A 3-D scan of hundreds of thousands of galaxies has confirmed the view that the expansion of the universe is speeding up, due to a mysterious factor called dark energy. The galaxy survey, described in a study set to be published by the journal Astronomy and Astrophysics, serves as one more line of evidence for dark energy's existence.
The idea behind dark energy cropped up 12 years ago when astronomers carefully measured how quickly supernovas were receding from us - and noticed that the speed was increasing with time. Since then, other types of evidence have piled up, including a survey of 13,000 galaxies conducted using the European Southern Observatory's Very Large Telescope in Chile.
The latest study kicks it up a notch by drawing upon the Hubble Space Telescope's COSMOS survey of more than 446,000 galaxies. Using ground-based telescopes, researchers were able to determine the distances to 194,000 of those galaxies - and chart the distribution of matter out to about 12 billion light-years.
Astronomers even accounted for the presence of dark matter in all those galaxies by analyzing how the gravitational effect of that unseen material warped the light coming from even more distant galaxies. This effect is called weak gravitational lensing.
A similar technique was used three years ago to generate a 3-D map of dark matter disribution from COSMOS data. Yet another gravitational-lensing study along the same lines was published in The Astrophysical Journal this year. The latest study puts together the 3-D map with information about how fast galaxies are receding, giving astronomers a better understanding of how the cosmic expansion has changed over time.
"The sheer number of galaxies included in this type of analysis is unprecedented, but more important is the wealth of information we could obtain about the invisible structures in the universe from this exceptional data set," one of the authors of the study, Patrick Simon of Edinburgh University, said in a news release issued by the European Space Agency's Hubble team.
When the researchers compared their data with different computer-generated models of what the universe should look like, they found that the models without dark energy could not fit what they were seeing.
"Dark energy affects our measurements for two reasons," said another co-author of the study, Benjamin Joachimi of the University of Bonn. "First, when it is present, galaxy clusters grow more slowly, and secondly, it changes the way the universe expands, leading to more distant - and more efficiently lensed - galaxies. Our analysis is sensitive to both effects."
Harvard astronomer William High said that most of the previous studies of matter distribution have been done in 2-D, like taking a chest X-ray of a patch of the night sky. "Our study is more like a 3-D reconstruction of the skeleton from a CT scan," he said. "On top of that, we are able to watch the skeleton of dark matter mature from the universe's youth to the present."
The European Hubble team released a color-coded image (in 2-D) that charts the development of the cosmic skeleton. That's the image that appears at the top of this item. White, blue and green represent structures that are closer to us, while red and orange structures are farther away. In all these spots, dark matter accounts for most of the mass being mapped.
Such a breakdown is consistent with the current thinking that all the matter we can see accounts for only 4 percent of the universe's content, with dark matter making up 22 percent. The other 74 percent seems to be bound up in dark energy.
The Hubble study doesn't shed any new light on what dark matter or dark energy actually is. To determine the nature of dark matter, scientists will probably have to turn to the Large Hadron Collider or future astronomical observations.
And dark energy? Figuring out what that is poses an even bigger challenge. Scientists might just have to accept dark energy as a property of the universe where we happen to live, and add a cosmic "fudge factor" to the equations of general relativity. Or they might have to come up with something else entirely.
More on the dark universe:
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