The Hubble Ultra Deep Field; Credit: S. Beckwith & the HUDF Working Group (STScI), HST, ESA, NASA
In 1995, the Director (Robert Williams) and members of the Space Telescope Institute had an unusual idea --- they decided to point the Hubble Space Telescope for about 5 days at the blankest portion of the sky that they could find. They knew that because space was so vast, if they looked far enough across the universe, they would eventually see other, distant galaxies. The result was even more remarkable than they anticipated --- in a patch of sky in which astronomers had never seen anything interesting before, they found thousands of galaxies.
In the decade since those first observation Hubble, and other observatories, have returned to this field many times. The image above is the most recent. In an area covering 1/16,000,000 (one sixteen millionth) of the sky, the images reveals 10,000 galaxies. If this image could be taken to cover the entire sky, it would reveal that our Universe contains at least 160 billion galaxies.
To look at this image is to look back in time. Light does not travel across space instantly. Although light is the fastest thing in the Universe, it is limited to a speed of 186,000 miles per second (300,000 km/s). The light from some of these galaxies has been traveling for nearly 13 billion years. The most distant, and therefore oldest, galaxies we can see formed only 500 million years after the Big Bang. Nonetheless, they look similar in many ways to galaxies that we see nearby. It turns out that stars formed relatively quickly after the birth of the universe.
If you look closely, however, you will see that many of the larger galaxies appear blue, and the fainter ones appear red. The bright, blue galaxies are nearby, and the faint, red ones are more distant. However, the redness of the distant galaxies is not caused by anything different about their stars. The distant ones appear red because the Universe is expanding. Almost all galaxies (the exceptions being nearby ones that happened to be pulled around by the gravity of other galaxies) appear to be moving away from us. As a result, their light becomes shifted to lower frequencies, in the same way that the siren from an ambulance that is traveling away from someone sounds like it has a lower pitch. Lower frequency light appears redder, so this process is referred to as a redshift.
You might wonder, then, if all galaxies are moving away from us, why should the more distant ones appear redder than the nearby ones? Does this mean that the more distant galaxies are moving away from us faster? Yes, in fact, it does. The Universe is not expanding from a point centered on us. We are moving just like every other galaxy. It is space itself that is expanding, causing the distances between galaxies to increase. (Our galaxy does not expand, because gravity holds it together.) This causes the more distant galaxies to be moving away from us faster. Imagine three galaxies in a line: our Milky Way on the left, and then two more, one in the center, and one on the right. Imagine yourself at the position of the center galaxy, our Galaxy on the left will appear to move one way, and the galaxy on the right will appear to move the other. Say that our Galaxy and the right galaxy are moving at the same speed away from the center. Now move to our vantage point. The center galaxy is moving at the same speed that we seemed to be moving away from it. However, the right galaxy is moving away twice as fast!
If you extend this out to more distant galaxies, they start moving away faster and faster. If you look far enough, eventually galaxies will be moving away from us at faster than the speed of light! Indeed, the reddest galaxies in this image do appear to be traveling faster than the speed of light. The light we do see started traveling toward us when the universe was very much smaller, and so it had to travel a much shorter distance than what now separates our Galaxy from the distant ones. Nonetheless, millions of years in our future, those galaxies will become to far away for light to travel between us and them, and the galaxies will fade from our sight.