The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN) – active SMBHs at projected separations larger than several parsecs – and binary AGN – probing the scale where SMBHs are bound in a Keplerian binary – is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field.

中文翻译：

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对双双超大质量黑洞的探索：多信使视角

在多信使时代开始时，对双双超大质量黑洞 (SMBH) 的探索令人信服。检测双活动星系核 (AGN)——预计间隔大于几秒差距的活动 SMBH——和双星 AGN——探测 SMBH 绑定在开普勒双星中的尺度——是一项观测挑战。AGN 对（双星或双星）的研究也代表了宇宙学和天体物理学中的首要理论问题。触发 AGN 需要详细了解 SMBH 即将发生的环境中气体的流体动力学条件，同时，它们的二元性需要详细了解星系如何通过主要和次要合并组装并沿着细丝生长的宇宙网。这篇综述描述了在整个电磁频谱中用于检测双和双 AGN 候选者的技术，并提出了新的搜索途径。将当前的观测状态与最先进的数值模拟和模型进行比较，以形成双星和双星 AGN。双星 SMBH 是宇宙中最响亮的引力波 (GW) 源之一。从低红移的激励 SMBH 中寻找 nHz 频率的 GW 背景，以及在接下来的十年中通过激光干涉仪空间天线直接检测来自它们合并的信号，使这成为多信使天体物理学的主要兴趣主题。本综述讨论了可能显着推进这一迷人领域的未来设施和观察策略。