Diving into the deep end: The Hubble Deep Field

The Hubble Space Telescope is arguably the most successful and important mission NASA has ever undertaken. Sure, the moon landings and robotic exploration of our solar system are dramatic and possess a more human sense of immediacy, but when it comes to finding out what our universe is like on the grandest of scales, you can't compare to Hubble.

Just look at the pictures. If you've never browsed through the online archive of images available at the Space Telescope Science Institute (STScI and yes, the lower case "c" is deliberate) website ( www.stsci.edu ), do so today. Come over to my house and use my computer if you have to. I really and truly believe that everyone on earth should have a chance to see these images.

I have several favorite Hubble images, many of which I love for being great eye-candy: the color and texture of a giant star-birthing dust cloud, the elegance of giant spiral galaxies. But there's one image I keep coming back to again and again, whenever I need a dose of awe. It's actually not the most visually inspiring of images (kind of dark, just a scatter of dim, irregular specks of light), but the significance of this single picture is staggering. It's called the Hubble Deep Field.

The story actually begins with a very human-scaled situation: taking a risk. As you might imagine, observing time on the Hubble Space Telescope is rabidly sought after by almost every professional astronomer on earth. A small amount of observing time is pre-allocated to the scientists who actually work for STScI, a pretty major job perk. All the other astronomers know this, and often lobby the Science Institute employees with suggestions as to how best use their guaranteed observing time.

In late 1995, Robert Williams, the director of STScI announced something rather shocking. He and a team of his collaborators planned to use up almost all of his time (a little more than ten days) to observe an empty region of the sky. Huh?

There was, of course, some method to their madness. The idea is actually quite simple: the more distant a star or galaxy is from us, the smaller and fainter it appears in our sky. Very distant galaxies require extended time exposures to be seen at all. I've seen this happen many times myself: you swing your giant telescope over to a known coordinate on the sky, and then wait.

At first, the sky just looks empty, but over several hours of collecting light, a dim smudge begins to emerge and slowly take shape. It takes time to see galaxies that far away. The longer you look, the dimmer (and farther away) galaxy you might be able to detect. Was it possible that there were even fainter galaxies out there, but we'd just never spent enough time and energy to discover them? Is it possible to see every single galaxy in the universe if you spend enough time looking? Just how deep can you look out into the universe?

With that idea in mind, the Hubble Deep Field team decided to take the argument to its logical conclusion. Why not pull out all the stops and see what you find if you use the most sensitive telescope ever invented to stare at a tiny region of the sky for a full ten days. And I mean a tiny region of the sky, equivalent to the inside of a needle's eye held at arm's length, an area of the sky almost too small to see. Spending that much time looking at a seemingly empty, microscopic point on the sky was certainly risky; no one knew quite what to expect. But the project went ahead, and pretty soon astronomical meetings were buzzing with conjecture. What would they see? How many galaxies might really exist in each tiny pin-prick of the sky?

The answer (and the final image) blew everyone away. Hubble returned a lovely picture, which looked to all the world like diamond dust scattered on black velvet. But each tiny speck of dust (and there turned out to be over 1,500 of them visible in the picture), was a distant galaxy unto itself, spinning in the darkness of space. Each tiny speck was a collection of almost a trillion stars, 1,500 of them inside the eye of a needle at arm's length. Some of the larger specks were recognizable as spiral galaxies much like our own; others were elliptical galaxies, and many others were too small to make out any clear shape.

The smallest galaxies on the image, mostly just single pixels of dim, red light, turned out to be well over 10 billion light years away (after follow-up measurements were made to determine their distance). Whenever I look at this image, I remind myself that I'm looking at galaxies that existed when the universe was only a fraction of its current age and size, back to a time when the galaxies were gathered more closely together, almost snuggling up against one another.

Closer examination of the image revealed that some of the galaxies weren't quite whole yet, but composed of huge clusters of stars that were starting to stick together and organize themselves into a larger body. Indeed, the Hubble Space Telescope had seen back to the very birth of the galaxies themselves.

But there was one more discovery that still takes my breath away, one locked a bit more deeply in the data. You see, the night sky, although it appears dark to our eyes, glows very slightly in all kinds of light. Even in the darkest stretches of space between the galaxies, where no stars shine, there's a faint background glow of light.

Astronomers usually think of this glow as "noise" in their measurements, and try to calibrate it out of their data. We don't really know where this dim glow comes from, but we've assumed it was from objects too far away to see; from galaxies so close to the edge of the universe that we couldn't resolve them in our telescopes (at least not yet). After studying the Hubble Deep Field, astronomer found something astonishing: there was no background glow (in visible light, at least) between the galaxies in the image.

What that means, effectively, is that we have resolved every source of visible light in the universe, for that tiny dot on the sky that the Hubble Deep Field covers. There are no more stars or galaxies out there to find. We have seen as much, in visible light, as there is to see.

The visible light caveat is important to note; there's still a background of radio, microwave, and high-energy light like X-rays and Gamma-rays. We haven't seen to the end of the universe in those wavelengths of light yet, and astronomers are working on instruments that will probe those regions. The Chandra X-ray telescope has recently completed a deep scan of the Hubble Deep Field region of the sky, and the up-coming infrared telescope SIRTF will do the same.

But in the kind of light our eyes see in, we have seen to the edge of the universe. If we look any farther out, we look so far back into time that no stars or galaxies had yet begun to shine.

I've recently begun to use the example of the Hubble Deep Field when I'm asked about whether or not I believe there might be other life in the universe. My response is this: hold up a needle at arm's length toward the sky. Look through that needle, I say, and if your eyes were sensitive enough, you could see 1,500 galaxies. Each galaxy has a trillion stars. Even if life exists against overwhelming odds, even if the chance of life starting somewhere else is a million or billion against, think how many chances you have inside the eye of just one needle.

Just think of it.

Please see the Hubble Deep Field for yourself. Go to http://oposite.stsci.edu/pubinfo/PR/96/01.html