Nature News | Eugenie Samuel Reich | 24 Nov 2010
A geometric measure of dark energy with pairs of galaxies - C Marinoni, A BuzziGeometric test supports the existence of a key thread in the fabric of the Universe.
The claim that mysterious dark energy is accelerating the Universe's expansion has been placed on firmer ground, with the successful application of a quirky geometric test proposed more than 30 years ago.
The accelerating expansion was first detected in 1998. Astronomers studying Type 1a supernovae, stellar explosions called "standard candles" because of their predictable luminosity, made the incredible discovery that the most distant of these supernovae appear dimmer than would be expected if the Universe were expanding at a constant rate.1 This suggested that some unknown force - subsequently dubbed dark energy - must be working against gravity to blow the universe apart.
Since that time, studies comparing variations in the cosmic microwave background radiation — an echo from the Big Bang — with the distribution of galaxies today have allowed cosmologists to trace how the Universe has expanded, supporting the idea of dark energy. They have also suggested that the Universe is 'flat' — that is, it contains just enough matter to keep it delicately poised between collapsing in on itself and expanding forever2.
These two assumptions have become a fundamental part of cosmologists' understanding of the Universe. Now Christian Marinoni and Adeline Buzzi of the Centre for Theoretical Physics at the University of Provence in Marseilles, France, have independently checked these ideas by analysing the geometry of orbiting pairs of galaxies. Their study is published this week in Nature3.
The researchers used a version of the Alcock–Paczynski test, which relies on identifying symmetrical objects in space and using them as 'standard spheres'. Any distortions in space caused by the expansion of the cosmos would cause the most distant standard spheres to appear asymmetrical. "This provides a similar level of accuracy to supernovae," says Marinoni. "It's a direct proof of dark energy."
- Nature 468(7323) 539 (25 Nov 2010) DOI: 10.1038/nature09577
Scientific American | John Matson | 24 Nov 2010
Einstein's 'Biggest Blunder' Turns Out to Be RightThe orientation of hundreds of galactic pairs provides a new test of the standard cosmological view
In the late 1990s, two teams of astronomers stunned the scientific community with the finding that the universe is accelerating in its expansion, somehow overpowering the constant pull of gravity that should be slowing it down. The culprit pressing the cosmic accelerator goes by the name "dark energy," which is an appropriately enigmatic moniker for something that remains so poorly understood.
"We have an amazingly simple picture of the universe," says Princeton University astrophysicist Michael Strauss. "Of course, we don't understand that picture—we don't know what dark energy is, and we don't know what dark matter is." Dark matter, a mysterious entity of longer standing, is some invisible but common substance that reveals itself only through its gravitational pull.
But dark energy—whatever it is—is there, according to a number of measurements taken in the years since its influence was first detected. Now a pair of researchers at the University of Provence in France has added to the body of evidence by confirming dark energy's presence through an independent test that verifies the impact of cosmic parameters on the appearance of pairs of distant galaxies. The research appears in the November 25 issue of Nature. (Scientific American is part of Nature Publishing Group.)
Space.com | Clara Moskowitz | 24 Nov 2010
Galactic illusion helps refine dark energy abundanceWhat Einstein called his worst mistake, scientists are now depending on to help explain the universe.
In 1917, Albert Einstein inserted a term called the cosmological constant into his theory of general relativity to force the equations to predict a stationary universe in keeping with physicists' thinking at the time. When it became clear that the universe wasn't actually static, but was expanding instead, Einstein abandoned the constant, calling it the '"biggest blunder" of his life.
But lately scientists have revived Einstein's cosmological constant (denoted by the Greek capital letter lambda) to explain a mysterious force called dark energy that seems to be counteracting gravity — causing the universe to expand at an accelerating pace.
A new study confirms that the cosmological constant is the best fit for dark energy, and offers the most precise and accurate estimate yet of its value, researchers said. The finding comes from a measurement of the universe's geometry that suggests our universe is flat, rather than spherical or curved.
New Scientist | Physics & Math | 24 Nov 2010
AN OPTICAL illusion has enabled the most precise measurement yet of the abundance of the mysterious dark energy, which is thought to be accelerating the universe's expansion.
The more dark energy there is, the faster the expansion should be, so measuring the universe's expansion provides an estimate of the abundance of dark energy.
Christian Marinoni and Adeline Buzz at the University of Provence in Marseille, France, realised they could fine-tune such estimates by observing distant galaxy systems in which two galaxies orbit each other. For far-off pairs, assessing the separation of the two galaxies relies on knowing how fast the universe is expanding. Get this wrong, and the result is an optical illusion: the distance between the galaxies appears longer or shorter than its true value.
Reasoning that, on average, the distances between galaxies in such pairs should be the same throughout time and space, the team measured the separation of 700 nearby pairs, which they could do directly without reference to the expansion of the universe. Then, taking the observations of 500 distant pairs, they adjusted the value of the rate of expansion until the average separation appeared the same in the distant ones as in the nearby ones. The resulting dark energy value, 74 per cent of the universe's energy, had a potential error one-third the size of previous estimates.
