Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=200741
Story Retrieval Date: 4/17/2015 11:42:49 AM CST
Courtesy of Argelander-Institut für Astronomie der Universität Bonn.
An international team of more than 120 scientists will take the next step Monday toward solving the mystery of dark energy, one of physics’ most perplexing conundrums. At Cerro Tololo, an American-operated observatory in the Chilean Andes, the high-powered telescope called Blanco will be shut down to prepare for the placement of the team’s huge, new state-of-the-art camera.
The camera is a crucial component in the Dark Energy Survey, said Josh Frieman, a cosmologist at the University of Chicago and the Fermi National Accelerator Laboratory in Batavia, who heads the project. Through their study, Frieman and his team aim to understand the properties of dark energy, the mysterious substance theorized to comprise more than 70 percent of our universe.
The initial steps Monday represent the culmination of years of work, said Brenna Flaugher, who managed the creation of the five-ton, 570-megapixel camera behemoth.
We started talking about the project at the very end of 2003,” she said. “We eventually got the funding in place from the Department of Energy about 2008 and we finished building the camera about a year ago.”
In August, the team will begin the five-year process of photographing our universe.
“The most exciting time will be when we take our first snapshots of the night sky,” Frieman said. Over the next few years, the camera will allow scientists to see about 300 million galaxies.
But this project is about more than pretty pictures. It’s about figuring out what Carlos Cunha, a Kavli fellow at Stanford University’s Kavli Center for Particle Astrophysics and Cosmology and project collaborator calls “perhaps the biggest mystery in all of physics.”
This mystery began in 1998. That year, the results of two separate studies shook a key assumption of physics like a rag doll in a hurricane. It was research so field-altering that Saul Perlmutter of the Lawrence Berkeley National Laboratory, Brian Schmidt of the Australian National University Mount Stromlo Observatory and Research School of Astronomy and Astrophysics and Adam Riess of Johns Hopkins University won the Nobel Prize in 2011. They had demonstrated that the expansion of the universe was, in fact, speeding up.
Physicists first learned about universal expansion thanks to Edwin Hubble’s work in 1929. Hubble, the American astronomer, became a household name with NASA’s launch of the Hubble Space Telescope in 1990. Hubble confirmed the existence of galaxies outside the Milky Way and found a way to measure distance in our universe, Cunha said.
In the process, however, Hubble realized something fascinating: The farther an object in the universe is from Earth, the faster it moves away. Based on his findings, scientists realized the universe is expanding and theorized the Big Bang. Applying Einstein’s theory of General Relativity, which describes how gravity works – with matter attracting and pulling other matter including space itself, they thought universal expansion would slow down over time.
To understand the concept, think of throwing a baseball, said Tim Eifler, a CCAPP fellow at the Ohio State University's Center for Cosmology and Astro-Particle Physics and project collaborator. Here on Earth, when you throw a ball up, it starts off fast and begins to slow down as it goes higher and loses momentum. When it reaches its highest point, the ball then falls back to Earth. Why? Because Earth's gravity pulls it back.
“Now, suppose one day you throw the ball up into the air and at first it slows down making you think everything is as it should be,” Eifler said. “But suddenly it goes faster and faster, keeps speeding up and just disappears out of sight.” That is essentially what Perlmutter, Riess and Schmidt realized was happening with the universe.
In the original theory, scientists thought the universe received its initial momentum from the Big Bang (the ball was thrown) and that it should react to gravity much like the baseball. Instead, the physicists discovered, about 5 billion years ago, it sped up and continues to accelerate to this day.
"This indicates a new area of physics, something that has never been seen before,” Eifler said. “And we have no idea of the underlying mechanism. Now this accelerated expansion of the universe needs a very different kind of physics to explain it.”
Scientists already have several potential theories to explain the new phenomenon. These include that laws of gravity, described by Einstein's General Relativity, are modified on very large scales (much larger than the size of the solar system or even the Milky Way), or that there is a mysterious energy component of the universe that causes the acceleration, according to Eifler. But no one knows for sure. Physicists have named the mysterious thing at the heart of the question “dark energy,” Eifler said.
Physicists said the new camera will provide information to help them better understand the composition of dark energy and the laws that govern it. The study is critically important given dark energy’s primacy and therefore potential implications, Frieman said.
“We have relatively little knowledge about this thing that makes up the majority of the universe,” he said. Not understanding dark energy, in other words, is like not understanding water on our own planet.
"It’s the properties of dark energy that will determine the future evolution of the universe,” he said. “If we want to understand where the universe is going on the timescale of billions of years, we need to understand dark energy.
