Searching for the Stochastic Gravitational-Wave Background with Advanced LIGO

爱因斯坦广义相对论的关键预言之一是引力波（GW）的存在：由大质量的快速运动引起的在时空几何中以光速运动的涟漪。虽然引力波的间接影响已经得到了很好的验证，但直接探测地球上的引力波仍然是一个突出的挑战。实现这种能力的科学回报将是巨大的——从探测爆炸恒星的极端动力学到收集关于宇宙大爆炸时刻的状态的信息。几十年来，为了探索宇宙的这个新窗口，科学家们进行了实验和技术开发，突破了物理科学不同领域的界限。随着第二代引力波探测器在先进激光干涉仪引力波天文台（aLIGO）投入使用，2015年将标志着引力波直接探测目标的一个备受期待的分水岭时刻。新的aLIGO探测器的灵敏度将提高到比第一代探测器好10倍左右，为观测GW源开辟了比以前大1000倍的空间体积。aLIGO的数据将用于寻找各种天体物理和宇宙引力波源。这项特别的奖励将支持搜索aLIGO数据的研究，以寻找来自许多独立来源的引力波信号产生的背景信号。aLIGO的第一批观测数据预计将于2015年获得，在接下来的三年里，探测器的灵敏度和占空比将稳步提高。这个项目的重点是寻找随机引力波背景（SGWB），它是由许多独立源产生的引力波信号的非相干叠加产生的。SGWB可能是宇宙学的起源：例如，在暴胀模型中，SGWB是在大爆炸之后的瞬间产生的，这意味着SGWB可能包含关于宇宙演化的最初时刻的独特信息，以及在实验室中无法获得的能量尺度物理。类似地，SGWB可能是天体物理学的起源：例如，总结宇宙中所有磁星或所有双中子星和/或黑洞的合并的贡献，导致随机引力波“前景”，其中包含有关宇宙中这些最猛烈的物体和事件的信息。使用aLIGO数据进行的SGWB搜索将使用相互关联技术，该技术已被开发并成功用于使用初始LIGO数据进行的SGWB搜索。对各向同性SGWB的搜索预计将导致比目前最佳上限高1000倍的测量灵敏度。同样，寻找各向异性的SGWB将导致比目前最佳上限高100倍的灵敏度测量。这些重大进展将导致一系列有趣的宇宙学和天体物理学意义，包括早期宇宙宇称违反的探测或新约束，大爆炸后第一分钟宇宙的演化，中子星的状态方程，紧致双星合并的速率及其随红移的演化。

Among the key predictions of Einstein's General Theory of Relativity is the existence of gravitational waves (GW): ripples moving at the speed of light in the geometry of space-time caused by the fast motion of large masses. Although well tested in terms of their indirect effects, the direct detection of gravitational waves incident on Earth poses an outstanding challenge. The scientific rewards from achieving this ability would be enormous - ranging from probing the extreme dynamics of exploding stars to gleaning information about the state of the Universe almost at the moment of the Big Bang itself. The effort to enable this new window on the Universe has occupied several decades of experimental and technological developments that have pushed the boundaries across diverse fields in the physical sciences. The year 2015 will mark a highly-anticipated watershed moment for the goal of direct detection of gravitational waves, as the second generation of gravitational-wave detectors is commissioned at the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO). The sensitivity of the new aLIGO detectors will be ramped up to become about ten times better than that of the first-generation detectors, opening up a spatial volume for observing GW sources that will be 1000 times larger than before. The aLIGO data will be used to search for a variety of astrophysical and cosmological gravitational wave sources. This specific award will support research that searches the aLIGO data to look for the background signals that arise from gravitational-wave signals from many independent sources. The first observation data from aLIGO is expected in 2015, with the sensitivity and duty-cycle of the detectors steadily increasing over the following three years. This project focuses on the searches for the stochastic gravitational wave background (SGWB), which arises from an incoherent superposition of gravitational-wave signals generated by many independent sources. The SGWB could be of cosmological origin: for example, in inflationary models the SGWB is generated just moments after the Big Bang, implying that the SGWB may contain unique information about the first moments in the evolution of the universe and about the physics of energy scales not accessible in laboratories. Similarly, the SGWB could be of astrophysical origin: for example, summing up contributions from all magnetars or from coalescences of all binary neutron stars and/or black holes in the universe leads to a stochastic gravitational-wave "foreground" that contains information about these most violent objects and events in the universe. The SGWB searches with aLIGO data will be conducted using cross-correlation techniques that were developed and successfully used in the SGWB searches with initial LIGO data. The searches for isotropic SGWB are expected to result in a measurement about 1000x more sensitive than the current best upper limits. Similarly, the searches for anisotropic SGWB will result in about 100x more sensitive measurement than the current best upper limits. Such substantial advances will result in a series of interesting cosmological and astrophysical implications, including detection or new constraints on parity violation in the early universe, evolution of the universe during the first minute after the Big Bang, equation of state in neutron stars, rate of compact binary coalescences and its evolution with redshift.