Physicists construct SPIDER detector to study theories of universe expansion

A team of University scientists has been working with NASA's Jet Propulsion Laboratory, the California Institute of Technology and the University of Toronto to build an instrument that will help cosmologists gain more insight into the expansion and homogenization of the very early universe.

Physicists put on hard hats on Thursday and used a crane to hoist the instrument into a carbon fiber gondola, which will act as a sturdy frame for the device as it flies over Antarctica in the austral summer.The instrument, known as SPIDER, will be entirely built on campus and then shipped to Texas, where the finishing touches of the building process will be made in June. SPIDER will depart for its final destination in November, when it will travel by boat to Antarctica. From the frozen continent, it will be launched in an enormous balloon to gather data from the sky for about 20 days in December.

SPIDER, which is being constructed in the basement of Jadwin Hall, is a balloon-borne polarimeter, a device that uses extremely sensitive cameras to collect information on the spatial orientation of photons of light. With this information, physicists and cosmologists will be able to extrapolate information on the origins of the universe, especially the nature of the gravitational waves produced by the Big Bang.

A window into the origins of the universe

Current theories hold that during the earliest period of the universe, an extremely rapid explosion of space commonly called the Big Bang was followed by a period of inflation. While there are whole classes of theories of inflation, the most accepted ones hold that gravitational waves were generated in those first moments of expansion.

“You can imagine that, if the universe is sort of ripping itself apart,that it might generate a bunch of these gravitational waves. Space-time itself is being ripped apart, so maybe you’ll get fluctuations in space-time, and those fluctuations in space-time are gravitational waves,” physics professor William Jones explained. Jones has been leading the construction of SPIDER at Princeton.

According to inflation theory, these fluctuations propagated through the primordial plasma, squishing and bending the thick, burning soup of subatomic particles. This process creates the right environment for what is called “polarized” light. SPIDER will observe this particular light coming from the Big Bang, specifically from the Cosmic Microwave Background. CMB can be detected coming from all directions at almost the same wavelength, suggesting that at one time long ago, it was emitted from the source of the universe. For this reason it has been affectionately dubbed by cosmologists "baby picture of the universe."

When scientists detect this light after it has been traveling for more than 13 billion years, the signal actually detects and characterizesinflation by showing the energy of its gravitational waves. According to theories of the early universe, light became polarized or rotated in a certain direction because of the way it radiates through matter. Therefore, polarized light might be called a “smoking gun”of inflation.

As Zigmund Kermish, a postdoctoral fellow working on the project, explained, “If we find this signal, it really is a very important thing for scientists to say that this type of generic inflationary model is right, and then we can start narrowing in on exactly what that model is.”

Jones said the instrument will be able to clarify the distinctions between many current theories of inflation. “SPIDER is designed explicitly to inform us about the nature of those early universe theories — to try and make up rigorous observational tests of which of those classes of theories are consistent with the data and which aren’t," he said.

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The data collected will augment previous studies on the CMB, such asthe Planck Experiment, which also involved members of the SPIDER team.

“Other Cosmic Microwave Background signals give us information about inflation, but this [particular type of polarization] is the strongest indicator,” explained Jon Gudmundsson GS, a physics student who has worked on both projects.

“The combination of the Planck and SPIDER data will be very powerful,” said Jones.

A national collaboration

During the next few months, the team at Princeton will finish building and collecting parts from its collaborating institutions. The physicists have been slowly accumulating different parts of the instrument, including the six detectors that collect the data and operate collectively as a state-of-the-art polarization detector, or polarimeter.

As Kermish explained, the detectors work as extremely sensitive cameras so that the light is focused onto an intricate cross-hatching of tiny antennas, which allow the team to observe how intensely the photons of light are oriented in any particular direction. The six detectors are essentially telescopes that are arranged like 1.3-meter bullets in a pistol, making the entire apparatus itself cylindrical — the size of a small cubicle.

The detectors must be kept cool as they collect data to avoid contamination data that might come from either the smallest vibrations of heat or waste particles, called phonons, coming from one detector to another. Vibrations from heat can send a false signal to the detector.

In order to lower the temperature in the apparatus, SPIDER is equipped with a main tank of very cold liquid helium. The physicists have connected another tank, which holds a substance called “superfluid helium-4” that flows like a liquid but creates no friction and can be used as an even colder bath for the detectors.

The project is the result of collaboration by many University scientists, including undergraduates. Physics major Will Taylor ’14 worked in the University's machine shop this past summer, making the straw-like system that transports the liquid helium from one tank to the other. He said that he found SPIDER’s ability to push the boundaries of this type of experiment exciting. “It’s quite neat to see all of this science come together,” he added.

Dust grains

SPIDER will not only study the polarization of light from the early universe, but it will also collect data about light that has been polarized by dust grains, or random molecules, in the outer atmosphere, according to Dr. Aurelien Fraisse, a postdoctoral fellow in the physics department who has been working on SPIDER.

Scientists know that when light encounters a dust grain, some of the light is absorbed, but some is emitted with the same wavelengths as that coming from the Cosmic Microwave Background. But if the dust grain has a particular shape or orientation, the light shining through it will be rotated and differing amounts of light will be emitted or absorbed.

By looking at the kind of the light coming from these dust grains, physicists will be able to tell what general orientation and composition the dust grains have, according to Fraisse. Because dust in the outer atmosphere actually positions itself according to the magnetic field of the galaxy, the polarized light can inform scientists about the universe as well as the Earth’s own galaxy, the Milky Way.

As the team continues to build cables, testing equipment, thermal contacts and windows, Gudmundsson saidthat while the entire process of building SPIDER has been rewarding, they’re looking forward to collecting the data successfully and safely in Antarctica this winter.

“The best thing about it is when you get a problem that you think you can maybe solve, and you think you can solve it, and maybe someone has solved it before ... you start, and you have noidea how it’s going to come out," said Gudmundsson. "You attack it once or twice and learn from it, but after a few weeks and a lot of hard work you eventually have something that just works."

"That’s a very fulfilling experience — starting with a piece of paper and ending with a scientific instrument," he added.