Planck data supports view of simple universe

Princeton researchers working with the European Space Agency have received groundbreaking data from the Planck satellite launched by the ESA in May 2009. The Planck data are unprecedented in accuracy and precision, receiving worldwide press coverage and public attention.

In processing the Planck data, scientists including Princeton physics professor William Jones ’98 have been able to confirm an existing hypothetical framework for how exactly the universe behaves. This model was based on research done before the satellite, including the Sloan Digital Sky Survey, which gave astrophysicists a clearer understanding of the distribution of galaxies in the universe, and Wilkinson Microwave Anistropy Probe, which gave the world the now-iconic “baby picture” image of the universe, displaying the Cosmic Microwave Background Radiation. With these previous projects, scientists had formulated a simple six-parameter model for the entire universe, otherwise known as the Lambda Cold Dark Matter model. Big Bang cosmology followed this model and was the dominant theory until Planck.

While the Planck scientists’ model worked as far as the measurements could tell, Jones explained, “There are some very natural things that you might think are missing.” Planck was able to go even further than WMAP by answering the question: Are there other extensions to that model – are scientists missing anything in their explanation of the large-scale structure of the universe? Planck uses much higher statistical power and much finer angular resolution. “You can ask about some basic questions – about the initial conditions of our universe but also the constituents of our universe that the previous data couldn’t constrain at all,” Jones said.

The satellite is so sensitive that the scientists were expecting to see something weird, Aurelien Fraisse, a postdoctoral fellow in the physics department working on Planck, explained. “The first shocker is how beautifully simple the model of the universe we need to describe the data is.” According to Fraisse, there is a whole host of variations to this model that would seem very reasonable. “You add them one by one, and you see that you don’t need any of them.”

The data from Planck are able to confirm this simple model. “It isn’t just consistent with a simple universe, it is a simple universe, and it’s not complicated. And that’s new. And that’s quite remarkable that we’ve got such a fundamental understanding of so many areas of physics,” Jones said.

The statistics of the project arise from the assumption that the universe we have today is one possibility in a host of possibilities given these six parameters. Jones likened it to tossing dice, not knowing exactly if or how the dice are loaded. One way to understand this property of the dice would be to toss a whole bucket full of the same kind of dice, or the same one repeatedly, and see exactly the distribution of values.

“The problem with the universe is that we only have essentially one roll of that die. We only get to observe our universe, not a bunch of similar universes. And so the thing that fundamentally limits our ability to make inferences about the underlying theories is that: Our sky, which we believe is a Gaussian random process, is just like rolling a die,” Jones explained.

A second difficulty in analyzing the Cosmic Microwave Background Radiation is the imperfection of the instruments. Planck sharpened the image that scientists had received from WMAP and essentially eliminated all the “noise” or “blurriness” from the data. “One of the fundamentally different things about the Planck instrument is its angular resolution, so what that provided was a consistent measurement of these fluctuations on scales that extend from the full sky all the way down to the amplitude of fluctuations that are separated by a few arcminutes,” Jones said.

The team at Princeton worked extensively on data collected from Planck’s detectors. When a signal from the background radiation of the Big Bang was picked up, it behaved the way a flashlight shines in the dark, except that the filament, instead of giving off light, received it. The intensity is brightest at the center and falls off with distance.

“We’re trying to actually map out that intensity profile as well as possible,” said Jon Gudmundsson, a physics graduate student who worked with Jones. Gudmundsson established the framework for calibrating the detectors from the very large scale to the small scale. He did so by using in-flight data from planets in our own solar system, which shine with known values.

Alexandra Rahlin, also a graduate student with Jones worked on ensuring the robustness of the system itself, said, “It’s amazing. Questions like these are why I became interested in cosmology.”

The data from Planck are now helping to discriminate between a class of theories that explains the universe more fully. With the satellite’s accurate reading of the universe, scientists must now examine what seeded the initial conditions of the Big Bang and must explain the cause of those early quantum fluctuations.

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A class of theories called single field inflation theory spans from quite simplistic models to more contrived models. Planck has begun the process of determining how these theories work. Fraisse explained that “Because it’s so simple, it makes it very hard to build some models that require some more complications,” referring to the early stage of the universe that dictated this six-parameter universe.

Scientists are now using the amplitude of the gravitational waves produced in the early universe to pare down these theories. According to Jones, the simplest of the models tends to predict more gravitational waves than are seen, but there is no solid limit on that.

In the basement of Princeton’s Jadwin Hall, this team is now building a new instrument called Spider, to be launched in Antarctica this December. Using thousands of detectors instead of Planck’s eight, Spider is designed to be a very sensitive probe of this gravitational wave background and focuses on larger scales on the sky.

Jones and his team will present results from the Planck satellite and introduce Spider in a talk on April 16 in McDonnell Hall.