To do so, physicists will take the pictures from the camera and turn them into the largest map of the universe ever created, Cunha said. Cunha and other collaborators’ work on the mapping, which uses the colors of distant galaxies to tell how fast they are moving away from us, will help make the rest of the project possible, Eifler said.
A few special features on the camera allow the creation of these critical maps. The camera’s large field of view, about eight times the size of the moon as seen from Earth, will allow each, 500-megabyte photo to cover a relatively large swath of sky, Flaugher said. (To compare, an iPhone 4S takes an average 1.8-megabyte photo). In addition, five filters provide the efficiency required to photograph and infer colors for hundreds of millions of galaxies and the camera’s high sensitivity to red light allows physicists to “probe further back toward the Big Bang,” she said. "That lets us understand the evolution and expansion of the universe over a longer time scale.”
To understand how this works, one fact is critical: photographs are not of the galaxies, per se, but of the light these star-lit figures emit, Cunha said. Because galaxies are so far away, their light takes a long time to reach Earth. When physicists look at pictures of the night sky, they are therefore seeing pictures of the past.
The trick to converting the 2D photographs taken by the camera into a 3D map lies in Hubble’s law, Cunha said.
“As the light from distant galaxies traverses the universe, it gets stretched with the expansion,” he said. “The stretching causes the light to become progressively redder, which we call the redshift. The farther away a galaxy is, the more its light will get stretched on its way to us, so the bigger the redshift.”
The redshift thus tells us how big the universe was when the light from the galaxies was emitted, Cunha said. And by logging these hundreds of millions of redshifts, the Dark Energy Survey team can reconstruct the history of the expansion of the universe.
With this understanding, Cunha considers the galaxies’ colors as measured through the camera’s five filters. Redshifts measured in this way are known as photometric redshifts and tell the team how fast a galaxy is moving away from Earth and what type of galaxy it is: elliptical, irregular, or spiral (our Milky Way falls into the last category).
By combining the redshift with the 2D photographs, Cunha and his colleagues create a time-lapse, 3D model of the night sky against which different teams can test theories about dark energy and how it affects the universe.
“It’s a very big collaboration, so a lot of people use that information in different analyses,” Cunha said. Cunha himself is interested in the clustering of galaxies – one of the project’s four main research areas or “probes.” The four probes are Type Ia Supernovae, Baryon Acoustic Oscillations, Galaxy clusters and weak gravitational lensing.
Eifler’s specialty, gravitational lensing, is a technique equally sensitive to luminous matter, such as stars and galaxies, and to dark matter and dark energy, whose nature is basically unknown, he said.
Gravitational lensing describes the effect on light rays emitted from very distant galaxies. On its way through space, the light is deflected by the intervening matter structure of the universe. While the deflection is very weak, it can cause the shape of the galaxy to be distorted when viewed from earth. By measuring the distortion effect over millions of galaxies, Eifler and his teammates will have a better understanding of the properties of the universe's structure and its dynamics.
"In combination with other probes, gravitational lensing enables us to distinguish whether it is the laws of gravity that need to be altered or whether it is an additional mysterious energy component that drives the universe's accelerated expansion,” Eifler said.
To gain a better understanding of the huge forces at play requires an in-depth investigation of each probe.
“The key point is to have many very different types of predictions,” Cunha said. “The more predictions that you make, the more likely it is that you can narrow down certain laws and not be fooled by systematics or problems in an experiment.” For each probe, “a few hundred” predictions will be made.
The information scientists glean from the data could have far-reaching implications. Ultimately, Cunha said, it could “help illuminate the path toward the Holy Grail: the unified theory of everything – from how subatomic particles work to how the universe began.”
Other people may enjoy benefits as well, Frieman said. Once tools such as the advanced camera go out into the world, “people find different ways to use them,” he said.
“It will have many technological spinoffs,” Cunha added. “We have developed this very advanced technology and it eventually trickles down.” After all, it was a physicist at CERN in Geneva, Switzerland who invented the World Wide Web, Frieman said.
And even if hand-held space cameras don’t make it onto store shelves any time soon, there is still the excitement of learning more about our universe.
“Just understanding where we come from, how the universe began – that’s why I started in the field,” Cunha said. “I think that interests a lot of people.”
There is something inherently alluring about this mystery, Frieman agreed.
“It seems like an intuitive thing that you would want to know what most of the universe is made of, why it acts the way it does,” he said. “Knowing what dark energy is will not solve any basic human problems, but to me it’s 73 percent of the universe. We should try to figure out what it is.”