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"Dark Side" of the Universe Is Coming to Light

By John Roach
for National Geographic News
June 19, 2003
 
For centuries astronomers have trained their gaze on the matter that brightens the universe: the moon, the planets, the stars, and the galaxies. But these bright spots only comprise four percent of the cosmos.

The rest is seemingly a void, nothing but darkness. But the darkness is not empty: It is filled with dark matter and dark energy that has, over the course of the 14 billion years since the big bang, molded the universe into the dynamic structure and shape it holds today. The big bang is the name given to the theory that the universe started with a single cosmic explosion.

Welcome to the dark side.


In a series of papers presented in the June 20 issue of the journal Science, astrophysicists document what is known and waiting to be discovered about the 96 percent of the cosmos that can't be seen.

"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," writes Robert Kirshner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, in the paper on dark energy.

Black Holes

Any exploration on the dark side of the universe would not be complete without a look at black holes: the mysterious voids that seem to suck in all matter that comes within their reach and are blamed for the disappearance of everything from a favorite pair of earrings to notorious criminals.

In reality, black holes are places where gravity is so strong that once something crosses their horizon it will never escape, not even light. But in the process of their consumption they may have helped develop galaxies and the structure of the universe.

Mitchell Begelman, an astrophysicist at the University of Colorado at Boulder and author of the review paper on black holes says that to understand the stars, galaxies, and the gas that lies between them, it is necessary to understand black holes.

"Black holes are not really pathological places that are strange in the normal scheme of things," he said. "In fact, the structure of the galaxies and how they and the planets and ultimately life form could be directly affected by what happens close to black holes."

While it is true that once something crosses their horizon it will never escape, only a small fraction of the material that comes under the influence of a black hole is actually sucked in. The rest is expelled as excess energy.

Research shows that black holes can only consume material that spins at a relatively slow speed, so as an object falls into a black hole it gives off all the excess energy to material that is farther out.

"One of the surprising facts of black holes is the regions around them can generate enormous amounts of energy and can affect the entire galaxy by the jettison of wind or gas," said Begelman.

Black holes are divided into two classes. The most common are remnants of massive stars that have collapsed. All stars that are considerably more massive than the sun will eventually meet this fate, said Begelman, meaning that galaxies such as the Milky Way have millions of these types of black holes.

The other kind of black holes are known as super-massive black holes. Their formation process is less clear than that of stellar-mass black holes, but according to Begelman they seem to be at the center of every galaxy.

According to theory, super-massive black holes were formed either by a collapse of debris left over from when the galaxies formed, or perhaps they "could have preceded the galaxy and provided a seed around which the galaxy coalesced," said Begelman.

Determining which came first, the galaxy or the black hole, is one of the major unsolved mysteries of black hole research, said Begelman. Driving the research quest to determine the answer is the curious fact that the masses of the super-massive black holes are correlated with the masses of galaxies. Again, further research may demonstrate why.

"Most of us are convinced that this correlation is very important," said Begelman. The answer may hold the key to how galaxies and black holes form.

Researchers are also testing Albert Einstein's 1915 theory of general relativity to find out if the principle developed to explain apparent conflicts between the laws of relativity and the laws gravity holds up at the most extreme of environments known—the region right next to black holes.

Dark Age

Before there were black holes and stars and galaxies to consider, the universe went through a period just after the big bang known as the Dark Age of the Universe that lasted until evolving structure ultimately led to the first stars.

Jordi Miralda-Escudé, an astronomer at Ohio State University in Columbus, describes the Dark Age as the epoch when the universe had become completely dark as it gradually cooled following the big bang and "all the matter that forms us today was spread around space and uniformly distributed."

Little differences in the density of matter that had been present since the beginning of the universe gradually grew larger as a result of gravitational attraction. Eventually the density was large enough to cause a gravitational collapse and the first star was born.

These first stars, according to Miralda-Escudé, were likely quite massive and when they exploded as supernovae they littered the universe with atoms heavier than hydrogen and helium which were required for the formation of planets in the next generation of stars.

"In the beginning, the universe contained only hydrogen and helium, but at the present time planets like the Earth are made of heavier atoms," he said. "They all come from stars. So all the atomic nuclei on which life is based—except for hydrogen—were made in stars."

One of the curiosities Miralda-Escudé is trying to pin down is when after the Dark Age began was the first star born?

