“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 result gave us the information that a very large number of galaxies exist in a minuscule region of the sky.
By extrapolating these results over the entire sky, we were able to figure out — at minimum — how many galaxies there are in the entire Universe. I even made a video about it.
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. Ladies and Gentlemen, may I present to you the Hubble Extreme Deep Field!
This picture may look familiar to you, even though you’ve probably never seen it before. The Extreme Deep Field (or XDF) is actually a part of the Ultra Deep Field, which you can see for yourself if you rescale both images and rotate them at 4.7 degrees relative to one another!
The XDF has far more galaxies in it than the HUDF does in a comparable region of space. Take a look for yourself at a small portion of these images, compared top-to-bottom with one another, and you can clearly see how many more galaxies there are in the XDF with your own eyes.
Sure, the Ultra-Deep one (atop) is very impressive, but there are maybe 75% more galaxies in the XDF! If we apply these results to the entire sky, we find that there are more like 200 billion galaxies in the entire Universe, around double what we got from the HUDF.
How do we estimate that there are so many? For starters, the area of the XDF is just a tiny, tiny fraction of the full Moon.
If you assume that the XDF is a typical region of outer space, you can calculate how many XDFs it would take to fill the entire night sky; it’s about 32 million. Multiply by the number of galaxies you find in the XDF, and that’s how you arrive at about 200 billion galaxies in the Universe.
But there’s more to the story than that.
We’re taking a region of space that has very few nearby galaxies, or galaxies whose light takes less than a few billion years to reach us. We’ve selected a deliberately low-density portion of the nearby Universe. The XDF has found many more galaxies whose light has traveled between 5 and 9 billion years to reach us, which are relatively dim galaxies that the HUDF simply couldn’t pick up. But where it really shines is in the early Universe, at finding galaxies whose light has been on its was for more than 9 billion years, finding the majority of new galaxies there.
But even the XDF is not optimized for finding these galaxies; we’d need an infrared space telescope for that, which is what James Webb is going to be. When that comes around, I wouldn’t be surprised to find that there are maybe even close to a trillion galaxies in the Universe; we just don’t have the tools to find them all yet. In the meantime, I thought it would be fun to allow you to compare the old HUDF image, rotated and cropped to XDF size:
with the new XDF image itself!
And for those of you who’d rather see the same chunks of these images side-by-side, I’ve broken them up into four chunks, each of which has the (old) HUDF image on the left and the (new) XDF image on the right.
The scale may be slightly off, but it still provides an excellent visual comparison between the two.
The way light-gathering works is you can typically see 41% as deep when you observe for twice as long, something astronomers are intimately familiar with.
It makes me so impatient for a more powerful telescope with the ability to see far into the infrared, because I can’t help but wonder what’s still invisible to even the XDF.
And there you have it: the deepest view of the Universe. Ever. What else is there to say? Enjoy them, discover them, and see what you can find in them. It’s just the tiniest fraction of the whole Universe, but like you’ve never seen it before.