Probing the Universe’s ‘Dark’ Secrets
A team of physicists and astronomers from around the globe are on a five-year mission to answer fundamental questions about our universe: Why are galaxies clustered the way they are? Why are they moving apart faster and faster, instead of gravity slowing them down?
Researchers from the University of Pennsylvania are playing an integral role in answering these questions, through a project known as the Dark Energy Survey. Now in its third year, the project is the culmination of 10 years of planning, building, and testing by scientists from 25 institutions in six countries.
“Every week, images of several million distant galaxies are being recorded by our camera and processed by our software,” says Bhuvnesh Jain, a professor in the School of Arts & Sciences’ Department of Physics and Astronomy and leader of the Penn contingent. “The cumulative analysis of this data will lead to novel results in multiple branches of cosmology.”
Other members of the team include professors Larry Gladney and Gary Bernstein, associate professor Masao Sako, and research-staff member Mike Jarvis. Postdoctoral researchers, graduate students, and undergraduates are contributing to their research.
The survey is designed to systematically map one-eighth of the sky in unprecedented detail, with the goal of finding out why the expansion of the universe is speeding up, instead of slowing down due to gravity.
At the heart of this mystery is dark energy, the force believed to be causing that acceleration. Called “dark” because it can’t be directly observed, team members are learning about this force by measuring its influence on the largest structures in the universe: galaxies and galactic clusters. The role of dark matter—the invisible substance that makes up roughly a quarter of the universe—must also be taken into account.
To measure the subtle signature of dark matter and energy on these enormous objects, the researchers need to record data with unprecedented precision. This is done by the Dark Energy Camera, a 570-megapixel digital camera mounted on the 4-meter Victor M. Blanco telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory in the Andes Mountains in Chile. The most powerful survey instrument of its kind, it is able to see light from more than 100,000 galaxies up to 8 billion light years away in each snapshot.
As co-leader of the Survey’s science verification team, Bernstein was responsible for fine-tuning this instrument, aligning the car-sized camera to tolerances thinner than a human hair. The Survey’s science verification and data management groups have also prepared software to collect and measure the nearly one terabyte of image data that will be produced every night of the survey—far more than the scientists could ever examine by eye.
The survey will obtain color images of 300 million galaxies and 100,000 galaxy clusters, and will discover 4,000 new supernovae, many of which were formed when the universe was half its current size. These observations will not be able to see dark energy directly, but, by studying the expansion of the universe and the growth of large-scale structures over time, the survey will give scientists the most precise measurements to date of its properties.
The survey will use four methods to probe dark energy: counting galaxy clusters, measuring supernovae, studying the bending of light through a technique known as gravitational lensing, and analyzing patterns in the distribution of galaxies.
Jain is the co-coordinator of the gravitational lensing group. Lensing is a phenomenon in which light from distant galaxies is bent by the gravity of massive objects on its way to Earth. When light from distant galaxies encounters dark matter in space, it bends around the matter, causing those galaxies to appear distorted in telescope images.
Jain, Bernstein, Jarvis, and their colleagues from around the world are comparing hundreds of thousands of galaxies, looking for patterns in such distortions that would reveal dark energy at work. Jarvis is in charge of the Survey’s “lensing pipeline.” His work involves reducing each of the galaxies imaged with the Dark Energy Camera to their vital statistics via computer algorithms, so they can be more easily compared.
This type of analysis has already helped produce maps of dark matter, which help corroborate theories about the overall shape of structures in the universe. They show large filaments of matter along which visible galaxies and galaxy clusters lie and cosmic voids where very few galaxies reside.
High concentrations of dark matter are shown in red and sparse areas in blue. Galaxies tend to form in the matter-rich areas.
“The cosmic background radiation left over from the Big Bang has ‘hot’ and ‘cold’ spots over the whole sky,” Jain says. “Starting from these primordial ripples in the early universe, gravity led to the growth of filaments and the emptying out of the voids we see in the maps.”
“As the universe ages,” Bernstein says, “these structures get more prominent. Mass attracts mass, and so the rich gets richer. And because most of this mass is dark matter, its concentration tells you where galaxies are more likely to form. That we see galaxy clusters in the same places where the gravitational lensing analysis tells us the dark matter is most concentrated demonstrates that this story makes sense.”
The Survey will continue to expand these maps, which currently cover only about 3 percent of the area of sky that will be documented over its five-year mission. As the researchers expand their search, they will be able to better test current theories on these cosmological mysteries.
