Because of the time it takes for light in the distant universe to reach Earth, when we look at, for example, a star a billion light-years away, we're actually seeing what a portion of the cosmos looked like a billion years ago.
So, by combining pictures of various reaches of a small patch of sky, Hubble's new eXtreme Deep Field (XDF) image, released Tuesday, essentially flattens time. Primitive, 13-billion-year-old galaxies—born just 450 million years after the big bang—seem to float side-by-side with closer, more advanced galaxies.
As "the deepest image of the sky ever obtained," the XDF picture "reveals the faintest and most distant galaxies ever seen," Hubble astronomer Garth Illingworth, of the University of California, Santa Cruz, said in a statement.
And some of the prettiest. "It's beautiful," said Geza Gyuk, director of astronomy at Chicago's Adler Planetarium. "The level of detail is amazing, and being able to look that far back in time is incredible. You can begin to really see the vast majority of the history of the universe in a single image."
To create the deepest ever picture of the universe, a NASA team combined more than 2,000 images of the same bit of southern sky in the constellation Fornax.
The images had been captured over ten years by two Hubble cameras, which together capture light in ultraviolet, visible, and near-infrared wavelengths. Total exposure time: two million seconds, or 23 days.
The combined view sparkles with some 5,500 galaxies, including spirals like our own Milky Way and fuzzy red giants—the aging remnants of ancient galactic collisions where no new stars are being born. Without Hubble's help, the faintest would have to be ten billion times brighter to be seen by the naked eye.
(Related: "Massive Star Collision Spotted by Hubble.")
Though the new Hubble picture is itself exceptional, the area it depicts is anything but, Gyuk said.
"It's probably a fairly average portion of the universe," he added. "It's a tiny portion of sky, but since it goes so far back it's actually a very large volume.
"Also, it does kind of flatten out time, and you get this impression of incredible richness, which is a bit of an illusion, because these galaxies are spread out over billions of light-years—the universe is not quite so crowded."