Probing Fundamental Physics with Gravitational-Wave Observations

2015年9月，当激光干涉仪引力波天文台（LIGO）首次探测到引力波事件GW150914时，爱因斯坦广义相对论的另一个非凡预测得到了证实。引力波是由致密物体（如黑洞）在时空中产生的涟漪，它在宇宙中传播，就像海浪或声波在水或大气中传播一样。对LIGO结果的仔细分析表明，GW150914的起源是两个黑洞的吸入和合并，每个黑洞的质量约为太阳的30倍。这些引力波几乎畅通无阻地在宇宙中传播了12亿年，直到2015年到达两个LIGO探测器，开启了物理学的新时代。LIGO现在已经加入了欧洲Virgo和日本KAGRA探测器的行列，形成了一个全球性的观测网络。这个网络已经探测到几十个事件，包括中子星合并GW170817也在整个电磁波谱中被观测到，并且预计在即将到来的第四次观测运行O4中会有数百个新的探测。引力波观测是我们为寻求当代物理学中最紧迫问题的答案而量身定制的工具。神秘的暗物质和暗能量构成了我们宇宙中90%以上的质能，它们的本质是什么？我们是否需要扩展爱因斯坦的理论，因为它与量子物理定律不相容？我们的宇宙在其演化的早期阶段是什么样子的？物质在密度高于核物质时的行为是什么？是否存在更多奇异的致密天体，比如虫洞？这些都是STFC战略愿景的核心问题，而利用引力波观测来寻求答案是我们项目的中心主题。确定引力波信号的来源及其性质的过程类似于分析犯罪现场的指纹。指纹（信号）与可能的罪魁祸首数据库（引力波模板库）进行比较，如果匹配，就可以识别出来源。然而，与指纹类比相反，在有限和离散的候选集合中识别单个源，我们正在处理引力波物理学中无限数量的可能源，这些可能源具有许多参数的特征，每个参数都可以在很宽的范围内连续变化。例如，黑洞双子星的特征（除其他参数外）是两个黑洞的质量，本质上可以取任何正值。因此，我们的结果不是以单一精确答案的形式出现，而是在参数空间上的概率分布。为此目的，在各种可能的辐射源范围内进行高精度的理论预测是至关重要的。我们将产生大量这种类型的新预测，并用现在和未来的探测器数据对它们进行分析。更具体地说，我们将通过对爱因斯坦引力理论进行特定修正的黑洞建模来扩展广义相对论的检验，并在观测数据中寻找这些修正的线索。我们将改进现有的大量黑洞模型，这些模型为我们提供了一种测量宇宙膨胀的统计方法。我们将从所谓的宇宙弦中寻找引力波，据预测，宇宙弦是在宇宙早期形成的。在极端密度下，物质的本质可以通过中子星的潮汐变形来探测，这种变形类似于地月系统中的潮汐，以及它们在引力波信号中引起的变化。最后，我们将寻找更奇特的致密物体的高度特征特征，例如可能来自虫洞的引力波回声。

When the Laser Interferometer Gravitational Wave Observatory (LIGO) made the first ever detection of a gravitational-wave event, GW150914, in September 2015, yet another remarkable prediction by Einstein's theory of General Relativity was confirmed. Gravitational waves are ripples in spacetime generated by compact objects, like black holes, that travel through the Universe like ocean waves or sound wave propagate in water or the atmosphere. Careful analysis of the LIGO result demonstrated that the origin of GW150914 was the inspiral and merger of two black holes, each with mass about thirty times that of the sun. These waves traveled for 1.2 billion years almost unimpeded through the universe until they hit the two LIGO detectors in 2015, ushering in a new era of physics. LIGO has now been joined by the European Virgo and the Japanese KAGRA detectors, resulting in a global network of observatories. This network has detected dozens of events, including the neutron star merger GW170817 also observed across the entire electromagnetic spectrum, and is expected to make hundreds of new detections in the upcoming fourth observation run O4. Gravitational-wave observations are a tailor made tool in our quest for answers to the most pressing questions in contemporary physics. What is the nature of the enigmatic dark matter and dark energy that make up over 90% of the mass-energy of our universe? Do we need to extend Einstein's theory as suggested by its incompatibility with the laws of quantum physics? How did our universe look like in the early stages of its evolution? What is the behaviour of matter at densities above that of nuclear matter? Do more exotic compact objects, such as wormholes, exist? These are all questions at the heart of STFC's strategic vision and the exploitation of gravitational-wave observations in the quest for answers is the central theme of our project. The determination of the source of a gravitational-wave signal and its properties proceeds in a manner similar to the analysis of finger prints on a crime scene. A finger print (the signal) is compared with a data bank of possible culprits (a gravitational-wave template bank) and, in case of a match, identifies the source. In contrast to the finger-print analogy where an individual source is identified among a finite and discrete set of candidates, however, we are dealing with an infinite number of possible sources in gravitational-wave physics that are characterized by a number of parameters, each of which can vary continuously over a wide range. For example, a black-hole binary is characterized (among other parameters) by the two hole's masses which can take on essentially any positive value. Our results therefore do not emerge in the form of a single exact answer, but a probability distribution over a parameter space. It is critical, for this purpose, to have theoretical predictions of high precision across the spectrum of possible sources. We will generate a wealth of new predictions of this type and analyse them with present and future detector data. More specifically, we will extend the tests of general relativity by modeling black holes for specific modifications to Einstein's theory of gravity and look for hints of these modifications in the observed data. We will improve existing models of large populations of black holes that provide us with a statistical way to measure the expansion of the universe. We will search for gravitational waves from so-called cosmic strings, which are predicted to form in the early universe. The nature of matter at extreme densities can be probed through tidal deformations in neutron stars akin to the tides in the Earth-Moon system, and the modifications they induce in the gravitational-wave signals. Finally, we will search for highly characteristic signatures of more exotic compact objects, such as gravitational-wave echoes which may arise from wormholes.