We are all familiar with maps of the globe, as though seen from space, unfolded onto a flat page with America on the left and China on the right. This map, with its glowing web of blobs and veins like so many city lights at night, shows space seen from Earth similarly unfolded. But unlike those two-dimensional maps, this one allows us to see in three dimensions: up and down, left and right, and then out through the universe. It’s a map of galaxies, bundled into clusters and superclusters.
The haze running through the middle is our own galaxy, the Milky Way. At the top is a bright density of blue—the Coma Cluster, which contains 1,000 galaxies. Below it there’s a purple patch called the Virgo Cluster. The purple is important. The galaxies are coloured according to their distance from us: the purple bits are closest, the red farthest away. But they are all in what astronomers, with their vertiginous sense of scale, call the “local universe”—which stretches for 380m light years.
It was published in 2011, the result of a project led by John Huchra, who until his death in 2010 was a professor at the Harvard-Smithsonian Centre for Astrophysics. “The idea”, says Professor Robert Kirshner, his friend and colleague, “was that while it’s easy to map the two-dimensional position of something on the sky—how far east, west, north or south it is—finding its distance from us is much harder. But once you find out where something is, you’re on the trail to knowing how it got there.”
Since the 1970s Huchra had been calculating the distances of galaxies using a phenomenon called redshift, discovered in the 1920s by Edwin Hubble, who found that when light is emitted by a moving object, its wavelength changes. If the object is moving towards you the light shifts to the blue end of the spectrum; farther away it shifts to the red end. Hubble saw that light from almost every galaxy is redshifted. Then he found something else: “Hubble’s really big discovery”, Kirshner says, “was that the velocities of those galaxies and their distances from us were proportional. By itself that’s astonishing, the story of an expanding universe. But as a practical matter, by measuring the wavelength of the light you get a number that’s proportional to the distance. And that tells you which ones are nearby and which are far away.” That proportion is called the Hubble Constant.
When Huchra and his colleagues began measuring the redshifts of galaxies, they found extraordinary cosmic structures. Mapping the northern sky in the 1980s, they found a wall of galaxies 500m light years long, 200m light years wide and 16m light years thick. Called the Great Wall, it‘s been likened to a giant cosmic duvet. They saw voids between clusters of galaxies hundreds of millions of light years across. They began to see that the universe is clumpy rather than smooth, and that galaxies are strung out on filaments—“like the bubbles in a soap dish”, Kirshner says—and that the best theories of how the universe was formed couldn’t account for what they were seeing.
While those earlier maps had been of portions of the sky, in 2005 work began on this map of the entire local universe. The first step was a 2D map of the sky, but there was one thing blocking the view: our own galaxy. The Milky Way is full of dust, which obscures large parts of the cosmos and creates what astronomers call the “Zone of Avoidance”.
To get round that problem, the astronomers worked in infra-red. “In the infra-red, the dust from our galaxy is easier to see through,” says Karen Masters, who worked with Huchra. Then they added redshift measurements to give the map its third dimension. “One of the big advantages of this map”, Masters says, “is that it reveals structures that, because we’re embedded in them, are difficult to see by looking in any one direction. Wherever there’s a very large concentration of galaxies, all the galaxies round it move towards that point. And it seems like the biggest concentration in our part of the universe is just behind the Zone of Avoidance.” In other words, it helps to show us where the Milky Way is going.
So how did the universe come to look like this? The answer appears to be hiding in plain sight. “Clusters of galaxies seem to be held together by gravity,” Masters says. “But we know they couldn’t be held together by their own gravity, by the gravity of the stars within them. We know there’s a material out there which we can’t see but which has gravity: dark matter. It’s easy to simulate on a computer, and if you make simulations of dark matter and what it does to the universe, you end up with structures that look a lot like what you see here. So this is the current thinking: that the galaxies you’re looking at are just lighting up the underlying field of dark matter.”