Watching the cosmic web unfold

Craig Hogan

The grandest spectacle in the universe is the structure defined by galaxies on the largest scales. Called the cosmic web, it is an enormous, perhaps even infinite structure, with a transcendent beauty. Dense clusters, each one swarming with thousands of orbiting galaxies, connect with each other over gossamer filaments and delicate sheets of galaxies, stretching across tens to hundreds of millions of light years. Between the sheets loom large, nearly empty voids. Similar clusters, filaments, sheets and voids extend as far as we can see in all directions, and probably far beyond that.

This majestic structure has been created from a nearly uniform expanding gas of matter by the simple force of gravity, the same process that earlier created galaxies. Using powerful computers, scientists can simulate its evolution over billions of years, as the cosmic web emerges from uniform matter: voids expand and galaxies stream, swarm and merge into larger galaxies and clusters. The simulations reproduce the properties of this complex structure very well, using remarkably few assumptions. The detailed evolution over time does depend on some unknown cosmic physics, in particular, the behavior of the dark energy that is accelerating the cosmic expansion.

For a physicist, it is deeply moving, as well as scientifically instructive, to compare the views of the computer simulation with the real cosmic web. Our first glimpses of the cosmic web— the closest clusters, filaments, sheets and voids— started to come into view about 30 years ago, when galaxy redshift surveys started to give us a three-dimensional map of the cosmos. The Sloan Digital Sky Survey, Fermilab’s first foray into large spectroscopic surveys, used a system of optical fibers to collect spectra and make a breathtaking 3D map with about a million galaxies, and hundreds of sheets, filaments and clusters. The real cosmic web, as seen by SDSS, that you can virtually fly through online (or view as part of a tour of the known universe), looks remarkably like the computer-generated universe based on the physics of gravity, dark matter and dark energy.

The SDSS map is still a relatively local snapshot of the cosmic web today; the universe has not changed much during the time light has travelled to us from most of those galaxies. We want to make a bigger, deeper map with many more galaxies, perhaps 10 million, and look much farther into the universe and back in time than SDSS. That way, the depth dimension of the map also shows directly how the web evolves with time—from a much younger universe, at the far edge of the map, to the universe today, in the nearby part. The reason is not just to enjoy the view, although there will be undeniable grandeur in the visualizations of this dataset. The deep view will tell us in precise detail about the mysterious physics of dark energy as it influenced the evolution of the cosmic web. For that reason, DOE has recently included the possibility of such a project in a recent Mission Need (CD-0) statement for a new dark energy experiment.

To go deeper than SDSS, we’ll need a bigger telescope. To collect 10 million galaxy spectra, we’ll also need a spectrograph with many more optical fibers. Fermilab scientists are working on two designs for such a system. One of them– called BigBoss– is led by scientists at Lawrence Berkeley National Laboratory and is destined for the Kitt Peak 4-meter Mayall telescope in Arizona. The other– called DESpec and led by Fermilab scientists– is proposed for installation in the southern hemisphere on the Blanco telescope at Cerro Tololo in Chile. If we do both of these surveys, we’ll have a deep map that covers the sky in all directions, including both the northern and southern hemispheres–a comprehensive view of most of our observable universe, and its emergence over most of cosmic time. That will be wonderful to see, whatever we learn about dark energy.