Analysis by Jennifer Ouellette
Wed Sep 12, 2012 01:13 AM ET
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Our universe is a mysterious place. Only 4 percent or so is made up of the ordinary matter we see around us, including all those galaxies filled with stars. The rest, physicists believe, is made of up weakly interacting dark matter, and a mysterious substance called dark energy that is causing the cosmos to expand at an accelerating rate.
At least, that's the working hypothesis, and it seems to fit the data, although there are scientists who question its existence and tout alternatives to explain that accelerating expansion.
A new, two-year study by scientists at the University of Portsmouth and Ludwig-Maximilians-Universitat Munchen concludes that dark energy does, indeed, exist. Those results just appeared in the journal Monthly Notices of the Royal Astronomical Society.
In the early 20th century, scientists believed the universe was in a steady state. But when Albert Einstein was working on his theory of general relativity, the math just didn't add up: the universe should have been expanding. So he invented something called the cosmological constant -- a mathematical trick to balance everything out so that the equations described a static universe, rather than an expanding one.
But then astronomer Edwin Hubble discovered the universe actually was expanding -- Einstein's original equations were correct. He dubbed the cosmological constant (lambda) his "greatest blunder."
The universe wasn't done surprising us, however. In 1998, astronomers studying distant exploding stars called a Type 1A supernovae discovered that not only was the universe expanding, but that the rate of expansion was accelerating due to some type of unknown force or dark energy. And one of the explanations for this effect is -- you guessed it -- Einstein's cosmological constant.
While that discovery snagged its team leaders the 2011 Nobel Prize in Physics, it's not the only evidence in favor of dark energy. For instance, last May, a team of scientists from Melbourne's Swinburne University announced their independent confirmation of both the existence of dark energy and its rate of expansion, based on four years of data collected by a powerful spectrograph at the Australian Astronomical Observatory.
That study included more than 240,000 galaxies going back over seven billion years and showed that the growth of galaxy clusters and super clusters has slowed down. This means that in the most distant parts of universe -- those further back in spacetime -- gravity dominates. It's only in our current part of the cosmos where dark energy dominates, and hence we are seeing accelerated expansion.
The Swinburne researchers also looked at the distances between pairs of galaxies, and the ripples in the cosmic microwave background radiation (CMB). They found that the average distance between galaxy pairs (about 500,000,000 light years) has been growing because of the expansion of space-time, providing further confirmation of dark energy.
One of the strongest pieces of evidence can be found in a unique feature of the CMB, known as the Integrated Sachs-Wolfe Effect, based on a 1967 prediction that light from the CMB would show a redshift -- i.e., it would become slightly "bluer" -- as it passed through gravitational fields of clumped matter.
Scientists didn't detect the Sachs-Wolfe Effect until 2003 -- Science magazine deemed it the "discovery of the year." It showed up as tiny gains in energy among photons in the CMB, based on comparing the temperature of the CMB with maps of galaxies in our local part of the universe.
As exciting as that discovery was, it was pretty weak signal, and might have been caused by something else -- space dust, for instance. So Tommaso Giannantonio and Robert Crittenden took the lead on a two-year study to re-examine that data and improve the galactic maps used in the original work.
Their conclusion: there is a 99.996 percent chance that dark energy is responsible for those observed variations in the CMB where the photons are just a wee bit hotter than the rest. That's equivalent to the level of certainly of the recent announcement of the discover of a Higgs-like particle.
We still don't know what exactly the dark energy is, of course, and what all this will ultimately mean for modifications of general relativity. But its existence is looking like much more of a sure thing, although we can expect to see some pretty close scrutiny of these new results from those who remain skeptical.
This isn't the final word, is the point -- science marches on. As Giannantonio said via press release, "The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works."
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Dark energy: The mysterious force causing the universe to expand
Scientists confirm with 99.996 percent certainty the existence of a little-understood substance that is "one of the great scientific mysteries of our time"
POSTED ON SEPTEMBER 14, 2012, AT 7:10 AM
Physicists believe that only 4 percent of the universe is made up of ordinary matter like stars and planets, with the remaining 96 percent composed of dark matter and dark energy. Photo: ThinkStock/HemeraSEE ALL 124 PHOTOS
Physicists have long understood very little about dark energy, a mysterious substance thought to be stretching out the cosmos. But thanks to a new two-year study from researchers in Germany and the U.K., we can at least confirm scientists' longstanding dark-energy theories with 99.996 percent certainty. But what is this stuff, exactly? Here, a handy guide:
First things first: What is the universe made of?
