astrophysical processes such as dynamically formed black-hole binaries or fragmentation in accretion disks around black holes. Cross-correlation techniques will be applied to the first data to be acquired by the second-generation detectors in search for these long-lasting signals. With over 1000 times better sensitivity than the latest published results, these searches will probe and constrain a number of proposed SGWB models. Furthermore, the SGWB models will be systematically studied to precisely identify the open science questions within these models that could be pursued by the second-generation detectors. Examples of such open questions include: What is the equation of state in neutron stars? What is the coalescence rate of binary neutron stars? Which cosmological models of SGWB are accessible to the second-generation detectors?This project is expected to have a strong impact on cosmology and astrophysics. For example, the SGWB is expected to be generated just moments after the Big Bang, and hence may contain unique information about the first moments in the evolution of the universe and about the physics of highest energy scales, which is otherwise not accessible in laboratories. Similarly, the SGWB could be of astrophysical origin: for example, summing up contributions from all binary neutron stars, binary black holes, magnetars, or Supernovae in the universe leads to a stochastic gravitational-wave "foreground" that contains information about these most violent objects and events in the universe. The long-lasting transient gravitational-wave signals will also provide information about the astrophysical objects and processes that generated them. For example, direct detection of gravitational-wave signals produced by dynamically formed black hole binaries would shed light on the population of black holes that lurk in the inner regions of nearby galaxies, revealing their density and velocity distribution.