Probing the Large Scale Structure of the Universe

According to astrophysicist Naoki Seto of the California Institute of Technology, “Large angular CMBR fluctuations contain precious information of the largest spatial scale fluctuations, but they are also contaminated by the (less interesting) small spatial scale power. Therefore, if we can remove the small spatial scale ones, we can get a cleaner picture of the potentially anomalous features of our universe.”

It all comes down to filtering out the distractions. Say someone from another country asks you about where you live and you describe the cracks in the front driveway and the angle of the sign perched on the pole at the end of the street. Not very helpful you say – especially to someone living in an entirely different part of the world. Data from WMAP is like that. Although it reveals slight temperature related fluctuations in the CMBR across the sky, these fluctuations are mostly associated with scattering of CMBR by “nearby” matter. As a result they are “contaminated” by the expansionary influence of dark energy associated with galaxies as far off as several billions of light-years. From an astronomical point of view, CMBR fluctuations are caused by nearby cracks in the pavement. Ultimately the goal is to see the “big picture” of the entire universe. It’s all a matter of scale…

What will we learn about the Universe based on such large scale variations? “You can study interesting behaviors of the inflation that might generate seed perturbations for cosmic structure, like galaxies”, says Naoki.

Early on, a curious form of energy dominated the universe (during the so called hyper-inflationary phase). In this period the attractive influence of matter was not a factor and the universal balloon expanded incredibly fast. Later as matter dominated, gravitation put the brakes on things, the Universe decelerated and the balloon may have barely managed to keep expanding at all. After deceleration, another engine kicked in – the mysterious force called “dark energy”. The constraining influence of gravity was overcome and the Universe resumed expansion, but at a more leisurely rate. In our current epoch, studies of the light of distant supernovas have shown that the expansion of the universal balloon is accelerating again. We live in an era of universal inflation and questions about inflation, along with the possibility of dark energy driving it, can best be answered by studying previous cycles of slower expansion.

Naoki and Caltech associate Elena Pierpaoli hope to eliminate the effects of dark energy by studying the polarization of microwave radiation arriving at our solar system from the direction of older galaxy clusters. One possibility is to use a future WMAP-like probe capable of higher resolution of detail to collect microwave radiation from regions where the CMBR was once scattered by distant clouds of free electrons in space. Since electron scattering naturally occurs where matter is found, galaxy clusters make ideal candidates. The catch is that such clusters must be far enough away to provide a picture of scattering as it occured long ago. By focusing on galaxy clusters seven billion light years away, we could see the CMBR as it appeared from clusters when the universe was half its current age. Dark energy at work then would not be as strong as it is now.

The resulting picture could provide important clues related to insights coming out of the WMAP project group. There is a possibility that, at the very largest scales, the universe is quite different from what was originally thought to be true. “Very roughly speaking,?, says Naoki, ?we expected that there would be no characteristic length in the largest-scale observable universe. This includes the spatial spectrum of the fluctuations and the shape of the universe.”

Other researchers have considered the use of galaxy clusters to probe large scale structure in the universe as well. But these researchers were not convinced the approach would work. Naoki and Elena found two important factors not sufficiently emphasized in earlier studies. First, they linked the obscuring small scale fluctuations in CMBR anisotropy to the influence of dark energy associated with the current accelerating era. Second, they determined that this obscuration could be minimized by exploiting scattering effects projected from galaxy clusters 7 billion light years away. Together these two insights could make it possible to see the largest scale universal structures influencing things today.

According to Elena: “The beauty of what we showed is that the observable quantity we propose to use is a function that varies very slowly on the sky. In order to map it observationally, you don’t need a high-resolution all-sky experiment, but you need to observe targeted objects uniformly spaced on the sky. This is, observationally, a much easier task than mapping the whole sky with that resolution.”

Unfortunately it is not possible for WMAP to achieve the degree of resolution needed to bring out the largest scale structures hinted at in the original data. For this reason, it may be several years before information needed by Naoki, Elena, and other astrophysicists is collected. The next probe scheduled for launch is ESA’s Planck in 2007. Despite Planck’s increased sensitivity and resolution, the signals needed are so weak that it will be difficult to eliminate other competing signals from those polarized by distant galaxy clusters. However future high-altitude ground-based instruments, such as ACT, APEX-SZm, and SPT, may provide the aperture needed to resolve the 1 arc minute sized regions needed to bring out the largest scale structures of the Universe. The Cornell-Caltech Atacama Telescope – a 25 meter sized submillimeter-wave instrument currently undergoing feasibility study – could be sensitive to these effects. The CCAT is expected to collect first photons in the early part of the next decade. Such an instrument should be able to resolve signals separated by as little as .5 arc minutes (1/60th the diameter of the Moon).

Ah, what irony! To map the largest scale structures of 7 billion years past we still need to be able to see a few cracks in the pavement…

About The Author:
Inspired by the early 1900’s masterpiece: “The Sky Through Three, Four, and Five Inch Telescopes”, Jeff Barbour got a start in astronomy and space science at the age of seven. Currently Jeff devotes much of his time observing the heavens and maintaining the website Astro.Geekjoy.