This week the number of known galaxies in the universe just increased by a factor of 10! Let's dig into this news.

First off, this is the Hubble Ultra-Deep Field that astronomers previously used to estimate the number of galaxies in the universe:

The Hubble Ultra-Deep Field consists of a combination of images from ultra-violet to infrared light taken over a nine-year period of time. There are 10,000 galaxies in this image. The oldest of which is only a couple of million years younger than the age of the universe. For context, the universe is 13.8 billion years old. (Image Credit: NASA, ESA, H. Teplitz and M. Rafelski, A. Koekemoer, R. Windhorst, Z. Levay)

The goal of the Hubble Ultra-Deep Field (HUDF) was to observe the earliest and farthest away galaxies in the universe. It did this job extremely well.

To understand what astronomers were trying to do with the HUDF, we have to understand how age and distance work with galaxies in our universe. Because of the finite speed of light, the farther away you look, the farther back in time you are looking. In other words, you are seeing earlier times in the universe because the light has taken so long to travel to us from those distant galaxies. It's a cool way to achieve time travel.

Astronomers are always trying to find the oldest galaxies and to do this they have to look as far away as possible. But, the farther away you try to look, the fainter the light becomes because light travels in straight lines and so the total brightness falls off with distance. Overall, it's extremely challenging to observe very old galaxies (in other words the earliest ones that formed in the universe) because not only are they far away, they are also intrinsically fainter.

But let's return to the HUDF. From the HUDF observations, astronomers learned that the oldest galaxies are undergoing extremely rapid star formation. They also got a large enough sample of very old galaxies to understand that galaxies in the early universe are messy. They are in the stages of evolution where they are merging together. They are baby Milky Ways on their way to becoming larger and more ordered like the beautiful spiral galaxies we're used to seeing in the local universe (local=nearby in space and time).

A recreation of the cosmic web, which is composed of galaxies, clusters of galaxies, and dark matter in filaments (blue stuff) and the empty voids in between (dark). (Image Credit: NASA, ESA, E. Hallman)

But, here's the kicker. The HUDF also allowed astronomers to count the number of galaxies in the universe. They estimated that there were ~100 billion galaxies in the universe.

A week ago, another group of astronomers found that there are 2 trillion galaxies in the universe. Using a technique which uses the measured densities of galaxies in the universe, they found that there are many smaller galaxies that are unaccounted for in the Hubble images. These galaxies are too faint to observe with the HUDF but incredibly important for describing the population of very very old galaxies. 

Think about this for a second. There are 10 times the number of galaxies in the universe than we previously thought. The sheer scale of astronomy already blows my mind, but this week astronomers proved that there is still so much to learn!