Almost 400,000 years old after the Big Bang, the primordial plasma of the nascent universe cooled enough for the first atoms to fuse, leaving room for embedded radiation to fly free. This light – the Cosmic Microwave Background (CMB) – continues to streak across the sky in all directions, broadcasting a snapshot of the early universe that is picked up by dedicated telescopes and even revealed in static on old ray televisions. cathodic.

After scientists discovered CMB radiation in 1965, they meticulously mapped its tiny temperature variations, which showed the exact state of the cosmos when it was just foaming plasma. Now they’re repurposing the CMB data to catalog large-scale structures that grew over billions of years as the universe matured.

“This light has experienced much of the history of the universe, and by seeing how it has changed, we can learn more about different eras,” said Kimmy Wu, a cosmologist at SLAC National Accelerator Laboratory.

During its journey of almost 14 billion years, the CMB light has been stretched, squeezed and distorted by all the matter in its path. Cosmologists are beginning to look beyond the primary fluctuations of CMB light to the secondary imprints left by interactions with galaxies and other cosmic structures. From these signals, they get a more accurate view of the distribution of ordinary matter — anything made up of atomic parts — and mysterious dark matter. In turn, these insights help solve some long-standing cosmological mysteries and pose new ones.

“We realize that the CMB does not only tell us about the initial conditions of the universe. It also tells us about the galaxies themselves,” said Emmanuel Schaan, also a cosmologist at SLAC. “And that turns out to be really powerful.”

A world of shadows

Standard optical surveys, which track the light emitted by stars, overlook most of the underlying mass of galaxies. Indeed, the vast majority of the total matter contained in the universe is invisible to telescopes, hidden in the form of clumps of dark matter or diffuse ionized gas that connects galaxies. But dark matter and scattered gas leave detectable imprints on the magnification and color of incoming CMB light.

“The universe is really a shadow theater in which the galaxies are the protagonists and the CMB is the backlight,” Schaan said.

Many shadow players are now in relief.

When bright particles, or photons, from the CMB scatter electrons into the gas between the galaxies, they are shot to higher energies. Additionally, if these galaxies are in motion relative to the expanding universe, the CMB photons experience a second energy shift, either up or down, depending on the relative motion of the cluster.

This pair of effects, known respectively as the thermal and kinematic Sunyaev-Zel’dovich (SZ) effects, were first theorized in the late 1960s and have been detected with increasing accuracy over the course of the last decade. Together, SZ effects leave a characteristic signature that can be extracted from CMB images, allowing scientists to map the location and temperature of all ordinary matter in the universe.

Finally, a third effect known as weak gravitational lensing distorts the path of CMB light as it travels near massive objects, distorting the CMB as if viewed through the base of glass at wine. Unlike SZ effects, the lens is sensitive to any material, dark or otherwise.

Taken together, these effects allow cosmologists to separate ordinary matter from dark matter. Then, scientists can overlay these maps with images from galaxy surveys to measure cosmic distances and even track star formation.