Making sense of two trillion galaxies

Sébastien Carassou is an independent science communicator. He holds a Ph.D. in astrophysics from the Paris Institute of Astrophysics, where he developed an innovative approach to infer the luminosity and size evolution of various galaxy populations using simulated images of galaxy surveys. He is now involved in several science outreach projects, including “le Sense Of Wonder”, a French Youtube channel about life, the universe and everything (but mostly the universe), and @Endirectdulabo, a rotational Twitter account featuring French-speaking scientists.


In less than a hundred years, our vision of the cosmos has dramatically shifted from the comforting sight of the Milky Way to an ever-expanding universe filled with two trillion galaxies, each filled with hundreds of billions of stars like the Sun. It is difficult to wrap our minds around such enormous numbers. It is also difficult to remain insensitive to the beauty of these colorful stellar megalopolises.

Galaxies are complicated beasts. Click To Tweet

Galaxies are complicated beasts. These marvelous structures have lives of their own, governed by mutual attractions, and a complex cycle of birth and death of their stars. They evolve embedded in a giant cosmic web made of filaments of hot primordial gas, as well as a mysterious invisible substance called dark matter. The nature of this dark matter is still unknown, but it appears to be a necessary ingredient in the making of galaxies, binding them together and making up most of their mass.

These “island universes” (as Kant would call them) can display a huge range of colors and shapes, from bright reddish rugby balls of stars to beautiful blueish spiral structures, and everything in between. These colors and shapes are the indirect witnesses of their history. Giant and cold filaments from the intergalactic medium can feed galaxies in hydrogen and helium gas, which can then be used to form stars. When galaxies are close to one another, their mutual gravity engages them in a cosmic ballet that can last for billions of years and usually results in the birth of a new galaxy when they merge. This is considered to be the ultimate fate of our own Milky Way, which is on a collision path with Andromeda, our galactic neighbor. And it seems to be the fate of most galaxies, as the big ones are thought to be the result (in part) of the merging of smaller ones in the past.

The Hubble Ultra-Deep Field, one of our deepest image of the universe to date. On this photo, apart from 5 or 6 stars, each patch of light is a galaxy. Credit:

To reconstruct the history of galaxy populations, astronomers must think like detectives, collecting evidence to build and test plausible scenarios to explain what is observed on telescope images. The problem is, these scenarios rely on many characters. Hundreds of billions of them, actually; stars of various masses, interstellar gas, and dust, dark matter, black holes, the filaments of the cosmic web, they all interact with one another to give rise to the structures we detect today. Understanding quantitatively the phenomena that shape galaxies, requires simulating the history of the cosmos in our best supercomputers, and comparing the virtual galaxies that emerge from these simulations to real images of galaxies taken by our best telescopes.

The history of the field of galaxy evolution is a fascinating one because it is deeply rooted in our quest to unravel the evolution of the universe itself. Click To Tweet

The history of the field of galaxy evolution is a fascinating one because it is deeply rooted in our quest to unravel the evolution of the universe itself. A field which has grown from the ever-increasing sizes of our mechanical eyes as well as the ever-increasing computational power of our supercomputers. Catch me at the Pint of Science festival, where I will explore the milestones that shaped the history of this fascinating human endeavor and show that we are living in a golden age for astronomy.

Astronomy and beer! Truly one of the best ways to engage with the public in a relaxed environment. Which makes me proud and excited to be part of the Pint of Science world domination plan. My sense of wonder is ready. What about yours?

Opinions in this blog post are that of the author, and not necessarily that of Hindawi. Profile photo credit: E. Ledolley and Sébastien Carassou. The text in this blog post is by Sébastien Carassou and is distributed under the Creative Commons Attribution License (CC-BY). Illustration by Hindawi and is also CC-BY.