Finding our Place in the Universe How We Discovered Laniakea – the Milky Way's Home

The images that astronomers produce can shape whole generations. Based on the Pale Blue Dot photo taken by the Voyager 1 space probe, Carl Sagan’s moving speech in Cosmos highlighted how small and insignificant we appear in the vastness of the universe. But we are not alone, being part of the solar system which is part of the Milky way galaxy. And ours is but one of billions, possibly trillions, of galaxies in the universe that, interestingly, are not scattered at random in space. In this compact and engagingly written book, French cosmographer Hélène Courtois shows you the next level up: superclusters. When it was published in 2014, the image of the supercluster to which our galaxy belongs for me was another one of those generation-defining images. It was of such stunning beauty that it stopped me in my tracks. Welcome to Laniakea, our home amidst the stars.

Finding Our Place in the Universe was originally published in French as Voyage sur les Flots de Galaxies in 2016, followed by a second edition in 2018. This English translation updates the story of how we determined the size and limits of Laniakea with subsequent findings of what lies beyond our local supercluster. But first, what is a cosmographer? In essence, Courtois is a star mapper. The same way geographers make maps of Earth, cosmographers make maps of our universe. One of the strong points of this book is its explanations. The book is liberally illustrated with diagrams and uses side boxes for more technical matters such as the dual nature of light or the value of the Hubble constant (the rate of cosmic expansion).

Courtois starts with the earliest attempts to determine the distances of the heavenly bodies such as the moon and nearby planets. Where it really gets interesting where this book is concerned is April 26th, 1920. This was when hundreds of scientists attended an academic debate, the Great Debate, at the Smithsonian Museum of Natural History. Two opposing ideas that had been around since the mid-18th century came to a head. One viewpoint, defended by Harlow Shapley, held that the universe is limited to our Milky Way galaxy – it is enormous, but it is all there is. The other viewpoint, here defended by Heber Curtis, held that our galaxy is but one of innumerable others.

Further research by Edwin Hubble would prove Curtis right and lead to the acceptance of the notion of a universe filled with galaxies. From here, we fast forward to the early ’90s where Courtois’s career starts. Work up to this point had already established the existence of the so-called Local Group (our galaxy and its neighbours) and nearby (super)clusters that surround us. Courtois brought further clarity to the positions of these groups of galaxies.

Stars and galaxies, however, are not motionless. Hubble had already shown that our universe is expanding. But even when accounting for that motion, later research revealed residual motion. Astronomers call this "peculiar" motion (a slightly confusing term given the word’s everyday connotations) and Courtois uses all her explanatory power in words and diagrams to clarify this. It turns out that gravitational forces are pulling our galaxy and all the neighbouring clusters to a region in space with the wonderfully mysterious name "The Great Attractor". Frustratingly, we cannot directly observe this region of space as it lies behind the disc of our Milky Way galaxy.

To determine how far the influence of this Great Attractor reaches, Courtois and colleagues teamed up with an ever-expanding international collaborative network. Through painstaking observations made over years, they build several iterations of an ever-larger database dubbed Cosmicflows that contained the positions and "peculiar" motions of as many nearby galaxies as possible. First 1,800 galaxies spanning 130 million light-years. Then 8,000 galaxies spanning 250 million light-years. It was not until the third iteration, the Cosmicflows-3 database that encompassed 18,000 galaxies spanning 600 million light-years, that Laniakea emerged. Next to a beautiful name (it is Hawaiian for "immense heaven"), this work also offered a clear definition of what a supercluster is, as the term had been used loosely up to this point. One useful analogy is the way a lake is fed by streams and rivulets and collects water from a large area around it, its watershed. Courtois and colleagues had now mapped the entire outline of the galactic watershed of which our galaxy was a part. It led to a well-deserved Nature publication in 2014 and the publication of that iconic image. A short video on Nature’s YouTube channel helps visualise it all.

As Courtois takes readers on this story, there are short personal asides about life as an astronomer, the people you get to work with, and the wonderful places you get to visit. Other technological developments and important discoveries that bear on her work are also covered, including a very clear explanation of the still mysterious nature of dark matter and dark energy – and the distinction between the two. A final thing to mention is the boxes that celebrate female scientists past and present, from Henrietta Swan Leavitt, who Edwin Hubble thought deserved a Nobel Prize, to Wendy Freedman, who led efforts to pinpoint the value of the Hubble constant and won the Gruber Cosmology Prize in 2009. Next to their work as human computers in the early years of astronomy, many women made ground-breaking discoveries themselves which were either downplayed until accepted decades later, or for which men took the credit. This book thus joins the fray to set this record straight and celebrate the underrecognized role of women in astronomy.

The discovery of Laniakea is not the end of the story, however, but merely the beginning of the next chapter. This is, I guess, the section that was added in the English translation. Work is already underway on Cosmicflows-4, which will encompass over 50,000 galaxies spanning more than a billion light-years and map neighbouring superclusters. The final thrill this book offers is the glimpse we are getting of the larger structure of our universe, where condensing superclusters form a filamentous cosmic web with cosmic voids forming in between.

A love letter to cosmology, Finding Our Place in the Universe is a short but thrilling read that gives the scientific backstory to one of astronomy’s most striking images. MIT Press did an excellent job released this in translation.