Using a technique similar to medical imaging, astronomers have constructed one of the most complete 3D maps of what the young universe looked like just three billion years after the Big Bang.
An international team led Khee-Gan Lee from the Max Planck Institute for Astronomy used extremely faint light from galaxies that are 10.8 billion light years away to backlight primordial hydrogen gas left over from the Big Bang. As starlight from distant (background) galaxies travel towards Earth, the vast clouds of hydrogen gas that exist in between (foreground) will leave a characteristic imprint—called “absorption signature”—in the spectrum of those galaxies. The density of hydrogen can then be determined based on this absorption of light.
By observing 24 background galaxies in a small patch of the sky using Keck I in Hawaii, the team mapped the web of hydrogen gas at a time when the universe contained a fraction of the dark matter we see today. This 3D map of the cosmic web—the backbone of cosmic structure—at a distance of nearly 11 billion years from Earth is the first time large-scale structures in such a distant part of the universe have been directly mapped. Here, brighter colors represent higher hydrogen gas densities:
Similar to how a CT scan constructs a 3D image using x-rays passing through a patient, Lyman-alpha tomography uses background light passing through hydrogen gas. Until recently, astronomers relied on the bright light of quasars. But these are scattered and few, and the density of hydrogen based on the absorption of quasar light only provides information about the presence of hydrogen along the line of sight—and not over a larger volume of space. “It’s a pretty weird map because it’s not really 3D,” Berkeley Lab’s David Schlegel explains in a news release. “It’s all these skewers; we don’t have a picture of what’s between the quasars, just what’s along the skewers.”
So rather than rely on the light of sparse quasars, the new technique uses faint light from numerous distant galaxies. Before this study, no one knew if galaxies farther than 10 billion light years away could provide enough light to be useful. After all, that light is exceedingly faint. So the team had to first develop algorithms to subtract light that would otherwise drown out the galactic signals.
This glimpse of early cosmic structure formation during a time when galaxies were undergoing a major growth spurt will help us understand how the universe has changed since the Big Bang 13 billion years ago. The findings will be published in Astrophysical Journal Letters.
Images: Casey Stark (UC Berkeley) and Khee-Gan Lee (MPIA)