"Our universe is a mysterious place," says Jennifer Ouellette atDiscovery News. Roughly 4 percent of it is made of ordinary matter like stars and planets. The rest, according to physicists, is a combination of mysterious dark matter — which functions as an invisible glue that holds the universe together — and an even lesser-understood substance call dark energy, which is "causing the cosmos to expand at an accelerating rate."
"Our universe is a mysterious place," says Jennifer Ouellette atDiscovery News. Roughly 4 percent of it is made of ordinary matter like stars and planets. The rest, according to physicists, is a combination of mysterious dark matter — which functions as an invisible glue that holds the universe together — and an even lesser-understood substance call dark energy, which is "causing the cosmos to expand at an accelerating rate."
Hold on. What exactly is dark energy?
Scientists aren't sure, but they have a few ideas. According to NASA, dark energy could be a "property of space" that helps create more space. Another possibility: Dark energy is an energy fluid or field that "fills all of space," but has the opposite properties of matter. Yes, it's confusing. And the truth is, scientists still don't know what dark energy is — only that it exists. Dark energy is now believed to make up 73 percent of the universe.
Scientists aren't sure, but they have a few ideas. According to NASA, dark energy could be a "property of space" that helps create more space. Another possibility: Dark energy is an energy fluid or field that "fills all of space," but has the opposite properties of matter. Yes, it's confusing. And the truth is, scientists still don't know what dark energy is — only that it exists. Dark energy is now believed to make up 73 percent of the universe.
How was dark energy discovered?
When Albert Einstein was working on his theory of relativity, "the math didn't add up," says Ouellette. According to his calculations, the universe wasn't a static size as he'd assumed — it was actually expanding. To account for this curveball, Einstein invented a mathematical trick to balance the equations, something he called the "cosmological constant," which serves as a placeholder for what's essentially empty space. Years later, astronomer Edwin Hubble confirmed the cosmological constant with his own observations, and astronomers in the late 20th century corroborated the theory with even more evidence that the universe was growing.
When Albert Einstein was working on his theory of relativity, "the math didn't add up," says Ouellette. According to his calculations, the universe wasn't a static size as he'd assumed — it was actually expanding. To account for this curveball, Einstein invented a mathematical trick to balance the equations, something he called the "cosmological constant," which serves as a placeholder for what's essentially empty space. Years later, astronomer Edwin Hubble confirmed the cosmological constant with his own observations, and astronomers in the late 20th century corroborated the theory with even more evidence that the universe was growing.
And dark energy makes the universe grow at a faster rate?
Yes. Most astronomers used to assume that the universe's expansion would eventually begin slowing down, or that the universe would even begin shrinking, because of another well-known force: Gravity. In 1998, however, two separate teams of researchers — who went on to win the Nobel Prize in physics — found that the universe's expansion was actuallyspeeding up. Their theory: That a unique force, dark energy, was outperforming gravity's pull and making the universe expand faster and faster.
Yes. Most astronomers used to assume that the universe's expansion would eventually begin slowing down, or that the universe would even begin shrinking, because of another well-known force: Gravity. In 1998, however, two separate teams of researchers — who went on to win the Nobel Prize in physics — found that the universe's expansion was actuallyspeeding up. Their theory: That a unique force, dark energy, was outperforming gravity's pull and making the universe expand faster and faster.
So... what did this new study prove?
Researchers looked at the distance between two galaxies and measured cosmic microwave background radiation, or CMB, which ripples as the two galaxies grow farther and farther apart. Researchers spent two years analyzing the data, and found that the two galaxies were indeed separating. Their conclusion? There's a 99.996 chance dark energy is real. Of course, skeptics say the work is very preliminary, and much more research will be needed to confirm that dark energy actually exists. "Dark energy is one of the great scientific mysteries of our time," says researcher Bob Nicholfrom the University of Portsmouth, "so it isn't surprising that so many researchers question its existence."
Researchers looked at the distance between two galaxies and measured cosmic microwave background radiation, or CMB, which ripples as the two galaxies grow farther and farther apart. Researchers spent two years analyzing the data, and found that the two galaxies were indeed separating. Their conclusion? There's a 99.996 chance dark energy is real. Of course, skeptics say the work is very preliminary, and much more research will be needed to confirm that dark energy actually exists. "Dark energy is one of the great scientific mysteries of our time," says researcher Bob Nicholfrom the University of Portsmouth, "so it isn't surprising that so many researchers question its existence."
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