An Alternative Channel For High-Mass Binary Black Holes

By a method of population synthesis we construct a model of dark matter?DM?accretion onto binary black holes?BHs?,and investigate the merger rate of the binary BHs.It assumes that the binary stars evolve into binary black holes via normal evolution,and the binary black holes accrete dark matter and merge due to gravitational wave radiation.We find that the merger rate can weakly increase?less than 10%?.However,the DM accretion can efficiently enhance the masses of binary BHs.For the model without the DM accretion,the results for the models with low metallicities?Z=0.0001 and 0.001?can cover all observational data?the binary black mass in GW150914,GW151226,GW170104,GW170608,GW170729,GW170809,GW170814,GW170818 and GW170823?,while these for higher metallicities?Z=0.01 and 0.02?can not explain the massive binary BHs.The main reasons are that the chemical homogeneous evolution is more efficient for the rotating stars with low metallicity.For the model with the DM accretion,the masses of binary BHs increase.For the results of the models with Z=0.0001,0.001,and0.01 can cover all observations well.The result for Z=0.01 is a good model to explain the effect of dark matter,the model Z=0.01 without the DM accretion can not explain the GW150914,GW170104,GW170729,GW170809,GW170814,GW170818 and GW170823,while with the DM accretion,it can cover all observations very well.For the higher metallicity?Z=0.02?,our model can not explain the mergers of high-mass binary BHs like GW150914,GW170104,GW170729,GW170809,GW170814,GW170818?GW170823.We estimate that the merger rate of binary BHs lies between 55Gpc^-3yr^-1and 197Gpc^-3yr^-1.The gravitational waves signals were detected by LIGO?GW150914,GW151226,G-W170104,GW170608,GW170729,GW170809,GW170814,GW170817,GW170818 and GW170823?,because the sensitivity of the detector has improved.Memory effects is a spe-cial character of gravitational waves.It is need more sensitive detector to detect the mem-ory effects.Study the nature of the singularities of plane gravitational waves spacetime is necessary before study the memory effects.In this thesis,we study the properties for sin-gularities of plane gravitational waves spacetime.Similar to the Schwarzschild coordinates for spherical black holes,the Baldwin,Jeffery and Rosen coordinates for plane gravitation-al waves are often singular,and extensions beyond such singularities are necessary,before studying asymptotic properties of such spacetimes.We find that the singularities can be in general characterized by??u??e^-U?u?/2=?u-u_s?^????u?,???u?=^??_n=0?_n?u-u_s?ⁿ.Then,we find that the tidal forces and distortions are finite across the singular surface u=u_son-ly in two particular cases,in which we have?i??=₂¹or?ii??=1.We find that A?u?is finite and well-behaved across u=u_sfor?=1/2,and A?u?is a coefficient in Brinkmann coordinates(ds²=-2d?ud?v+d?y²+d?z²+₂¹A??u?^?y2-?z2d?u²).However,in the case?=1,we found that A?u?is finite and well-behaved across u=u_sonly when?₁=?₂=?₃=0,where?_nare the expansion coefficients of???u?.If any of these three coefficients is not zero,A?u?will be singular across u=u_s,although the distortions of the freely falling observers consid-ered in this paper are finite.There are two possibilities for these cases:?i?The corresponding spacetimes are indeed singular;?ii?The corresponding singularities are coordinate ones,which can be removed by proper coordinate transformations.In this thesis,we study the nature of the singularities and find that some of them actually represent real spacetime singularities,and any extension beyond them is impossible physically.In fact,they already represent the boundaries of such spacetimes.As a result,such spacetimes cannot be used to study their asymptotical properties,such as memory effects and soft graviton theorems.