Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
The Sunyaev-Zeldovich (SZ) effect is a spectral distortion in the Cosmic Microwave Background (CMB), due to up-scattering of CMB photons by high energy electrons in clusters of galaxies or any cosmic structure. The Planck satellite mission has measured the spectral distortion with great sensitivity and has produced a full-sky SZ (y) map, which can be used to trace the large-scale structure of the Universe.In this dissertation, I construct the average SZ (y) profile of ∼ 65,000 Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey Data Release 7 (SDSS/DR7) using the Planck y map and compare the measured profile with predictions from the cosmo-OWLS suite of cosmological hydrodynamical simulations. This comparison agrees well for models that include feedback from active galactic nuclei (AGN feedback).In addition, I search for the SZ signal due to gas filaments between ∼260,000 pairs of LRGs taken from the Sloan Digital Sky Survey Data Release 12 (SDSS/DR12), lying between 6-10 h −1 Mpc of each other in the tangential direction and within 6h −1 Mpc in the radial direction. I find a statistically significant SZ signal between the LRG pairs. This is the first detection of gas plausibly located in filaments, expected to exist in the large-scale structure of the universe. I compare this result with the BAHAMAS suite of cosmological hydrodynamical simulations and find that it predicts a slightly lower, but marginally consistent result.As an extension of my MSc. thesis work, I study CMB polarization. The B-mode component of CMB polarization is an important observable to test the theory of inflation in the early universe. However, foreground emissions in our own galaxy dominates the B-mode signal and therefore multi-frequency observations will be required to separate any CMB signal from the foreground emission. I assess the value of adding a new low-frequency channel at 10 GHz for the foreground removal problem by simulating realistic experimental data. I find that such a channel can greatly improve our determination of the synchrotron component which, in turn, significantly improves the reliability of the CMB separation.
Master's Student Supervision (2010 - 2020)
In this MS.c. thesis, we demonstrate a method for estimating the expansion history of the universe using the hydrogen intensity map from the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which will be generated in the near future. This map will be in angular and redshift space, where redshift of the hydrogen due to the expansion serves as a time variable. The expansion history, dependent on cosmological parameters via the Einstein equations, determines the distance away from us in units of a grid comoving with the expansion at which light of each redshift was emitted. We use knowledge of the fixed comoving distance, approximately 150 megaparsecs, that baryon acoustic oscillations, or primordial sound waves, traveled away from the centers of matter perturbations, where there is a corresponding peak in the matter correlation function subject to uncertainty of the initial quantum mechanical fluctuations. We explain the method by which we fit the correlation function to a model for expansion determined by the equation of state of dark energy, to constrain this parameter. We test the method using a three-dimensional realization of the theoretical matter power spectrum calculated from CAMB (Code for Anisotropies in the Microwave Background), providing an estimate of constraints obtained from a small redshift region spanning one sound horizon diameter in redshift space assuming a constant equation of state of dark energy and fixed values of the other cosmological parameters. We explain how to generalize this method to a more complete analysis.
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a transit interferometer located at the Dominion Radio Astrophysical Observatory in Penticton, BC. It is designed to map large- scale structure in the universe by observing 21 cm emission from the hyperfine transition of neutral hydrogen between redshifts 0.8 and 2.5. CHIME will perform the largest volume survey of the universe yet attempted and will characterize the BAO scale and expansion history of the universe with unprecedented precision in this redshift range. CHIME achieved first light in the fall of 2017 and instrument commissioning is underway. In this work I present sensitivity forecasts and derive constraints on cosmological parameters given CHIME’s nominal survey. The broad redshift range of the observations will enable tight constraints to be placed on the Hubble constant H0 , independent of CMB or local recession velocity measurements. Precision measurements of this epoch will shed new light on the tension between direct measurements of the Hubble constant vs. those inferred from high-redshift observations, notably the CMB anisotropy. CHIME measurements together with a prior on the baryon density from measurements of deuterium abundance are enough to place constraints on H0 at the 0.5% level assuming a flat ΛCDM model, with uncertainty increasing to ∼ 1% if curvature is allowed to vary, or up to ∼ 3% for a dark energy equation of state with w/= −1. Including priors from CMB measurements, in the scenario where the datasets are consistent, narrows these uncertainties further, most significantly in the model where w is a free parameter.
