A background of primordial gravitational waves, if present at the time of last scattering, would leave an imprint in the form of a B-mode polarization pattern in the Cosmic Microwave Background (CMB) radiation. Detection of these waves would constitute the first direct evidence of quantum gravitational effects in the early universe, probing fundamental physics at energy scales far higher than what can be investigated in terrestrial laboratories. Furthermore, these gravitational waves are predicted to have been produced by inflation. Interpreted within the inflationary paradigm, observations of the B-mode polarization can be used to measure the tensor-to-scalar ratio, r, or to set a tight enough upper limit on it to usefully-constrain inflation models.
As a result of the profound significance such a detection would hold, the experimental search for primordial B-modes from inflation is proceeding apace! SPIDER is an ambitious balloon-borne CMB polarimeter consisting of six monochromatic, refracting telescopes, sensitive to degree angular scales. The telescopes have a stepped half-wave plate at each aperture, allowing the linear polarization of the sky signal to be modulated without varying instrumental polarization. The telescopes are entirely enclosed within and cooled by a large (~1300 L) liquid helium cryostat with a nominal hold time of 20 days. At the focal plane of each receiver is a large-format array of antenna-coupled, transition edge sensor (TES) bolometers. The detectors are fabricated with low thermal conductances (G) to take advantage of the low atmospheric loading in the stratospheric environment, giving the instrument a greater sensitivity per bolometer than ground-based arrays.
The SPIDER experiment hangs from the launch vehicle in Antarctica
SPIDER had its first successful 16-day flight around Antarctica in January 2015, observing from an average altitude of 35.5 km. Verifying a detection of cosmological B-modes requires 1) verifying the statistical isotropy of the signal using a large sky fraction to check many sub-regions, 2) verifying the form of the Cℓ angular power spectrum using many uncorrelated ℓ-bins, and 3) verifying the EM frequency spectrum, in order to distinguish the signal from astrophysical foregrounds. In this regard, SPIDER occupies a favourable niche, benefitting from the large angular coverage and the access to higher frequency bands afforded by a stratospheric balloon platform. Indeed, the first flight has produced a ~4500 sq. deg (12% sky fraction) map of the Southern sky at 150 GHz and 94 GHz, and achieved overall instrumental sensitivites (NETs) of ~6 μK·√s and ~7 μK·√s, respectively in each of those bands. A second flight, scheduled for the 2017-2018 austral summer, will see the addition of a 285 GHz band, allowing a better characterization of polarized Galactic dust foreground emission.
SPIDER 150 GHz CMB temperature map (equatorial coordinates)
As a graduate student, I carried out some of the systems engineering that enabled the payload to function completely autonomously in the stratosphere. In particular, I worked on the flight power systems, and on all aspects of the pointing control system, including azimuth and elevation drive motors, actuators, electronics, and control software. I also developed an observing strategy for CMB observations that allowed for good cross-linking and even coverage in declination. I am currently participating in analysis of data from the first flight.
The third-generation instrument on the 10 m South Pole Telescope, or SPT-3G, incorporates a new array of multichroic TES bolometers coupled by sinuous antennas. This design produces a broadband, polarization-sensitive receiver, in which each spatial pixel is sensitive to three bands (95, 150, and 220 GHz), and two orthogonzal polarization directions. The instrument also includes cryogenically-cooled re-imaging optics to widen the field of view. As a result, SPT-3G has an order of magnitude increase in the number of detectors (to ~15 000 bolometers) and a factor of ~20 increase in mapping speed over its predecessor, the SPTpol instrument. Having such a sensitive instrument mapping the CMB sky with arcminute resolution opens up the prospect for exciting new physics such as precise constraints on the sum of neutrino masses, new constraints on the Epoch of Reionization, and potentially even a separation of the lensing and primordial contributions to the B-mode angular power spectrum.
SPT-3G detectors close up, as featured on Gizmodo
At Case Western, I operated a cryogenic testbed for the 3G detector wafers, consisting of a mechanical pulse tube cooler and a three-stage Chase Research helium sorption fridge. The combination of these elements allowed us to cool the detector tiles to as low as 0.28 K, which is necessary for their operation on the superconducting transition. In concert with testing groups at Colorado, Chicago, Fermilab, Toronto, and elsewhere, we measured (and continue to measure) optical and dark detector device properties. This detector characterization was crucial to deploying the 3G instrument to the South Pole in time for the 2016-2017 observing season. Ongoing detector characterization to improve instrumental performance in subsequent observing seasons is underway.
BLASTPol was a balloon-borne submillimetre polarimeter consisting of a 1.8 m Cassegrain telescope and three arrays of bolometric detectors with bands centred at 250, 350, and 500 μm. BLASTPol's main purpose was to measure polarized thermal emission from dust in molecular clouds. This enables a measurement of the direction of the plane-of-sky component of the magnetic field in these regions. To date, among submillimetre polarimeters, BLASTPol has a unique combination of angular resolution, sensitivity, and mapping speed, allowing it to trace magnetic fields from dense cores out to the more diffuse surrounding structure. Hence, BLASTPol is suited to examine the relationship between the morphology and the magnetic field direction in these regions. One goal is to understand better the role played by magnetic fields in regulating gravitational collapse during the earliest stages of star formation.
Contours: BLASTPol-measured 350 μm intensity of the Carina Nebula. Line segments: BLASTPol-measured 350 μm polarization amplitude and direction.
BLASTPol data can also be used to investigate the dust polarization spectrum in molecular clouds, i.e. the variation of the dust linear polarization fraction with wavelength. Such measurements can potentially be compared with theoretical models to test various proposed dust grain alignment mechanisms, some of which depend on the dust radiative environment. In graduate school, I investigated the polarization spectrum in one of BLASTPol's more dynamic targets: the Carina Nebula. I am continuing this work, with the inclusion of a fourth waveband from Planck 353 GHz (~850 μm) observations of the same region.