approximately 400,000 years old After the Big Bang, the primordial plasma of the infant universe cooled enough for the first atoms to combine, making room for the embedded radiation to liberate it. This light – the cosmic microwave background (CMB) – continues to stream across the sky in all directions, broadcasting a snapshot of the early universe that was captured by dedicated telescopes and even revealed in stills on old cathode ray televisions.
After scientists discovered the CMB radiation in 1965, they meticulously mapped the minute temperature differences, which showed the exact state of the universe when it was just a frothy plasma. Now they are reusing the CMB data to catalog large-scale structures that evolved over billions of years as the universe matured.
“This light has witnessed a large part of the history of the universe, and by seeing how it changes, we can learn about different eras,” said Kimi Wu, a cosmologist at the SLAC National Accelerator Laboratory.
Over the course of its roughly 14-billion-year journey, light from the CMB has been stretched, compressed, and distorted by everything in its path. Cosmologists are beginning to look beyond the initial fluctuations in the light of the CMB radiation to the secondary imprints left by interactions with galaxies and other cosmic structures. From these signals, they gain a clearer view of the distribution of both ordinary matter–everything made up of atomic parts–and the mysterious dark matter. In turn, these insights help settle some old cosmic mysteries and put forward some new ones.
We realize that the CMB doesn’t just tell us about the initial conditions of the universe. It also tells us about the galaxies themselves, said Emmanuel Chan, a cosmologist also at SLAC. “And that turned out to be really powerful.”
A universe of shadows
Standard optical surveys, which track light from stars, overlook most of the core mass of galaxies. That’s because the vast majority of the universe’s total matter content is invisible to telescopes – hidden from view either as clumps of dark matter or as the diffuse ionized gas that binds galaxies. But both the dark matter and the scattered gas leave detectable fingerprints on the magnification and color of the incoming CMB light.
“The universe is really a shadow theater in which the galaxies are the protagonists, and the CMB is the backlight,” Shan said.
Many of the shadow players were now starting to rest.
When particles of light, or photons, from the CMB scatter electrons in the intergalactic gas, they collide at higher energies. In addition, if those galaxies are in motion relative to the expanding universe, the cosmic microwave background radiation photons get a second shift of energy, either up or down, depending on the relative motion of the cluster.
This pair of effects, known respectively as the Sunyaev-Zel’dovich (SZ) thermal and kinetic effects, was first theorized in the late 1960s and has been detected with increasing precision in the past decade. Together, the SZ effects leave a distinct signature that can be removed from CMB images, allowing scientists to map the location and temperature of all ordinary matter in the universe.
Finally, there is a third effect known as weak gravitational inversion that distorts the path of CMB light as it travels near massive objects, distorting the CMB as if viewed through the base of a wine glass. Unlike SZ effects, the lens is sensitive to all materials—dark or otherwise.
Combined, 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.