Observational Constraints On Screened Modified Gravity

Einstein's theory of general relativity has been the accepted theory of gravity.De-spite this,there are still some mysteries it cannot account for.As we know,GR is incomplete in the ultraviolet regime where it should be replaced by a(still unknown)quantum theory of gravity.In addition,the evolution of the universe cannot be properly described by GR unless one adds the dark matter and dark energy as the supplementary materials in our Universe.Therefore,alternative theory of gravity is the direction that is supposed to be worth a try.This thesis explores a class of theories of screened modified gravity(SMG)which is a kind of scalar-tensor theory with screening mechanism.SMG can generate the screening effect to suppress the fifth force(scalar force)in high density regions and allow theories to evade the solar system and laboratory tests.Meanwhile,the potential of scalar field in the theories can drive the acceleration of the late universe.In this thesis,we calculate the parameterized post-Newtonian(PPN)parameters,the effective gravitational constant and the effective cosmological constant for SMG with a general potential and coupling function.The dependence of these parameters on the model parameters of SMG and/or the scalar charge of the source object are clearly presented.The original chameleon has been ruled out by the combined constraints of the solar system and cosmology.We investigate how the screening mechanisms in SMG affect the post-Keplerian effects in binary pulsar system,and calculate in detail the five post-Keplerian parameters:?,the average rate of periastron advance;?,the amplitude of the Einstein time delay of the pulsar signal;the range ? and shape s of the Shapiro time delay of the pulsar signal;Pb,the orbital period decay rate due to the emission of gravitational radiation.It turns out that these parameters depend not only on two masses of the binary pulsar system,but also on their two scalar charges.We investigate the five neutron star-white dwarf(NS-WD)binary pulsars:PSRs J1141-6545,J1738+0333,J0348+0432,J1012+5307 and J0751 + 1807.By performing Monte Carlo simulations,we obtain the constraints on the system parameters for these five binary pulsars,respec-tively.We find that SMG can pass the combined constraints from the post-Keplerian effects,and PSR J1738+0333 is still the most constraining system for the scalar charge.In particular,we find that the scalar charges of the five white dwarfs all are not only of the same order of magnitude(?10-3),but also marginally inconsistent with zero at 95.4%CL,which could be a slight indication of departures from GR.Furthermore,we find that for low-mass white dwarfs there exists a possible inverse correlation between mass and scalar charge,as predicted by the screening mechanism in SMG.In addition,two independent constraints on the scalar background ?VEV from PSRs J1738+0333 and J0348+0432 are consistent with each other,we derive the combined constraints on?VEV = 2.30-1.83+2.07×10-8MPI(95.4%CL)by the observations of these two systems.Finally,as an application of these results,we focus on three specific theories of SMG(chameleon,symmetron and dilaton models),and derive the constraints on the model parameters by combining the observations of binary pulsar and solar systems.