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,