The polarization of Cosmic Microwave Background can help us probe theearly universe. The polarization pattern can be classified into E-mode and B-mode. The B-mode polarization is a smoking gun of cosmological inflation.PIXIE is an in-proposal space telescope observing CMB polarization. Itis extremely powerful to extract CMB polarization signal from foregroundcontamination. The second chapter of this thesis summarizes my work onoptimizing the optical system of PIXIE. I run a Monte-Carlo Markov Chainfor the instrument parameters to maximize the value ”Good” which judgesthe behavior of the instrument. For the optimized instrument, with all kindsof noises from inside instrument and wrong polarization taken into account,good rays from the sky make up of 15.27% of all the rays received by thedetector. The instrument has a 1.1° top-hat beam response.The third chapter summarizes my work on studying the potential con-tamination in the reconstructed y map by doing cross-correlation betweentSZ signal and weak lensing. The weak lensing data is the convergence mapfrom the Red Sequence Cluster Lensing Survey. I reconstruct the tSZ mapwith a Needlet Internal Linear Combination method with 6 HFI sky mapsmade by Planck satellite. The reconstructed cross correlation is consistentwith Planck NILC SZ map. I take Cosmic Infrared Background (CIB) andgalactic dust as two potential source of contamination in the reconstructedmap. I find that κ × CIB contributes (5.8 ± 4.6)% in my reconstructed NILCy map for 500
The following document describes pursued studies to understand the properties of radio frequency interference (RFI) which affects the quality of the data of the Canadian Hydrogen Intensity Mapping Experiment Pathfinder at the Dominion Radio Astronomy Observatory in Penticton, British Columbia. The Canadian Hydrogen Intensity Mapping Experiment is a challenging project aimed to trace large scale structure by observing the 21cm emission line of neutral hydrogen in the frequency spectrum 400-800MHz to research the nature of Dark Energy.RFI is terrestrial signal caused by radio bands, TV stations, satellites etc. that produces unwanted disturbances in the frequency spectrum which adds power to the data. It represents a challenge to measure faint sources in the sky and we seek ways to identify it based on its statistical properties such as non-Gaussianity.We have designed algorithms that aim to identify and flag RFI in our data. Digital TV bands cause permanent corruption in the affected frequency bins and account for a 19% loss of bandwidth.The 5 sigma threshold cut searches for time-varying RFI in each frequency bin. Outliers above 5 standard deviations are iteratively flagged but not all of the occurring RFI were recognized due to non-Gaussianity.The median absolute deviation cut is a robust statistical method that uses sky data only. Identification of short-lived and long-lived RFI occurrences originating mainly from the sky has been successful.A correlation coefficient algorithm uses a combination of a reference RFI antenna sensitive to the horizon and a sky antenna to find correlated signals that are significantly above expected thermal noise of the radiometer while disregarding correlation due to sky signal. RFI at the horizon is well recognized by this method.
The fluctuations in the cosmic microwave background(CMB) contain a lot of information on the history and composition of our universe. In particular, the rich detail about our early universe is included in the angular power spectra of the CMB fluctuations, which constrains the cosmological parameters in current models of the universe. The latest cosmological data strongly support an inflationary Lambda CDM cosmology with a minimal six parameters to describe our universe. The next challenge in cosmology is to probe the physics of the inflationary period by looking for the signature of primordial gravitational waves in the polarized CMB. CMB polarization was generated at last scattering by scalar and tensor perturbations in the primordial fluid. The tensor perturbations are produced by the stretching of space-time by gravitational wave fluctuations, while scalar perturbations are produced by density fluctuations in the primordial fluid. The ratio of the tensor to scalar perturbation amplitude, r, is a key tracer of the physics of the inflationary epoch, which is deeply connected to the energy scale of inflation in a standard inflationary model. A local quadrupole anisotropy in the radiation field at the time of decoupling causes the linear polarization in CMB through Thomson scattering by electrons. The CMB polarization can be decomposed into two rotationally invariant quantities, called E and B. The CMB B-mode is a direct tracer of the tensor perturbations caused by gravitational waves in the inflationary period of the universe. Thus, the detection of B-mode has currently been dubbed the ''smoking gun'' of inflation. However, the galactic foreground emissions also have much stronger E- and B-modes polarization. We intend to produce half-sky maps of total intensity and linear polarization at 10 GHz. This data would probe galactic synchrotron emission and also can help constrain the so-called anomalous emission. Therefore, the maps can be used with other surveys such as WMAP and Planck to subtract galactic foreground emissions and obtain more precise CMB data. In addition, the data will give us information about galactic emission components such as synchrotron, free-free, thermal dust and anomalous emission in the microwave range.