Accurate Weak Lensing Measurement And Its Application

When the photons emitted from the background galaxies pass through the foreground density field,their trajectories will be distorted under gravity,leading to the deformation of these background galaxies,which is called the gravitational lensing.According to the degree of galaxy deformation and the masses of the foreground lenses,we can distinguish three regimes:strong lensing,weak lensing and microlensing.Weak lensing is a powerful tool in cosmology.In this paper,we mainly focus on the accurate measurement and application of weak lensing.In the first chaper,we introduce the corresponding cosmological backgrounds.In chapter 2,we do the shear measurement based on the galaxy images.Currently,there are already some popular shear measurement methods,for example,KSB[1-3],Shapelets[4,5],Lensfit[6]and so on.In this paper we use the FQ shear measurement method.We construct a pipeline,including the source detection,the background removal,the star-galaxy seperation,the noise reduction,the PSF reconstruction and the shear measurement.In the chapter two,I would focus on introducing the works that I was involved in druing the early stage of the development of the pipeline.Starting from the images provided by the CFHTLS Wide Survey,we have sucessfully measured the weak lensing signals using the pipeline.Also,we have tested the accuracy of the shear measurement pipeline in the paper.After generating the shear catalogue,we have done some primary scientific calculations.In chapter 3,we use weak lensing to detect the low density regions.Low density regions are ideal places for probing the nature of dark energy,since they are less affected by the nonlinear structure evolution,as well as baryonic physics.In this work,we define a new cosmological probe:low-density-positions(LDPs).Unlike identifications of individual voids,LDPs are defined by excluding the foreground bright galaxies from the sky with a critical radius in projection.We then study the stacked lensing signals around the LDPs.We find that this method can lead to lensing signals that are significant enough for differentiating several dark energy models.In this work,we use the CFHTLenS catalogue to define LDPs,as well as measuring their background lensing signals.Besides,with subhalo abundance matching(SHAM),we do the similar caculations in the N-body cosmological simulations.By comparing the LDP lensing signals from observation and simulations,we make preliminary constraints on the dark energy equation of state.In chapter 4,we use weak lensing to reconstruct the halo mass functions.In the Cold dark matter model,the halo mass function is sensitive to the cosmological parameters.However,due to the uncertainty in the observable-mass relation in observations,biased halo mass function is measured,which is called the Eddington bias.Unless the scatter in the observable-mass relation is well known,which is difficult even with numerical simulations,exact correction for the bias is not easy.In this work,we find an interesting feature for the halo mass function in the ACDM model:the total halo mass within each evenly-spaced logarithmic mass bin is almost the same over a large mass range.This feature make us able to construct an almost bias-free halo mass function,with the halo mass estimator and stacked weak lensing measurements.We test this idea using cosmological simulations.In chaper 5,we apply LDP to the measurement of the Integrated Sachs-Wolfe effect(ISW)effects.In fact,the direct evidence for the dark energy in observation is the ISW effect.Currently,when directly stacking the temeprature fluctuations(?T)around the voids,people found that the observational signals are much larger than the signals from simulations.Here we treat the LDPs as the matter density tracers,and calculate the correlations bettwen the LDP number fluctuations and the CMB ?T map.In this way,we gain signals with high signal-to-noise ratio,which is much larger than the one measured in the traditional ways for the voids.In the N-body simulations,we construct the ?T field making use of the density field,and do the similar measurement.After considering the observational effects,we get signals that are consistent with the one from observation.This work is still in progress,and we hope to use more simulations with large volumes to study the cosmic variance.At last,we summarize the whole paper in chapter 6.We construct several new cosmological probes in this paper,and have done preliminary tests.We plan to apply these method to larger surveys with more accurate redshift measurements in the next step,helping us to learn more about our universe.