Illinois Physics Professor and Illinois Center for Advanced Studies of the Universe (ICASU) member Jeff Filippini is part of a team of scientists who recently received a five-year, multimillion dollar grant from NASA. The team will build and launch an instrument called Taurus to measure the glow of the early universe from a balloon high in the stratosphere.
Filippini explains, “The cosmic microwave background, or CMB, is the glow of the early universe. When we look out into space, we’re looking back in time. As the universe expands and cools, we see a glow from the moment when the universe went from being opaque plasma to transparent neutral gas. This glow is the oldest light in the universe. It's behind everything, a sort of backlight for all the things that have happened in the universe since then.”
In the more than 50 years since its discovery, observations of the CMB have taught us an enormous amount about our universe’s history and composition. However, we don't see the CMB as it was—we see it slightly fogged. As the light from the CMB travels to instruments on Earth or in space it may bounce off free electrons and scatter, thus appearing slightly blurred.
According to Filippini, “When the very first stars were born, the ultraviolet light they generated ionized the surrounding gas. The ‘bubbles’ of ionized gas around each star grew, until eventually the bulk of the universe was re-ionized, as we find it today. And that means that the universe is once again full of free electrons, so a light ray from the CMB has a small probability of getting scattered on its way to our instrument. If we know that probability, we can factor this blur into our interpretation of CMB measurements and gain greater precision.”
The probability that this scattering will occur is characterized by a number called tau, the universe’s “optical depth.” (The name “Taurus” is a play on "Tau ‘R’ Us.”) Taurus will measure tau through the faint polarization that this scattering process induces in the CMB’s light. Current measurements tell us that tau is around 5%, but a much more precise measurement of this probability will provide new information about how quickly the first stars formed.
A more precise measurement of tau will also help scientists attempting to measure the mass of neutrinos through their effect on the CMB. Massive neutrinos leave their signature through a different kind of blurring of the CMB: gravitational lensing. Light bends as it passes by massive objects in the universe on its way to Earth. This bending, or lensing, distorts the light we see from the CMB. A precision measurement of tau is critical to interpreting this lensing signal, and thus to measuring the neutrino mass.
In addition, Taurus will measure the polarized glow of dust in our own galaxy with high sensitivity, at observing frequencies that are largely blocked by Earth’s atmosphere. The maps generated by the Taurus mission will be critical to Taurus’s data analysis, and they will provide lasting value to other researchers exploring CMB polarization and galactic dust.
Taurus’s measurement requires many weeks of observation from the space-like environment of the stratosphere. Previous long-duration balloon missions—including SPIDER, another mission Filippini has worked on—have typically flown from Antarctica. However, measuring tau will require a map of a much larger portion of the sky than can be observed from such high latitudes.
Taurus will be a mid-latitude mission, and its balloon must be able to withstand the day-night temperature changes that occur during a multi-day flight. This mission will be one of the first to fly on NASA’s new Super Pressure Balloon, which was designed for these long-duration, mid-latitude flights. When it is complete, the balloon will be roughly the size of a football stadium and roughly the thickness of a plastic sandwich bag. The payload it carries will weigh around 3,000 pounds.
“This mission was not possible until recently, because we did not yet have the technology. With a Super Pressure Balloon, we can fly a balloon at mid-latitudes over multiple day-night cycles, which will allow us to map a large fraction of the sky” Filippini says.
Work on Taurus begins this fall, and Filippini is hiring students and postdocs to work on the project. He says, “I think there will be a nice synergy between the ground-based and space-based efforts here. My group has already been working on CMB-S4, a ground-based CMB telescope, and there will be overlap between the tools and techniques needed for these different experiments. Together, these separate efforts will be greater than the sum of their parts.”
This grant is part of NASA’s Astrophysics Research and Analysis (APRA) program, and it is shared among the University of Illinois, Washington University in St. Louis, Princeton University, and the National Institute of Standards and Technology (NIST).
Click here for further reading about the research happening in the Filippini group.