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In a special issue of Science, researchers explain that the presence of dark energy - with its unique density and negative pressure - is causing the universe to expand at a faster and faster rate.
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Science

Forget matching a pile of black socks in an unlit room: Cosmic acceleration caused by dark energy, and the black holes that lurk in almost every galaxy are two of the real dark mysteries of the universe. Scientists are using rapidly advancing observational tools, computer simulations and theoretical predictions to study how these dark mysteries impact the past, present and future of our galaxy and of the universe.

Articles describing “the state of the dark” appear in Friday’s “Dark Side” special issue of the journal Science, published by the American Association for the Advancement of Science.

Astrophysicists around the world are refining and improving their ability to study distant and ancient light. These advances are also helping scientists understand the dark energy, dark matter and black holes that make up the 96 percent of the cosmos that we can’t see — the dark side.

“Making precise measurements is the nature of the game … and a real observational challenge,” said Robert Kirshner from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. His article describes the connections between dark energy, exploding stars and cosmic expansion.

Our expanding universe
The expansion of the universe has been speeding up in the past 7 billion years, thanks to dark energy. Scientists report that the presence of dark energy — with its unique density and negative pressure — is causing the universe to expand at a faster and faster rate.

Imagine our Milky Way galaxy as a red grape suspended in the orange Jell-O of the universe. To say that the universe is expanding at an ever-increasing rate means that the amount of orange Jell-O expands more today than it did yesterday.

There are many billions of other grapes in the Jell-O — each representing a galaxy in the universe. As the universe expands, the Milky Way and the rest of the galaxies stay the same size because they are held together by gravitational forces. However, the distance between galaxies increases.

Exploding stars expose accelerations 
The acceleration of the universe provides indirect evidence for the existence of dark energy. But how did the researchers figure out that the universe is expanding?

The answer comes from exploding stars — specifically, a certain class of supernova explosions. Supernovae are the extremely bright explosions that mark the end of the life of some stars.

Scientists compare the light from supernova explosions from different ages in the past and use this light to measure changes in the rate of the expansion of the universe.

Light from explosions approximately one-third of the way back to the Big Bang or about 5 billion years ago are about 25 percent dimmer than they would be if the universe were expanding at a constant rate. From these kinds of observations, scientists demonstrate that for the past 7 billion years, the universe has been expanding at an increasing rate. These supernova explosions, about 4 billion times brighter than the sun, make it possible to back track through space and time and probe the history of cosmic expansion and the nature of dark energy.

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Kirshner writes that the current evidence points to a universe currently dominated by dark energy.

“You see a tree moving and you say, ‘Oh, it’s the wind,’” said Kirshner, who drew a conceptual comparison between the invisibility of both wind and dark energy.

“We can’t see the dark energy, but we can study the way it interacts with the aspects of space that we can see,” explained Kirshner.

Big roles for black holes
Scientists are also watching the lights of the cosmos to gain insights on another dark mystery of the universe — black holes.

In a separate article in the same special issue of Science, Mitchell Begelman from the University of Colorado at Boulder describes how black-hole research is informing scientists about the evolution of our 14-billion-year-old universe.

Unlike dark energy, which is sprinkled throughout the universe and is measured by its large-scale effect on the evolution of the universe, black holes are discrete places that scientists can identify with observations from telescopes.

Black holes can form when massive stars collapse. Begelman writes that the gravity of black holes is so strong that nothing that enters them — not even light — can escape.

Tornados of space and time
“We can study what happens around specific black holes. We can also observe things that are in the process of falling into black holes,” said Begelman. He explained that spinning black holes kick up a tornado of space and time. Matter gets caught up in spacetime tornadoes and swirls around the black hole as it falls in.

image: universe
Observations by the High-z Supernova Search Team hint that we live in a "stop and go" universe whose expansion slowed under the influence of gravity before accelerating again due to an unexplained dark energy. This artist's conception illustrates the history of the cosmos, from the Big Bang and the recombination epoch that created the microwave background, through the formation of galactic superclusters and galaxies themselves.
With rapidly advancing observational tools, scientists can observe far-away phenomena such as spacetime tornadoes and the X-rays emitted from discs of gas being swallowed by black holes. Phenomena observed near black holes can be compared with theoretical predictions and laboratory measurements of X-rays from hot gases. These sorts of comparisons yield insights on the nature of black holes.

In describing black holes, Begelman writes, “space and time are so distorted that there is literally no way out.”

Black holes as laboratories
There may be no way out of a black hole — but they are anything but a dead end for scientists.

“Black holes are laboratories for the most extreme conditions encountered in the post-Big Bang universe. Understanding black holes is crucial to understanding the Universe. They are much more than an oddity found in galaxies,” said Begelman.

Black holes come in at least two sizes. There are the smaller, stellar-size black holes, and there are supermassive black holes. The center of each galaxy, including our own Milky Way galaxy, contains a supermassive black hole.

Our supermassive black hole is 25,000 light years away from Earth. If you look in the direction of the constellation Sagittarius, the Archer, in the center of the Milky Way, you are looking toward our supermassive black hole. To study it, scientists must pierce the thick dust obscuring the Galactic Center.

The energy currently released from the Milky Way’s supermassive black hole is a thousand times greater than the energy emitted from the sun. However, researchers believe this amount of energy is just a dribble when compared with the energy it unleashed during galaxy formation.

“During the growth of our supermassive black hole, it released so much energy that it greatly impacted the development of the galaxy,” said Begelman. “But it’s hard to tell what role it played.”

Black holes and life
Scientists believe that supermassive black holes were crucial to the development of the structure of the universe.

“Black holes were involved in the accumulation of stars and gas into the compact regions of space that eventually formed planets and possibly life,” explained Begelman.

“We are incredibly lucky to be working just at the moment when the pieces of the cosmic jigsaw puzzle are falling into place, locking together, and revealing the outline of the pieces yet to come. Dark energy is the biggest missing piece and a place where astronomical observations point to a gaping hole in the present knowledge of fundamental physics,” Kirshner writes in Science.

© 2013 American Association for the Advancement of Science

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