NASA JPL-Caltech GALEX | 2011 May 19
Dark Energy Is Real, Galaxy Survey ShowsA five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, has led to one of the best independent confirmations that dark energy is driving our universe apart at accelerating speeds. The survey used data from NASA's space-based Galaxy Evolution Explorer and the Anglo-Australian Telescope on Siding Spring Mountain in Australia.
The findings offer new support for the favored theory of how dark energy works -- as a constant force, uniformly affecting the universe and propelling its runaway expansion. They contradict an alternate theory, where gravity, not dark energy, is the force pushing space apart. According to this alternate theory, with which the new survey results are not consistent, Albert Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances.
"The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into the sky faster and faster," said Chris Blake of the Swinburne University of Technology in Melbourne, Australia. Blake is lead author of two papers describing the results that appeared in recent issues of the Monthly Notices of the Royal Astronomical Society. "The results tell us that dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we wouldn't be seeing these constant effects of dark energy throughout time."
Dark energy is thought to dominate our universe, making up about 74 percent of it. Dark matter, a slightly less mysterious substance, accounts for 22 percent. So-called normal matter, anything with atoms, or the stuff that makes up living creatures, planets and stars, is only approximately four percent of the cosmos.
The idea of dark energy was proposed during the previous decade, based on studies of distant exploding stars called supernovae. Supernovae emit constant, measurable light, making them so-called "standard candles," which allows calculation of their distance from Earth. Observations revealed dark energy was flinging the objects out at accelerating speeds.
Dark energy is in a tug-of-war contest with gravity. In the early universe, gravity took the lead, dominating dark energy. At about 8 billion years after the Big Bang, as space expanded and matter became diluted, gravitational attractions weakened and dark energy gained the upper hand. Billions of years from now, dark energy will be even more dominant. Astronomers predict our universe will be a cosmic wasteland, with galaxies spread apart so far that any intelligent beings living inside them wouldn't be able to see other galaxies.
The new survey provides two separate methods for independently checking the supernovae results. This is the first time astronomers performed these checks across the whole cosmic timespan dominated by dark energy. The team began by assembling the largest three-dimensional map of galaxies in the distant universe, spotted by the Galaxy Evolution Explorer. The ultraviolet-sensing telescope has scanned about three-quarters of the sky, observing hundreds of millions of galaxies.
"The Galaxy Evolution Explorer helped identify bright, young galaxies, which are ideal for this type of study," said Christopher Martin, principal investigator for the mission at the California Institute of Technology in Pasadena. "It provided the scaffolding for this enormous 3-D map."
The astronomers acquired detailed information about the light for each galaxy using the Anglo-Australian Telescope and studied the pattern of distance between them. Sound waves from the very early universe left imprints in the patterns of galaxies, causing pairs of galaxies to be separated by approximately 500 million light-years.
This "standard ruler" was used to determine the distance from the galaxy pairs to Earth -- the closer a galaxy pair is to us, the farther apart the galaxies will appear from each other on the sky. As with the supernovae studies, this distance data were combined with information about the speeds at which the pairs are moving away from us, revealing, yet again, the fabric of space is stretching apart faster and faster.
The team also used the galaxy map to study how clusters of galaxies grow over time like cities, eventually containing many thousands of galaxies. The clusters attract new galaxies through gravity, but dark energy tugs the clusters apart. It slows down the process, allowing scientists to measure dark energy's repulsive force.
"Observations by astronomers over the last 15 years have produced one of the most startling discoveries in physical science; the expansion of the universe, triggered by the Big Bang, is speeding up," said Jon Morse, astrophysics division director at NASA Headquarters in Washington. "Using entirely independent methods, data from the Galaxy Evolution Explorer have helped increase our confidence in the existence of dark energy."
Australian Astronomical Observatory | 2011 May 19
WiggleZ galaxy project proves Einstein was right!Observations of more than 200,000 galaxies made with the Anglo-Australian Telescope in eastern Australia have shown that "dark energy" is real and not a mistake in Einstein's conception of gravity.
The result is conveyed in two papers written by Dr Chris Blake (Swinburne University of Technology, Melbourne, Australia) and colleagues, which have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.
The galaxy survey, called WiggleZ ("wiggles"), was set up to measure the properties of "dark energy", a concept invoked in the late 1990s to explain why the Universe seems to be expanding at an accelerating rate.
To account for the acceleration, astronomers had to either rewrite Einstein's theory of gravity or accept that the Universe is filled with a new kind of energy.
"Our new work shows dark energy is real," said Dr Blake. "Einstein remains untoppled."
