Imaging the Invisible: The Dark Energy Camera
A prototype of the Dark Energy Survey camera, DECam. The front ring holds the detectors and is 45cm in diameter.
As shown in this illustration, dark energy is estimated to contribute about 75% of the energy in the Universe, dark matter about 21% and normal matter about 4%. Only the normal matter can be directly detected with telescopes, and about 85% of this is hot, intergalactic gas.
DECam will be mounted on the Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory, shown here.
Photo by David Walker
The average digital camera is great for taking embarrassing pictures of friends and capturing a couple’s first kiss, but taking pictures of really faint galaxies that are millions of light years away requires some serious modifications.
Scientists from the United States, Brazil, Spain and the United Kingdom have been working to create just such a souped-up camera for the Dark Energy Survey (DES). With a price tag of $50 million, the survey will be carried out by a 570 megapixel camera (compare that to an iPhone 4 camera, which has five megapixels) mounted on a telescope in Chile. The camera, about the size of a smart car, will record light from distant galaxies and supernovae intermittently over five years. Scientists expect this information to shed light on a mysterious thing called dark energy.
Dark energy is called dark because so far it has been invisible to us - no one knows what it is, or if it even exists. In the 1920s, data collected by astronomer Edwin Hubble showed that the universe is expanding and has been since the big bang (for more information on this, see Catch a Cosmic Microwave). This led to debates among the science community: Will the universe continue to expand forever? Will gravity eventually reverse the expansion and cause a “big crunch”? Will the universe reach a state of equilibrium somewhere in the middle?
In 1998, two teams of astronomers announced a surprising discovery - the universe is actually expanding faster than it used to be. This revolutionized the way that scientists thought about the expanding universe, and they created new models to explain the phenomenon. One of the most widely accepted models includes a type of energy that has never been observed, and calculations show that this dark energy might account for 75% of the energy in the universe!
So, how can a giant digital camera help us study dark energy?
If you know how far a car is from a certain point and how fast it has been traveling, you can determine where the car was at any given time since it left. For example, if Bob is 50 miles from home and has been driving at a constant rate of 25 mph, we know that 30 minutes after he left home he was 12.5 miles from home (25 mph*0.5h). If Bob is traveling in a straight line, we can determine exactly where he was at any given time.
Similarly, if you want to know where a galaxy was in relation to the Earth millions or billions of years ago, you need to know how far away the galaxy is from the Earth, and how fast it is traveling away from the Earth. If you can figure this out for a bunch of galaxies, you can create a pretty good map of how the universe looked at any given time in the past. This is the goal of the Dark Energy Survey.
Astronomers will use information like the apparent brightness of Type Ia Supernovae and changes in the structure of the universe to determine the distance to galaxies. To determine how fast a galaxy is moving, they will compare its light spectrum to what the spectrum would look like if the object was not moving (its Doppler shift). All of this information, and more, will be captured by DECam, giving us a detailed look at how the universe has expanded over time.
DECam is essentially a really big, really sensitive digital camera. Like the Nikon Coolpix and Kodak Easyshare, the camera has lenses to enable focusing and detectors that turn incoming light signals into digital values that correspond to individual pixels. However, it has many special features, such as its sensitivity to red and infrared light and its wide field of view.
DECam will capture light from distant, and therefore faint, galaxies. Doing so requires extreme sensitivity. The average digital camera has one detector with 5-20 million pixels (1 million pixels = 1 megapixel), but DECam has 74 detectors composed of millions of pixels each. The detectors are so sensitive that they have to be kept at -100 degrees Celsius to reduce background noise.
The camera will be mounted on an existing 4-meter telescope at the Cerro Tololo Intern-American Observatory in Chile. During the next five years, DECam will operate for a total of 525 nights and image about 300 million galaxies in the southern hemisphere. A supercomputer will process this huge amount of data, identifying objects and creating a database of information about the brightness, location, and other properties of galaxies. Teams of researchers on the Dark Energy Survey project will analyze the results, and hopefully a clearer picture of how the universe has expanded over time will emerge.
The survey team will also make the data available to the public, so that people around the world can help with the analysis. Dark energy is one of the biggest mysteries of science today, and shedding some light on it will be a major achievement. Whether the survey results support the existence of dark matter or unearth another surprise that causes astronomers to rethink their models, the results are sure to be exciting.
Visit these sites for more information:
Cerro Tololo Inter-American Observatory
What is Dark Energy? (NASA)
The Dark Energy Survey
The Doppler Effect (University of IL)
How Digital Cameras Work (How Stuff Works)
Runaway Universe: The Birth of Supernovae Type Ia (NOVA online)