When you gaze up at the night sky, through the veil of stars and the plane of the Milky Way close by, you can’t help but feel small before the grand abyss of the Universe that lies beyond. Even though nearly all of them are invisible to our eyes, our observable Universe, extending tens of billions of light years in all directions, contains a fantastically large number of galaxies within it.
Just how many galaxies are out there used to be a mystery, with estimates rising from the thousands to the millions to the billions, all as telescope technology improved. If we made the most straightforward estimate using today’s best technology, we’d state there are 170 billion galaxies in our Universe. But we know more than that, and our modern estimate is even grander: two trillion galaxies. Here’s how we got there. science space science on earth science
In an ideal world, we’d simply count them all. We’d point our telescopes at the sky, cover the entire thing, collect every photon emitted our way, and detect every object that was out there, no matter how faint. With arbitrarily good technology and an infinite amount of resources, we’d simply measure everything in the Universe, and that would teach us how many galaxies are out there. science space science on earth science
But in practice, that won’t work. Our telescopes are limited in size, which in turn limits how many photons they can collect and the resolutions they can achieve. There’s a trade-off between how faint an object you can see and how much of the sky you can take in at once. Some of the Universe is obscured by intervening matter. And the more distant an object is, the fainter it appears; at some point, a source is far enough away that even observing for a century won’t reveal such a galaxy. science space science on earth science
So what we can do, instead, is to view a clear portion of the Universe without intervening matter, stars, or galaxies as deeply as possible. The longer you stare at a single patch of sky, the more light you’ll collect and the more you’ll reveal about it. We first did this in the mid-1990s with the Hubble Space Telescope, pointing at a patch of sky that was known to have practically nothing in it, and to simply sit on that spot and let the Universe reveal what was present. science space science on earth science
It was one of the riskiest strategies of all-time. If it failed, it would have been a waste of over a week of observing time on the newly-corrected Hubble Space Telescope, the most sought-after observatory to take data with. But if it succeeded, it promised to reveal a glimpse of the Universe in a way we had never seen before. science space science on earth science