Researches On Black Holes In The Early Universe

This thesis presents my researches,under the guidance of Professor Yang Zhang and Professor Yi-Fu Cai,on black holes in the early universe,including two works:1.the gravitational radiation from the super massive black hole binary(SMBHB)OJ 287,and its detection by pulsar timing array(PTA);2.the creation,abundance and evolutions of primordial black holes(PBHs)in bouncing universe,and the constraints on parameters of the bouncing cosmology through PBHs.OJ 287 is a candidate gravitational-wave(GW)source in the current PTA projects,and its periodic light variation allows us to calculate its orbital motions and GW sig-nals with high accuracies.Hence,the studies on OJ 287 has important meaning in the future PTA detections.Furthermore,bouncing cosmology is a paradigm of early uni-verse,alternatively to inflationary cosmology,and it satisfies the requirements of the present cosmological observations,such as the cosmic microwave background(CMB)detections.Further tests on bouncing cosmology is one of goals of future cosmological detections.PBH is the relic of early universe,and its mass function and abundance can trace the early evolutionary history.Therefore,PBH may present a new tool of testing bouncing cosmology,and the relevant researches are important.This thesis consists of 4 chapters.In Chapter 1,We shall briefly revisit the background knowledge on areas of black hole,cosmology and GW,including their developing histories,present states and future trends.We will concentrate on introducing the closely related parts of the areas,such as PBH,bouncing cosmology and PTA etc..In Chapter 2,we shall present the study on SMBHB OJ 287.Firstly,we will in-troduce the post-Newtonian(PN)dynamics which can describe the motion of binary system,including the PN equation of orbital motion,PN equation of spin precession and PN formulation of gravitational radiation.Secondly,we shall obtain the orbit of the binary system up to 3PN order,based on the mechanism of the periodic light varia-tion OJ 287.Then,based on the accurate orbit of OJ 287,we can present a forecast of its gravitational radiation around the earth for the next 10 years(2019-2029):the GW will be strong,with a peak amplitude 8 × 10-16,only when the two black holes are around their orbital periastrons(2019-2021),and will become weak after that(2022-2029).In 2022-2029,when the gravitational radiation around the earth is weak,the PTA signals will be dominated by the pulsar term,so it will be a good opportunity to observe pulsar terms.Furthermore,we shall select the most stable 26 millisecond pulsars and calculate their timing residual signals and signal-to-noise ratios(SNRs),induced by the gravi-tational radiation from OJ 287 in the next 10 years.The resulting total SNR of the 26 pulsars ranges from 1.9 to 2.9.Once the gravitational radiation from OJ 287 is observed by PTAs in the future,we will expect to explore OJ 287 through the gravitational wave window,but it requires pulsars with precisely measured distance,such as J0437-4715.Hence,we shall use J0437-4715 to constrain the polarization angle parameter of OJ 287,which is difficult to be obtained by optical observation,and the resulting measurement uncertainty is below 8°.In Chapter 3,we shall reseaches on PBH in bouncing cosmology.At first,we shall parameterize the bouncing model,with the universe devided to 3 stages:matter contraction,bounce and expansion,and then calculate the evolution of primordial den-sity fluctuation.According to our analysis,four parameters are required to describe the bouncing model:the sound speed in the contracting phase c,,the Hubble parameters in the begining and end of the bouncing stage respectively H_and H+,and the pa-rameter T which is related to the time of bouncing phase.Then we shall analyze the scales,abundances and evolutions of the PBHs in the bouncing cosmology,based on a linear gravitational approximation under the comoving gauge.Furthermore,based on the analyses above and the observational results of PBH,we will give 3 methods to constrain the bouncing model:1.calculate the mass fraction of PBHs at the end of the contracting phase,and make it much smaller than unity;2.calculate the energy den-sities of PBHs and Hawking radiations at the bounce point,and make it much smaller than the pressure of background;3.calculate the relic densities of PBHs and Hawking radiations at today,and make it below the observational constraints.Our results im-ply that the matter-bounce models favored by the current CMB observations are safe from creating a large number of PBHs to violate the background dynamics,but CMB observations cannot constrain the evolutionary process of bouncing phase,hence,the constraints of the paramter T from PBHs actually complements the CMB results.In Chapter 4,we shall present a summary and outlook on the thesis.