Applications Of Transients And Other Observations In Cosmology

Since the great discovery of the accelerating expansion of our universe,cosmol-ogy has become a hot research filed again.Cosmic transients,including supernovae,gamma-ray bursts(GRBs)and fast radio bursts(FRBs),are regarded as powerful tools for cosmological research,especially for high-redshift universe.Because they are so bright that we can detect them at very high redshift.Meanwhile,because those tran-sients are associated with the evolution process of stars,they are also useful tools for understanding the evolution of matter in the universe.In addition to those transients,other cosmic observations,such as Hubble parameters(H(z),baryon acoustic oscilla-tions(BAO)and strong gravitational lensing(SGL)systems,can also provide plenty of information about our universe.In chapter I,we will give a short introduction of the background of cosmology and the applications of different observations in cosmology.In chapter ?,we first analyze the cosmic star formation rate and stellar mass den-sity at different redshifts and find the inconsistency between them.Then we discuss the potential reasons for this inconsistency.In the second part of this chapter,we es-timate the luminosity function and formation rate of long-duration GRBs detected by Swift using Lynden-Bell c-method and find that their luminosity function evolves with redshift.And we also find that the expansion rate of long-duration GRBs has an unex-pected excess at z<1.In chapter ?,we propose a new method to measure the cosmic proper distance with the dispersion measure of FRBs.This is the first method which can measure the proper distance.With Monte Carlo simulations,we find that the relation between proper distance and redshift can be constrained tightly if there will be about 500 FRBs with redshift measurements in the future.Then we can also use the relation to constrain the cosmic curvature parameter independently.In chapter ?,we propose two new methods to test the weak equivalence principle(WEP)with the Shapiro time delay.They can overcome the effects of other factors in previous literature and give a more reliable result.Firstly,we introduce the method which uses a large sample of GRBs' time delay measurements to test the WEP and it shows no significant evidence for the invalidity of WEP.Then we introduce another method which uses strong lensed cosmic transients to test WEP.In chapter ?,we first propose a new model-independent method to constrain the cosmic curvature parameter.With current observations,we find the cosmic curvature parameter is about-0.09± 0.19,which is consistent with a flat universe.In the second section we extend our work with new H(z)and BAO observations.Finally,some open questions in cosmology are summarized and a short plan for my future research is also given in Chapter VI.