Research progress on gravitational lensing time delays
According to Einstein's general relativity(GR),light travels along a null geodesic.Therefore,when the inhomogeneous distribution of matter leads to the curvature of time and space,light rays will deflect as the light deflects in the optical lens.This phenomenon is called"gravitational lensing".The gravitational lensing effect is an important tool used in astronomical research.The history of gravitational lensing dates back to the observation of starlight deflection by the Sun during the solar eclipse in 1919 as strong evidences in favor of GR.A more interesting phenomenon in this effect is that the same celestial source can generate multiple images under certain circumstances.These images arrive on Earth in sequence and are observed by people.The difference in arrival time between them is called"time delay"and carries rich astronomical information.Specific observation strategies and algorithms have been developed to measure time delays for different celestial sources.In modern cosmology,the traditional time delay observation object is a distant bright quasar strongly lensed by the foreground galaxy or galaxy cluster,forming multiple magnified images of the point-like bright nucleus while the lensed dimmer parts of the host galaxy appear in the form of stretched arcs.Such lensed quasar systems have been attracting great attention as they are useful for efficient studies of either the nature of lens galaxies and quasars or the geometry of the Universe.Their time delay is measured by comparing the light variation curves.In the era of Webb telescope,the time delay of quasars will give accurate measurements of Hubble constant,in order to solve the current"Hubble constant issue".Meanwhile,with the opening of the era of time-domain astronomy,lensed transient sources such as various types of supernovae,gamma ray bursts(GRBs)and their afterglow,rapid radio bursts(FRBs),and gravitational waves(GWs)will also be widely observed.For these sources,time delay measurement may become easier and more accurate.The transient source lens system will bring us more information about the nature of gravity,the properties of dark matter,and the expansion of the Universe.Detecting and verifying these lensed transient signals are interesting and significant because they would take us to a new research field and bring more discoveries.Firstly,while lensing of traditional sources assumes the description of geometric optics by default,lensed transient signals should sometimes take into account wave optics due to long wavelengths,small source size or coherent emission.Secondly,compared to the traditional targets,lensed transients have many advantages in studying the Universe.For example,due to the well-known light curve template of type Ⅰa supernovae,strong lens time delay measurements may become easier and more accurate with shorter monitoring time,which benefits Hubble constant inference.This advantage becomes much more obvious for lensed FRBs,GRBs and GWs due to their transient nature(signal durations are much smaller than time delays themselves).Finally,studying the lensing effect may also shed light on the nature of transients themselves,especially at high redshifts where the magnification effect becomes non-negligible.This review is based on the latest research progress in the field of time delay both domestically and internationally.It introduces the measurement methods of time delay,the current status of measurement,and its applications in testing basic physics,researching astrophysics,and cosmology.
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