Measurements of the linear polarization of the cosmic microwave background (CMB) offer a unique probe of the early universe. The currently most-successful model of modern cosmology includes an epoch of rapid expansion of space-time shortly after the Big Bang. Gravity waves, excited during this inflationary epoch, propagate through the universe and interact at much later times to impart a distinctive polarization pattern to the CMB. Detection of this gravity-wave signature would have profound consequences for both cosmology and high-energy physics. Not only would it establish inflation as a physical reality, a detection would test physics at energies above 10^15 GeV, more than 12 orders of magnitude beyond what is accessible in particle accelerators, yielding new insight into the nature of Grand Unification and quantum gravity.
The predicted polarization signal in the CMB is weak and its measurement complicated by much brighter emission from the Milky Way galaxy. To effectively separate these two components, measurements must be accurate and made over a wide range in frequency. One of the most promising techniques for efficiently measuring the polarization of the CMB is in constructing many-pixel, integrated polarimeter arrays in which the signal in each channel is "chopped" electrically between opposing polarization states on time scales (~100 Hz) much shorter than those associated with the observing conditions. To date, these integrated polarimeters have lacked one key item: low-capacitance, low-insertion loss switches that can operate at cryogenic temperatures for long periods of time.
The MEMS devices to be developed by PI's Barker and Kogut are Micro-Electro-Mechanical switches that appear to satisfy all of the necessary criteria for completing these "polarimeters on a chip". Central to this specialized effort is the combination of state-of-the-art development facilities at the University of Virginia Microfabrication Laboratories and the Detector Development Laboratory at NASA's Goddard Space Flight Center. Together these facilities enable fabrication and cryogenic testing of sophisticated electrical networks operating at frequencies up to 250 GHz.
Going beyond the development of a technique for obtaining fundamental information about the early history of our Universe, the work to be carried out under this award will involve graduate and undergraduate students in the science and engineering of mm-wave instrumentation and produce new technologies that will benefit emerging applications in the fields of remote sensing and wideband communications.
Measurements of the linear polarization of the cosmic microwave background (CMB) offer a unique probe of the early universe. The "concordance" model of modern cosmology posits an epoch of exponential expansion of space-time shortly after the Big Bang. Gravity waves, excited during this inflationary epoch, propagate freely through the universe and interact at much later times to impart a distinctive pattern of linear polarization in the CMB. Detection of this gravity-wave signature of inflation would have profound consequences for both cosmology and high-energy physics. Not only would it establish inflation as a physical reality, it would also provide a direct, model independent determination of the relevant energy scales. A detection would test physics at energies above 1015 GeV, more than 12 orders of magnitude beyond those accessible to direct experimentation in particle accelerators, yielding new insight into the nature of Grand Unification and quantum gravity. Recent results from the Wilkinson Microwave Anisotropy Probe suggest that a detectable inflationary signal should exist. The expected signal is faint, however, and obscured by much brighter emission from the Galaxy. Reliable separation of a faint polarized signal from a bright unpolarized background requires some method of modulating only the polarized component of incident radiation. The current state of the art modulation techniques rely on macroscopic moving parts such as rotating half-wave plates or wire grids for the required signal modulation. The maximum practical size of such systems (∼50 cm diameter) limits the size of the instrument focal plane and hence the number of detectors, while the relatively slow speeds (a few Hz) restrict possible scan strategies. In addition, the requirement of macroscopic moving parts with high duty cycles creates reliability problems for cryogenic systems. Radio-Frequency Micro-Electro-Mechanical Systems (MEMS) technology presents a promising alternative. MEMS switches are miniature surface micromachined components providing controlled motion over short (micron) distances to create either an open- or a short-circuit across a microwave transmis- sion line. Embedded in an appropriate circuit, MEMS switches provide a critical signal-modulation technology with significant advantages over traditional modulation schemes. We propose to develop and demonstrate mm-wave MEMS switches suitable for inclusion in a cryogenic "polarimeter-on-a-chip" for the next generation of CMB and millimeter-wave polarimetry. RF MEMS (Radio Frequency Microelectromechanical Systems) offer low insertion loss, low power consumption, and high-linearity that makes them ideal devices for tasks like signal routing and impedance tuning. Our work investigates the integration of series DC-contact RF MEMS switches as shown in Figure 1 for use in low-loss superconducting circuits with applications for phase modulating weak signals in radio astronomy.
