By Ethan Siegel, over at Scienceblogs: The Deepest View of the Universe. EVER. – Starts With A Bang. Thinking about it for just a few seconds will make you dizzy:
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.
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.
You can find multiple resolutions for the XDF image, along with more technical information and a brief explanation of what you can see in the image, by visiting Hubblesite.