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The enigma of dark energy

Over the next five years, astronomers will meticulously photograph 300 million galaxies with the goal of discovering the nature of dark energy, a key to understanding why the universe is expanding at an accelerating rate. This ambitious project involves two Catalan research centers

ÀNNIA MONREAL | FEBRUARY 3th, 2011

Illustration of the Milky Way

In 1998, scientists discovered that the universe is not only expanding but, more surprisingly, this expansion is accelerating. This observation contradicts what physicists thought, as it was believed that gravity would gradually slow the expansion of the universe. Several theories have arisen to explain this apparent contradiction, including one which is based on the possible existence of so-called dark energy.

But what is dark energy? The truth is that nobody is really sure, and that is why astronomers in Catalonia have launched the Dark Energy Survey (DES). DES is building a 560-megapixels camera to photograph outer space for measurements on the position, distance and shape of about 300 million galaxies located in "about an eighth of the sky," explained Ramon Miquel, a physicist at the Institute for High Energy Physics (IFAE) at the Autonomous University of Barcelona and ICREA researcher. "This way we can study the geometry of the universe and the speed at which galaxies are formed, which will perhaps explain what dark energy really is.”

The Dark Energy Survey counts on the participation of the IFAE and the Institute of Space Sciences of Spain’s National Research Council (IEEC-CSIC) located in Blanes in northern Catalonia. These two Catalan centers are working on the creation of the electronics to read the 70 CCDs (charge-coupled device, which measure the light photons in each pixel) and the resulting images. At the end of next year, the camera will be installed in a telescope at the Cerro Tololo Observatory in Chile.

Why is the universe expanding?

Physicists believe that shortly after the Big Bang the amount of dark energy was "irrelevant and very small, but over the past five to six billion years it has begun to gain size," Miquel said. Although the antiquity of this energy is now known, physicists only began to think about it twelve years ago. In 1998, two teams of astrophysicists studying supernovas realized that the universe is expanding, and at a very fast pace.

"Over time, dark energy will occupy more than 70% of the universe"Among scientists, there are several lines of thought that try to explain dark energy. The leading theory suggests that "dark energy is probably the energy of a vacuum, which may be different from zero," said Miquel. "Everybody thinks that a vacuum has some energy, but nobody knows how to calculate it. There have been several attempts, but they have all failed catastrophically.” Theoretical physicists are perplexed because, currently, there is no explanation for the mystery of dark energy.

If tacking dark energy head on does not provide valid answers, then Miquel argues to approach it from other routes. "There are many projects whose goal is to observe the basic features of dark energy," including the DES, he said. "At the moment, we can only study dark energy by observing the geometry of the universe, because dark energy alters it.” The idea is that by studying how galaxies have been formed some of dark energy’s properties can be discerned.

"Over time, dark energy will make up more than 70% of the universe," said Miquel, and "there will come a time when the universe splits in two because with the accelerated expansion there will be distances which cannot be covered at the speed of light," the measurement of space. "This will mean that the visible universe will become increasingly smaller,” he said, adding that there will be more than one universe, ours and another one which we will no longer form a part of.

A PROJECT IN THE CANARY ISLANDS
Parallel to the Dark Energy Survey project (DES), the Institute for High Energy Physics (IFAE) and Institute of Space Sciences are working with the CIEMAT to build a camera capable of generating some additional results to those of DES. Under the name of Physics of the Accelerating Universe (PAU), they are developing a smaller camera with lower definition and monitoring capabilities than the DES, with 18 CCDs instead of 70, but with a greater ability to analyze the light beams.
"The camera to be installed at the William Herschel Telescope in the Canary Islands is smaller and should be operational in late 2012," explained Ramon Miquel of the IFAE "We will be mapping the universe in a different way from what has been done so far," he said. This is the main challenge of the project, which will photograph a smaller part of the universe compared to DES, but will have 48 filters to translate the light compared to the DES’s five. "This way we can better measure the distance to galaxies; we will gain in depth," he said.

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