Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=213650
Story Retrieval Date: 12/21/2014 10:58:11 PM CST
LA SERENA, Chile — Before 1962, Cerro Tololo was just another remote mountain near the Chilean town of La Serena. Then a group of astronomers decided that -– with the crisp views provided by its dry air, clear skies and high altitude -- it would be a good place to build an observatory. Astronomers flocked to the site.
By the 1974 there were multiple telescopes on the peak, including what was at the time the largest optical telescope in the southern hemisphere. In the ‘90s it was one of the sites where scientists discovered the universe is expanding at an increasing rate. Now, Cerro Tololo is home to the world’s largest digital camera.
The 570-megapixel Dark Energy Camera is scheduled to begin its survey of the driving force behind the universe’s accelerating expansion later this month. It is attached to the Victor Blanco Telescope, the large optical instrument that played a key role in the discovery of dark energy in the '90s.
“The last decade was the decade of confirming the universe was speeding up,” project director Josh Frieman said. “I’m hoping the next decade will be figuring out why it’s speeding up.”
Frieman, a theoretical physicist at the Fermi National Accelerator Laboratory near Batavia, leads the Dark Energy Survey, a project that brings together hundreds of scientists from around the world. The survey will spend 525 nights of observations over 5 years on Cerro Tololo probing the universe for explanations about this elusive force.
‘A very large digital camera’
Imagine a digital camera. There’s a system of mirrors, lenses and sensors that converts light into images. Now imagine a camera with a 15-ton mirror, a 4-meter aperture and 74 sensors. That is the device mounted on the Victor Blanco Telescope in Chile.
At the telescope, a constant hissing of the liquid nitrogen cooling system is amplified under the large silver dome. During the day, engineers tend to the telescope and make sure it is balanced so precisely it could be moved by hand. At sunset, researchers fill the control room and track the southern sky.
When the dome opens, the camera begins to capture images the same way smaller cameras do. Light from distant objects reflects off the massive mirror onto the camera’s sensors, which convert the photons to electrons.
“Fundamentally it’s a very large digital camera,” astronomer Chris Smith said. “Not that different, except in its size, from the ones you buy at Best Buy or your local electronics store.”
Smith is the director in Chile of the Association of Universities for Research in Astronomy, the organization that operates Cerro Tololo. He first arrived at Cerro Tololo in 1991, and hasn’t left Chile since.
The camera’s light-detecting “charge coupled devices” are like those found in digital SLR cameras. The difference is they are bigger and there are many more of them. Sixty-two sensors are dedicated to taking images, and the other dozen act as image-stabilization and auto-focus features found on its consumer counterparts, Smith said.
The 12 corrective sensors take reference images every few seconds, even if the primary exposure is minutes long. Software compares the reference images to the targeted exposure to ensure objects stay in the same position in each frame. If the objects are off by even a pixel, the computer will tell the telescope how to correct itself, Smith said.
The camera will take hundreds of images each night, and each image file is about 1 gigabyte in size, said William Wester, a particle astrophysicist at Fermilab. At this rate, he said, they would fill up a consumer computer hard drive in a matter of a few nights.
The data is transferred daily from the Chilean mountain, which is an hour-and-a-half drive up mostly dirt roads, to a supercomputer at the University of Illinois at Urbana-Champaign. The information travels via microwaves and the Internet to the supercomputer, which will catalog and process the images, Wester said.
“This will be the largest collection of galaxies by a single enterprise,” Wester said. “And when you have that many galaxies to study you can do all kinds of precision measurements on those.”
Four probes of dark energy
When teams of astronomers discovered in 1998 that the universe was expanding at an increasing rate, they concluded a force that acts against gravity must exist. This is called dark energy.
Ordinary matter, such as that made of atoms and molecules, is only about 5 percent of all the stuff in the universe. Less than a quarter is dark matter and the rest is dark energy, explained project scientist Darren DePoy.
“We didn’t even know it was there until 10 or 15 years ago,” DePoy said. “So it’s a pretty major constituent of the universe that we didn’t know about and we’d like to know more about it.”
Dark matter and ordinary matter are gravitationally attractive, while dark energy has the opposite effect. Scientists credit that attribute as the cause for the universe’s increasing expansion rate.
That’s about as detailed as the definition gets — for now. The mountain top camera could help.
“I think it is fair to say that we don’t understand the properties of dark energy, we don’t know what it is,” project director Frieman said. “And that’s the reason behind the DES — to try and carry out observations that we hope will ultimately give us a better physical understanding of what dark energy is.”
Because there’s no way to directly measure dark energy, the survey team will focus on four areas to study the effects it has on our universe.
“We don’t yet know how revolutionary the theory is going to need to be that explain these phenomenon,” Frieman said. “That means to me that we need to get more data, better data, to get a better physical understanding of what’s driving the expansion of the universe.”
Why is the universe speeding up?
When Albert Einstein worked out his theory of general relativity, he created a constant in his equations so they would yield a stationary universe. As cosmology progressed, it was evident the universe was not static, but expanding. Einstein abandoned this constant and called it his biggest blunder.
As it turned out, Einstein wasn’t completely wrong.
The scientists who studied the universe’s expansion in 1998 expected to find the universe was decelerating because of the slowing effects of gravity. When they discovered the rate of expansion was instead increasing, they determined there must be something in space that has anti-gravitational properties. This is where dark energy comes in.
“It’s an energy and we know it’s there because we can see the results,” Smith said. “Something is pushing the universe apart. It’s not just coasting, it’s pushing.”
The leading theory to describe dark energy is that empty space inherently has energy, Smith said. Energy of a vacuum, such as space, is what Einstein described with his cosmological constant. The constant was an anti-gravitational force, just like dark energy. He created it so his equations would produce a stationary universe, but it can also account for an accelerating one.
What they will find
Not much is certain about what dark energy is, so not much is certain about what the Dark Energy Survey will find. The study of dark energy is in its infancy, and the survey is meant to give scientists an idea on where to go next.
Einstein’s cosmological constant, the energy of empty space, is what most cosmologists use to describe dark energy, Smith said. This doesn’t mean it’s the only possible theory to explain this anti-gravitational force.
“We’re all about eliminating theories,” Smith said. “Not really about proving one, but eliminating others and leaving the ones that are consistent with our observations to do further tests on.”
Though it’s possible the survey will produce findings consistent with Einstein’s blunder, Frieman said, it could also give scientists something unexpected.
“My hope is that we find that it’s not vacuum energy, that it’s not the cosmological constant that’s causing the expansion,” he said. “Because any time you overthrow people’s preconceptions it’s even more interesting -- you learn more about the universe.”