Tag: Hubble telescope

How Many Galaxies Are There in the Universe? The Redder We Look, the More We See

By Guest Blogger | October 10, 2012 1:22 pm

Ethan Siegel is a theoretical astrophysicist living in Portland, Oregon, who specializes in cosmology. He has been writing about the Universe for everyone since 2008, and can’t wait for the launch of the James Webb Space Telescope. A different version of this post appeared on his blog, Starts With a Bang. 

“It is by going down into the abyss that we recover the treasures of life. Where you stumble, there lies your treasure.” –Joseph Campbell

One of the bravest things that was ever done with the Hubble Space Telescope was to find a patch of sky with absolutely nothing in it—no bright stars, no nebulae, and no known galaxies—and observe it. Not just for a few minutes, or an hour, or even for a day. But orbit-after-orbit, for a huge amount of time, staring off into the nothingness of empty space, recording image after image of pure darkness.

What would we find, out beyond the limits of what we could see? Something? Nothing? After a total of more than 11 days of observing this tiny area of the sky, this is what we found:

The Hubble Ultra Deep Field—the deepest view ever of the Universe, was the result. With all those orbits spent observing what appears to be a blank patch of sky, what we were really doing was probing the far-distant Universe, seeing beyond what any human eye—even one aided by a telescope—could ever hope to see. It took literally hundreds of thousands of seconds of observations across four separate color filters to produce these results.

What you’re seeing—in practically every point or smear of light—is an individual galaxy. The result gave us the information that a very large number of galaxies exist in a minuscule region of the sky: around 10,000 in the tiny volume surveyed by the Hubble Ultra Deep Field image, below.


Image credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

By extrapolating these results over the entire sky (which is some 10 million times larger), we were able to figure out—at minimumthat there were at least 100 billion galaxies in the entire Universe. I even made a video about it.

But that’s not the end of the story; not by a long shot. You see, there might be at least 100 billion galaxies, based on what we’ve observed, but there might be more. Galaxies that are too dim to observe with “only” 11 days of Hubble data. Galaxies that are redshifted too far for even Hubble’s farthest infrared filter to pick up. Galaxies that might appear, if only we had the patience to look for longer.

So that’s exactly what we did, looking for a total of 23 days over the last decade—more than twice as long as the Ultra-Deep Field—in an even smaller region of space. (There are over 1,000 observing proposals submitted to Hubble every cycle, so getting that much time, even spread over a decade, is remarkable.) Ladies and Gentlemen, may I present to you the Hubble Extreme Deep Field!

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