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There are voids in the universe, and we can't see them properly. And that's a good thing.
These voids — giant, irregular gaps in space that are empty of galaxies — are all over the cosmos. But, because they are empty, astronomers can't directly observe them. Instead, they spot them by mapping galaxies across space, and then marking the areas in between these areas. However, from our perspective on Earth, all those voids look distorted.
Areas - Places - Others - Consequence - Galaxies
These areas appear stretched in some places and squished in others. That’s a consequence of "redshifting" of galaxies at their borders, a visual distortion caused by the movement of these systems: As they move away from the viewer (Earthlings, in this case), the galaxies' wavelengths appear to stretch, becoming more red; those moving toward us would look more blue as their wavelengths get shorter. Dark energy is the name astronomers have given to an invisible force stretching our universe and causing galaxies to move away from one another.
"What we are actually measuring is the distortion in the positions of galaxies around void regions," said Seshadri Nadathur, a researcher at the University of Portsmouth in the United Kingdom, and lead author of the paper. "The cool thing about voids is that they are regions of space around which we can very accurately model galaxy motions."
Galaxies - Gravity - Regions - Matter - Problem
"Galaxies move because of gravity pulling them toward regions of excess matter, and the problem generally is that our theory of gravity — Einstein’s general relativity — is very complex, and the equations are hard to solve exactly," he said. "So most of the time in cosmology we use approximations — known as 'perturbation theory' — to help make the problem tractable. This perturbation theory works a lot better in void regions than it does in regions where there is lots of matter, so...
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