Phil Plait writes on Bad Astronomy:
Holy Haleakala! This picture is incredible. They pointed Hubble at a fairly empty region of space, one where very few stars are seen. Then they unleashed the new Wide Field Camera 3 (called WFC3 for short) on it, taking images in infrared wavelengths just outside what the human eye can see … and they let it stare at that spot for a solid 48 hours.
The result? This picture, showing galaxies flippin’ everywhere, some seen a mere 600 million years after the Big Bang itself. Because the Universe is expanding, distant galaxies appear to recede from us, and their light gets stretched out. This Doppler Effect — the same thing that makes the sound of a car engine drop in pitch when it passes you at high speed — changes the colors we see from these far-flung galaxies, so their ultraviolet light, for example, gets stretched into visible and even infrared wavelengths. What you are seeing here is actually more energetic light emitted by galaxies that’s lost energy traveling across the expanding Universe, so by the time it gets here it’s infrared.
So the colors are not “real” in this image; they’ve been translated into red, green, and blue so we can see them. The reddest objects in the image are most likely the farthest away, and may be as much as 13 billion light years away. Thirteen billion. With a B.