引力波视角下的世界图景

The gravitational-wave landscape

自古希腊以来,人们一直都在探索茫茫星空的无尽奥秘.1609年的秋天,45岁的意大利天文学家伽利略(Galileo Galilei)把望远镜指向了天空,打开了一扇观测宇宙的窗口,彻底改变了人类对世界的认识.伽利略本人也被后人尊称为"现代科学之父"和"观测天文学之父".如今,人们已经不再局限于通过肉眼和可见光来观测星空.

In the fall of 1609, Galileo Galilei opened a window to observe the divine heaven by pointing the telescope skywards. Since then enormous achievements were made in the field of astronomy with dedicated telescopes and satellites, towards a deep understanding of the "electromagnetic universe". About four hundred years later, the LIGO/Virgo Collaboration discovered, via operating the Michelson interferometers at an unprecedented sensitivity level, a gravitational wave(GW) event, the so-called GW150914, from the merger of a binary black hole. It opened a completely novel window to observe the "gravitational universe", which is vastly different from the "electromagnetic universe". The discovery triggered intense researches in the GW field, and many more GW events were observed since then. The summit of the field was delivered by the multimessenger observation of a binary neutron star merger, the so-called GW170817. The event was examined in great detail with joint observations from both GW and electromagnetic instruments. In electromagnetic bands, the counterpart of GW170817 was seen by observations from radio waves, to optical bands, and to X-rays and gamma-rays. The physics revealed by them is extremely rich, and it helped scientists to better understand the equation of state of the supranuclear matters of neutron stars, short gamma-ray bursts, kilonovae, alternative gravity theories, and so on. Despite of many exciting discoveries, the landscape of GWs is much broader than the kilohertz waveband as was already demonstrated. A new era of GWs with extended wavebands is yet to come, with a lot of promising scientific targets. The whole spectrum of GWs will be revealed by:(a) The ground-based laser interferometers that aim for kilohertz GWs,(b) the space-based laser interferometers that aim for millihertz GWs,(c) the pulsar timing arrays that aim for nanohertz GWs, and(d) the specific polarization pattern of cosmic microwave background that was influenced by GWs at the ultra-low frequency at today. To embrace the modern time of GWs, there are still catalogs of puzzles to handle with, in particular,(a) how to build accurate gravitational waveforms that cover a large parameter space with affordable computational costs,(b) how to inversely derive astrophysical information from GWs thus to interpret and learn the physical environments in an unbiased way,(c) how to deal with the increasing complexity of GW time-series data with the help of advanced data-analysis means. These aspects are intrinsically mutually interdependent and the solution of them requests close collaboration of GW scientists from different research sub-fields. A deep understanding of GWs from multiband, as well as multimessenger, aspects is desirable to extract the maximum science output from GW data. Fortunately, both domestically or internationally, great work is being done constantly by groups of researchers and collaboration of them. New patterns among GW scientists are emerging that try to coherently combine relevant knowledge from astronomy, physics, data science, and engineering science. This is crucial for building a big science field, namely, the GW science, to have a great impact on our understanding of the world. These excellent prospects will eventually open up the GW window to our "gravitational universe" thoroughly in the near future, with extremely promising scientific returns.