脉冲星计时阵列与纳赫兹引力波探测

Pulsar timing arrays and nano-hertz gravitational wave detection

阿尔伯特·爱因斯坦(Albert Einstein)在1915年提出广义相对论,并在一年后就预言了引力波的存在.引力波可被看作时空的“涟漪”,几乎不与其他物质相互作用,因此能够在宇宙中自由穿梭.20世纪60年代,作为引力波探测的先行者,约瑟夫·韦伯(Joseph Weber)曾宣称通过共振棒直接探测到了引力波.因为他的实验结果无法被他人重现,因而未被广泛承认.直到2015年9月14日,美国的激光干涉引力波天文台(Laser Interferometer Gravitational-Wave Observatory,LIGO)通过激光干涉的方法,首次直接探测到了引力波[1].

The detection of hecto-hertz gravitational waves from a stellar-mass binary black hole merger by ground-based laser interferometers in 2015 opened a new window to explore the gravitational Universe,largely complementing the existing means of observing the electromagnetic Universe.Extraordinary discoveries were made afterwards,including the entrance into the multi-messenger astronomy era with worldwide attention to a merger of a binary neutron star in 2017.In June of 2023,several international collaborations of pulsar timing array,including the Chinese Pulsar Timing Array,announced key evidence in revealing a new type of gravitational wave—The nano-hertz stochastic gravitational wave background,most likely originating from supermassive binary black hole mergers.These pieces of evidence were achieved with large radio telescopes—Like the five-hundred-meter aperture spherical radio telescope in Guizhou China—Monitoring in a dedicated way the spatially-correlated times of arrival of pulse signals from pulsars.Most of the pulsars are fast rotating neutron stars with strong magnetic fields.The radio wave beams emit along the magnetic axis of the rotating neutron star and they sweep past the Earth periodically,similar to a lighthouse’s flashes.We can establish a timing model for the arrival time of the pulsating signals to account for the effects in the generation and propagation of the pulses.At the same time,these signals are affected by the passing gravitational waves and in return,by analyzing times of arrival of pulsar pulses,pulsar astronomers are able to infer the properties of the stochastic gravitational wave background.Currently the evidence is still not strong enough to claim a solid detection,but is already telling us the bright future of using pulsar timing arrays to probe the nature of nano-hertz gravitational waves.These gravitational waves are most likely from supermassive binary black hole mergers.When two galaxies merge,the two central supermassive black holes can form a gravitational bound system.The rich information about the population of the supermassive binary black holes and the mergers of galaxies is encoded in the gravitational waves,from which we can enhance our understanding of the supermassive black holes and their host galaxies.On the other hand,these gravitational waves are also possibly from early Universe,including the inflation,phase transition of matter,and even new types of objects like the cosmic strings and domain walls.In addition,we can test gravity theories with gravitational wave backgrounds.The propagation speed and the polarization state of gravitational waves can be different in modified gravity theories from what are predicted by the general relativity,which affect the times of arrival of the pulses in different ways.Thus,precise measurements of the correlation of the times of arrival of pulses between pulsars are in principle possible to distinguish different gravity theories.Near-future pulsar timing array observations,equipped with remarkable timing and searching ability of,say,the upcoming Square Kilometre Array,will provide essential data to better disclose the nature of nano-hertz gravitational waves and also the fundamental properties of gravitation and spacetime.