A Camera for the Cosmos
You don’t need a time machine to see what the universe looked like more than 13 billion years ago. You just need the right camera, one good enough to take photos of the faint cosmic microwave background (CMB) radiation left over from the big bang.
Each photon in this radiation is a time capsule that carries information about the face of the universe in its infancy. Collect enough of them and you have a snapshot—specifically, a snapshot of temperature.
Portrait of a young universe
Just as a hot iron glows red with visible light, the early universe glowed with microwave radiation as it cooled. The peak wavelength of this glow, about 2 millimeters, corresponds to a specific temperature derived from the blackbody spectrum: about 2.7 Kelvin. This temperature is extremely consistent in all parts of the sky. Though it has interesting cosmological implications, it is very boring to look at.
Subtract this 2.7 Kelvin signal, and something exciting happens. Slight differences in the microwave radiation become visible, and the map of the CMB attains structure.
To take a picture of this structure, you need a very sensitive camera. The CMB signal is already very weak, and the interesting variations within the overall signal are fainter still. The polarization of light from the CMB is interestingly nonuniform as well, but measuring polarization anisotropy requires even more sensitive detection.
Condensed matter meets cosmic microwaves
As an undergraduate majoring in physics and mathematics at Drexel University, I spent two six-month internships at Argonne National Laboratory just outside of Chicago, Illinois, working on detectors for the South Pole Telescope. My devices will be deployed this year to map the CMB at small angular scales. There’s a very good chance that I will also be going to the South Pole in the future!
The pixels in this telescope’s cameras are similar to those in other telescopes investigating the CMB. They are superconducting devices called transition-edge sensors (TES). A TES detector is so sensitive that it can resolve the energy of a single photon.
My task as part of the detector fabrication team was to construct the next generation of detectors to be deployed in the telescope. I spent lots of quality time in a clean room forging 150-pixel TES arrays on six-inch silicon chips. I was also involved in the design and testing of the early devices.
A TES detector consists of three small parts, all with sizes on the order of microns. Detection occurs when a photon heats an absorber. The temperature is then measured by a thermometer. Finally, the heat is dissipated through a thermal link. Our thermometers are special because they are superconductors. The special properties of the superconducting transition give TES detectors their sensitivity.
Measuring the largest features in the universe, I’ve learned, requires a deep understanding of materials at much smaller scales.
Superconductors have zero resistance below the superconducting transition temperature. Cool aluminum down from room temperature, for instance, and at around 1.2 Kelvin the resistivity of the metal drops from a finite value to zero. This transition is often quick; it can occur in less than a millikelvin.
A TES thermometer is a superconductor cooled precisely to its transition temperature. Smack dab in the middle of its transition, the material is not quite superconducting and not quite normal metal. In this state a very small variation in temperature, such as that created by an incoming photon, generates a sizable shift in resistivity. Measuring this shift in resistance probes the temperature of the absorber. From the temperature of the absorber, the energy of the photon can be deduced.
My research has focused on controlling and tailoring this transition. I’ve worked on new TES designs that will be used in conjunction with antennae that couple to CMB radiation. The antennae are specifically designed to couple a broad band of frequencies and discriminate between different kinds of CMB polarization of interest to cosmologists. By incorporating band-pass filters into the new pixel designs, this broadband signal can be divided into smaller frequency bands, allowing each pixel to measure multiple frequencies.
Measuring the largest features in the universe, I’ve learned, requires a deep understanding of materials at much smaller scales. Condensed matter physics and observational cosmology are permanently entangled. This cosmic connection between two seemingly separate worlds has and will continue to provide insight into the beginning of it all.
