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John Groh

Graduate Student ResearchersUniversity of California BerkeleySpace Sciences Laboratory
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My work focuses on precision measurements of the Cosmic Microwave Background (CMB).  The CMB has provided a wealth of information about the nature and content of our universe, and continues to offer to provide many more insights.   Of particular interest today is the theory of cosmic inflation, which offers a simple explanation for many observed properties of the universe by postulating a new quantum field which drove a period of accelerated expansion during the first 10^-32s after the Big Bang.  Inflation generically predicts a background of stochastic gravitational waves, and the CMB polarization field is expected to encode information about these primordial gravitational waves.  Many experiments, both past, present, and planed, are attempting to search for this signal in the CMB polarization field.
I primarily work on the Simons Array, a series of 3 microwave telescopes under construction in the Chilean Atacama Desert at an altitude of more than 5000 meters.  Each telescope houses an array of thousands of polarization-sensitive antenna-coupled Transition Edge Sensor bolometers, all cryogenically cooled to less than 0.3 Kelvin.   To maintain high optical throughput, three reimaging lenses per telescope are also cooled to 4 Kelvin, and to mitigate (unpolarized) atmospheric thermal emission, a continuously rotating half-wave plate modulates the polarization signal to a frequency range with more stable noise properties.  The readout of such a large array of TES bolometers is particularly difficult, and we have implemented a custom frequency-division multiplexing scheme using SQUID amplifiers to overcome cryogenic wiring constraints. Our first telescope just came online in late 2018, the second should be operational in 2020, and the third is well into construction to follow soon after.
With the Simons Array data set, we will be able to further constrain theories of cosmic inflation via their imprint on the large-scale patterns in the CMB polarization field.  We can also tighten the upper limit on the sum of neutrino masses via their effect of cosmic neutrinos on the suppression of structure formation in the universe, which we are sensitive to via said structures’ gravitational lensing of the CMB.  More exotic physics, such as parity-violating cosmic birefringence can also be constrained with this dataset.  The Simons Array also will play an important role as a technology demonstrator for future satellite missions that will measure the CMB polarization field with exquisite precision, such as the LiteBIRD mission.