Mark Halpern

The standard model of cosmology (ΛCDM ) is able to statistically describethe observable Universe with a small set of parameters that have been mea-sured to a percent level precision through observations of the Cosmic Mi-crowave Background (CMB) [Aghanim et al. 2020]. While this model showsa remarkable agreement with the data, it still can’t explain all the featureswe observe in the CMB. One of the biggest mysteries resides on the factthat CMB radiation sky is extremely uniform in temperature, despite thatit was generated when the Universe was so young that just small patches ofsky could be in causal contact. As an add on to the Standard CosmologicalModel the theory of inflation [Guth 1981] was introduced, to solve this setof puzzles. One prediction from Inflation is the presence of a background ofgravitational waves, in the primordial Universe, that would have produceda unique parity-odd pattern in the polarization of the CMB, also known asB-Modes. Until now no CMB experiment was able to find B-Modes. AsB-modes are very low in signal (O(Bmodes) ∼ 10⁻⁷O(∆T )), finding themis one of the biggest challenges of Cosmology today and it requires an ex-tremely sensitive and low noise instrument, high level of systematics controland an accurate model for foregrounds subtraction. In this thesis I will gothrough the design of BICEP Array (BA), the latest multi frequency instru-ment part of the BICEP/Keck (BK) experiment, which is optimized for thesearch of B-Modes. I will show the instrument tuning and characterizationprocess and explain the observation strategy. I will also explain the latestresults from the BK data, show how these data allow a huge step forward inour understanding of inflation and what are the prospects in the search forB-Modes with BICEP Array. I will then show how, using a Water VapourRadiometer located next to the telescope, I developed a new method foratmospheric noise subtraction that could allow us to recover a fraction ofthe low-frequency B-Modes power spectrum, which is otherwise lost duringdata processing.

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Measurements indicate that at present the expansion of the universe is accelerating even though the long-distance gravitational force is attractive. To help understand this late-time cosmic acceleration, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is designed to measure the Baryon Acoustic Oscillations (BAO) scale, therefore the expansion history of the universe in the redshift range 0.8
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Observations suggest that almost half of the total light emitted by stars in the Universe is absorbed by dust, and the emission is re-radiated at far-infrared and submillimeter wavelengths. Dusty star-forming galaxies play a significant role in the stellar mass build-up at high redshift, but their contribution to the cosmic star formation rate density at z > 4 is still unknown, due to the currently limited availability of statistically significant high-redshift dusty galaxy samples. In this thesis we analyze data from two large area surveys, the HerMES Large Mode Survey (HeLMS) and the Herschel Stripe 82 Survey (HerS), observed with the Herschel-SPIRE instrument at far-infrared wavelengths of 250, 350 and 500 μm. We describe the process of constructing maps from detector data that provide an unbiased estimate of the sky signal, then we use a map-based detection method to assemble a large catalog of candidate z > 4 dusty star-forming galaxies detected in HeLMS. The large area of the survey allows us to detect a significant number of sources and we are able to determine the differential number counts of these galaxies at 500 μm. We find an excess of such high-redshift galaxies compared to model predictions, and our counts suggest strong evolution in their properties.We examine the properties of our sources at different wavelengths. Follow-up observations with ALMA, SCUBA-2 and ACT strengthen our initial assumption that the detected population consists of high-z dusty galaxies with their spectrum dominated by thermal dust emission, best fitted with an optically thick modified blackbody. These follow-up observations also allow us to examine the biasing effects in our number counts due to blending of nearby sources. We also investigate the mean dusty star formation activity in moderate redshift massive galaxy clusters detected by the Atacama Cosmology Telescope. We find that, on average, there is an excess of far-infrared emission in the line of sight of these clusters. Finding dusty star-forming galaxies in massive clusters implies that the environment can affect the star formation activity in galaxies.

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Observations of the Cosmic Microwave Background (CMB) are crucial components of our understanding of cosmology. Modern high resolution, ground-based CMB survey instruments provide important information about the mass and energy content of our present Universe and the high-energy physics of the Big Bang.In this work we present several aspects of our work on the Atacama Cosmology Telescope (ACT), a 6m telescope in Northern Chile that observed the CMB in three millimetre wavelength bands from 2007–2010. We begin with a description of the Multi-Channel Electronics readout system, an important component of the data acquisition systems for ACT and several other CMB observatories. The system provides room-temperature electronics and software for controlling and reading out arrays of Transition Edge Sensor bolometers via a cryogenic time-domain multiplexing system.We next present our measurement of the ACT point spread function, or beam, using observations of Solar System planets. An accurate understanding of the beam and its covariant error is essential for interpretation of astrophysical and cosmological signal in the ACT data. We then use our understanding of the beam and the instrument calibration to measure the brightness temperatures of Uranus and Saturn at millimetre wavelengths. Precise measurements of planetary brightnesses provide convenient calibration sources for other observatories at these wavelengths.Finally we present a sample of galaxy clusters detected in the ACT maps. We develop a new approach for the analysis of Sunyaev-Zeldovich signal that incorporates a model for the typical cluster pressure to better understand selection effects and evaluate cluster masses. Addressing the current level of systematic uncertainty in the overall mass calibration of clusters, we explore the cosmological constraints obtained when calibrating the mass relation based on pressure profile measurements from X-ray data and from models that take different approaches to the cluster physics. Ultimately we use dynamical mass estimates based on optical velocity dispersion measurements to obtain constraints on the amplitude of scalar fluctuations, the matter density, the Dark Energy equation of state parameter, and the sum of the neutrino mass species.

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