"Imagine you are an observer floating in space and looking through the universe in the Dark Age and around you it is completely dark," he said. "The temperature is decreasing so the cosmic background light has all shifted to infrared wavelengths, and there is nothing in visual light. Then, all of a sudden you see the first star appear in the distance."

According to theoretical models based on this random observer and the little areas where matter was clustered together, Miralda-Escudé estimates that the first visible star probably appeared about 75 million years after the big bang

Today, the universe is nearly 14 billion years old and contains billions of galaxies, each one with billions of stars.

Dark Matter

The first stars and galaxies formed only because there is dark matter, which is credited for providing the blueprint for the growth and structure of the universe as we know it today.

"I often point out that we would not exist if it were not for dark matter," said Paul Steinhardt, a physicist at Princeton University in New Jersey and co-author of the article on dark matter.

Dark matter is estimated to account for 26 percent of the universe and it is much more abundant than ordinary matter, which makes up just 4 percent of the universe. Structure growth is caused by gravity drawing together matter into clumps. The pace of growth depends on how much matter there is.

"Since there is more dark matter than ordinary matter, dark matter dominates the process," said Steinhardt. "The ordinary matter is like froth moving about on an ocean of dark matter."

Steinhardt adds that because ordinary matter closely interacts with light, it cannot clump together because the hot radiation that we see as light would blow apart any clumps.

"Dark matter can start clustering well ahead of ordinary matter because it does not interact with light," he said. "When the ordinary matter finally stops interacting with light strongly, it finds that dark matter has already collapsed gravitationally and formed clumps."

This dark matter pulls in the ordinary matter, allowing it to become concentrated much faster than it would if ordinary matter alone was clustering. Thus, Steinhardt says, "with dark matter we can go from the nearly uniform universe to the universe of galaxies and stars we see today, and we can exist."

The question remains, however: what exactly is dark matter.

The predominant theory is that dark matter is made up of cold, weakly interacting massive particles (called WIMPS), but there may be a problem in that the theory appears to predict more clumps and more concentrated clumps of matter than scientists have detected.

"For example, we see about a dozen dwarf galaxies orbiting our Milky Way but simulations suggest there should be thousands," said Steinhardt. "Also, the simulations predict highly concentrated dark matter in the cores of galaxies including our own Milky Way, but evidence suggests that very little dark matter is in the core of our Milky Way."

As a result, Steinhardt and colleagues are exploring other theories that can account for the discrepancy. The paper he co-authored with Jeremiah Ostriker, an astrophysicist at Princeton University, outlines how they can use the local universe—he volume around the Milky Way and nearby Andromeda Galaxy—to learn the nature of dark matter.

"We have seen that careful studies of the distribution, density, environment, and variety of subgalactic structures in our neighborhood might shed light on the nature of dark matter," said Steinhardt.

Once the nature of the dark matter is determined, future studies will be focused on its specific identity.

Dark Energy

The least understood component of the dark side is dark energy, which is believed to comprise 70 percent of the universe. But whatever dark energy is, it appears to be causing the accelerating expansion of the universe.

Based on observations of a range of exploding stars—or supernovae—out to about 7 billion light years, astronomers see hints that the universe was slowing down approximately 7 billion years ago, but they are confident that it has more recently begun to accelerate.

Kirshner at the Harvard-Smithsonian Center for Astrophysics said that for most of the 20th century scientists had assumed cosmic expansion would be slowing down, but that it now looks like dark energy is making the universe speed up.

This shift from slowing down to speeding up is a clue into the nature of dark energy, said Kirshner. Unlike dark matter, which is believed to have slowed down the expansion of the universe for several billion years owing to its gravitational tug, dark energy does not become diluted over time.

Dark energy may be a modern form of Einstein's lamented "cosmological constant." or it may be an energy that changes over time. Observation programs underway will help find out.

"It's a little like looking out the window and seeing a tree swaying, with its leaves fluttering. You don't see anything doing it but you say, 'oh, it's the wind,'" said Kirshner. "We see the acceleration, so we assume there's something doing it, and dark energy could be the source."

Kirshner and his colleagues plan to take advantage of new technologies such as electronic cameras on ground based telescopes like the Cerro Tololo telescope in Chile and the new Advanced Camera for Surveys on the Hubble Space Telescope to find and measure more supernovae and hence learn more about the nature of dark energy.
 

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