Dark energy was originally discovered by studying the brightness of distant supernovae — exploding stars.
The WiggleZ project has used two other kinds of observations that provide an independent check on the supernovae results. One kind involves measuring a pattern in how galaxies are distributed in space ("baryon acoustic oscillations"), and the other, measuring how quickly clusters of galaxies have formed over time. [For details, see Background Notes.]
Both tests have confirmed the reality of dark energy.
WiggleZ is one of several baryon acoustic oscillations experiments planned or in progress, and is the first one at high redshift to bear significant fruit.
The survey mapped the distribution of galaxies in an unprecedented volume of the Universe, looking eight billion years back in time — more than half the age of the Universe.
"This is the first individual galaxy survey to span such a long stretch of cosmic time," said Professor Michael Drinkwater (University of Queensland), one of the survey's leaders. "We've broken new ground."
University of Queensland | 2011 May 19
Dark Energy Is Real: ClaimAn Australian-based astronomy team, co-led by Professor Michael Drinkwater from the School of Mathematics and Physics (SMP) at The University of Queensland (UQ), has shown that the mysterious ‘dark energy' is indeed real and not a mistake in Einstein's theory of gravity.
Using the Anglo-Australian Telescope, 26 astronomers (from 14 different institutions) contributed to the ‘WiggleZ Dark Energy Survey', which mapped the distribution of galaxies over an unprecedented volume of the Universe.
Because light takes so long to reach Earth, it was the equivalent of looking seven billion years back in time – more than half way back to the Big Bang.
“This is the first individual galaxy survey to span such a long stretch of cosmic time,” said Professor Drinkwater said.
It was only possible thanks to new Australian technology.”
The survey, which covered more than 200,000 galaxies, took four years to complete and aimed to measure the properties of ‘dark energy' - a concept first cast by Einstein in his Theory of General Relativity.
The scientist adapted his original equations to include the idea and later ruefully admitted that it was "his greatest blunder".
Dark energy is the name astronomers gave in the late 1990s to an unknown cause of the Universe's accelerating expansion.
This mysterious energy, that defies gravity, makes up about 72 percent of the Universe, with the remaining 24 percent constituting dark matter, and 4 percent making up the planets, stars and galaxies that we normally hear about.
“The discovery of acceleration was an enormous shock, because it went against everything we thought we knew about gravity,” co-researcher Dr Tamara Davis from the University of Queensland said.
“The problem was, that supernova data couldn't tell us whether dark energy was genuinely there, or whether Einstein's theory of gravity itself was failing."
WiggleZ used two other kinds of observations to provide an independent check on the supernova results.
One measured the pattern of how galaxies are distributed in space and the other measured how quickly clusters of galaxies formed over time.
“WiggleZ says dark energy is real. Einstein remains untoppled,” said Dr Chris Blake, of Swinburne University, lead author of the recent findings, which will be published in two papers in the Monthly Notices of the Royal Astronomical Society.
According to Professor Warrick Couch, Director of the Centre for Astrophysics and Supercomputing, confirming the existence of the anti-gravity agent is a significant step forward in understanding the Universe.
“Although the exact physics required to explain dark energy still remains a mystery, knowing that dark energy exists has advanced astronomers' understanding of the origin, evolution and fate of the Universe,” he said.
The WiggleZ observations were possible due to a powerful spectrograph located at the Anglo-Australian Telescope.
The spectrograph was able to image 392 galaxies an hour, despite the galaxies being located halfway to the edge of the observable Universe.
“WiggleZ has been a success because we have an instrument attached to the telescope, a spectrograph, that is one of the best in the world for large galaxy surveys of this kind,” said Professor Matthew Colless, director of the Australian Astronomical Observatory.
Discovery News | Stuart Gary, ABC Science Online | 2011 May 19
Dark Energy Is Real, New Evidence IndicatesAlbert Einstein was right: Dark energy is constant throughout the universe and causing it to expand.
- Dark energy exists and is a cosmological constant throughout our universe as Einstein predicted, according to a new study.
- A hundred years ago scientists believed the universe was steady and unchanging.
- Einstein invented the constant to expand the fabric of space-time after his own equations wouldn't allow for the cosmos to remain static.
Space.com | 2011 May 19
The WiggleZ Dark Energy Survey: the growth rate of cosmic structure since redshift z=0.9 - C Blake et al
- arXiv.org > astro-ph > arXiv:1104.2948 > 15 Apr 2011
- arXiv.org > astro-ph > arXiv:1105.2862 > 14 May 2011