Greatest Mysteries: Where is the Rest of the Universe?

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Greatest Mysteries: Where is the Rest of the Universe?

By Dave Mosher
Staff Writer
posted: 20 August 2007
06:11 am ET


Scientists trying to create a detailed inventory of all the matter and energy in the cosmos run into a curious problem--the vast majority of it is missing.

"I call it the dark side of the universe," said Michael Turner, a cosmologist at the University of Chicago, referring to the great mysteries of dark matter and dark energy.

In fact, only 4 percent of the matter and energy in the universe has been found. The other 96 percent remains elusive, but scientists are looking in the farthest reaches of space and deepest depths of Earth to solve the two dark riddles.

Missing matter

Einstein's famous equation "E=mc^2" describes energy and matter (or mass) as one and the same--maps of the cosmos refer to the energy-matter combination as energy density, for short. The problem with detecting dark matter, thought to make up 22 percent of the universe's mass/energy pie, is that light doesn't interact with it.

But it does exhibit the tug of gravity.

Initial evidence for the mysterious matter was discovered 75 years ago when astrophysicists noticed an anomaly in a jumble of galaxies: The galactic cluster had hundreds of times more gravitational pull than it should have, far outweighing its visible mass of stars.

"We can predict the motions of the sun and planets very accurately, but when we measure distant things we see anomalies," said Scott Dodelson, an astrophysicist at Fermi National Accelerator Laboratory in Illinois. "Dark matter is currently the best possible solution, even though we've never seen any of it."

Another hallmark of dark matter is gravitational lensing, similar to the effect of light passing through a piece of polished glass. Massive objects like the sun can bend light, but colossal clouds of dark matter create "bubbles" in the cosmos that magnify, distort and duplicate the light of galaxies or stars behind them.

Gravitational lensing recently exposed evidence of the unseen mass in the Bullet cluster as well as in a ring around a cluster of colliding galaxies called ZwCl0024+1652.

Particle hunt

In spite of the ghostly evidence, pieces of dark matter have yet to be pinned down by researchers. "Until we actually discover particles, we're not home yet," Dodelson said.

Particle physicists have detected neutrinos, which are extremely lightweight particles that pour out of the sun and hardly interact into ordinary matter, but Turner said they make up an extremely small fraction of dark matter in the universe.

"We arrested one of the members of the gang, but not the leader of the gang," Turner said of neutrinos. He thinks the leader is actually a WIMP: a weakly interactive, massive particle. Unfortunately, WIMPS are just a theory so far.

The thinking goes that WIMPs are very heavy, yet like neutrinos they rarely bump into matter to produce a detectable signal. But the idea that WIMPS--such as theoretical axion or neutralino particles--can bump into visible matter at all gives scientists hope.

"This is a story that may soon be at its end," Turner said, noting that the Cryogenic Dark Matter Search in the Soudan mine of Minnesota and other experiments below the ground should be sensitive enough to detect a WIMP.

The anti-gravity

Perhaps the biggest mystery of all is dark matter's big cousin, dark energy.

The invisible force is thought to be a large-scale "anti-gravity," pushing apart galactic clusters and causing the unexplainable, accelerating expansion of the universe. Turner thinks dark energy is the biggest mystery of them all--and quite literally, since physicists predict that it makes up 74 percent of energy density in the universe.

"So far, the greatest achievement with dark energy is giving it a name," Turner said of the elusive force. "We are really at the very beginning of this puzzle."

Turner described dark energy as "really weird stuff," best thought of as an elastic, repulsive gravity that can't be broken down into particles. "We know what it does, but we don't know what it is," Turner said.

While astrophysicists look deep into space to gather more details about dark energy's effects, Turner noted that theoretical physicists are focusing on explaining how the force actually works. And at this point, he joked, any physicist's explanation for dark energy is probably good enough to consider.

"We're at this very early stage, at the crime scene of dark energy's existence, if you will," Turner said. "It's a highly creative period, and now is the time for ideas."

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Astronomers Find a Hole in the Universe



By SETH BORENSTEIN
The Associated Press
Friday, August 24, 2007; 7:45 AM

WASHINGTON -- Astronomers have stumbled upon a tremendous hole in the universe. That's got them scratching their heads about what's just not there. The cosmic blank spot has no stray stars, no galaxies, no sucking black holes, not even mysterious dark matter. It is 1 billion light years across of nothing. That's an expanse of nearly 6 billion trillion miles of emptiness, a University of Minnesota team announced Thursday.

Astronomers have known for many years that there are patches in the universe where nobody's home. In fact, one such place is practically a neighbor, a mere 2 million light years away. But what the Minnesota team discovered, using two different types of astronomical observations, is a void that's far bigger than scientists ever imagined.

"This is 1,000 times the volume of what we sort of expected to see in terms of a typical void," said Minnesota astronomy professor Lawrence Rudnick, author of the paper that will be published in Astrophysical Journal. "It's not clear that we have the right word yet ... This is too much of a surprise."

Rudnick was examining a sky survey from the National Radio Astronomy Observatory, which essentially takes radio pictures of a broad expanse of the universe. But one area of the universe had radio pictures indicating there was up to 45 percent less matter in that region, Rudnick said.

The rest of the matter in the radio pictures can be explained as stars and other cosmic structures between here and the void, which is about 5 to 10 billion light years away.

Rudnick then checked observations of cosmic microwave background radiation and found a cold spot. The only explanation, Rudnick said, is it's empty of matter.

It could also be a statistical freak of nature, but that's probably less likely than a giant void, said James Condon, an astronomer at the National Radio Astronomy Observatory. He wasn't part of Rudnick's team but is following up on the research.

"It looks like something to be taken seriously," said Brent Tully, a University of Hawaii astronomer who wasn't part of this research but studies the void closer to Earth.

Tully said astronomers may eventually find a few cosmic structures in the void, but it would still be nearly empty.

Holes in the universe probably occur when the gravity from areas with bigger mass pull matter from less dense areas, Tully said. After 13 billion years "they are losing out in the battle to where there are larger concentrations of matter," he said.

Retired NASA astronomer Steve Maran said of the discovery: "This is incredibly important for something where there is nothing to it."

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On the Net:

Rudnick paper: http://xxx.lanl.gov/pdf/0704.0908

National Radio Astronomy Observatory: http://www.nrao.edu/pr/2007/coldspot/