almost 400,000 years After the Big Bang, the primordial plasma of the infant universe cooled enough for the first atoms to fuse, leaving room for the embedded radiation to rise freely. That light, the cosmic microwave background (CMB), continues to stream across the sky in all directions, broadcasting a snapshot of the early universe that is captured by dedicated telescopes and even revealed in static on old cathode ray televisions.
After scientists discovered CMB radiation in 1965, they meticulously mapped its small temperature variations, which showed the exact state of the cosmos when it was a mere foamy plasma. Now they are reusing the CMB data to catalog the large-scale structures that developed over billions of years as the universe matured.
“That light experienced much of the history of the universe, and by seeing how it has changed, we can learn about different epochs,” he said. kimmy wucosmologist at SLAC’s National Accelerator Laboratory.
Over the course of its nearly 14 billion-year journey, light from the CMB has been stretched, compressed, and deformed by all the matter that got in its way. Cosmologists are beginning to look beyond the primary fluctuations in CMB light to the secondary traces left by interactions with galaxies and other cosmic structures. From these signals, they are getting a clearer view of the distribution of both ordinary matter, everything that is made up of atomic parts, and the mysterious dark matter. In turn, those insights are helping to solve some longstanding cosmological mysteries and raising some new ones.
“We are realizing that the CMB doesn’t just tell us about the initial conditions of the universe. It also tells us about the galaxies themselves,” he said. emmanuel schaan, also a cosmologist at SLAC. “And that turns out to be really powerful.”
a universe of shadows
Standard optical surveys, which track the light emitted by stars, miss most of the underlying mass of galaxies. This is because the vast majority of the total matter content of the universe is invisible to telescopes, hiding out of sight either as clumps of dark matter or as the diffuse ionized gas that binds galaxies together. But both the dark matter and the scattered gas leave detectable traces in the magnification and color of the incoming CMB light.
“The universe is really a shadow theater in which galaxies are the stars and the CMB is the background light,” Schaan said.
Many of the shadow players are now taking over.
When particles of light, or photons, from the CMB scatter electrons in the gas between the galaxies, they rise to higher energies. Furthermore, if those galaxies are in motion with respect to the expanding universe, the CMB photons get a second shift in energy, either up or down, depending on the relative motion of the cluster.
This pair of effects, known respectively as Sunyaev-Zel’dovich (SZ) thermal and kinematic effects, were first theorized in the late 1960s and have been more accurately detected in the past decade. Together, the 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 warps the path of CMB light when it travels near massive objects, distorting the CMB as if viewed through the bottom of a wine glass. Unlike SZ effects, the lens is sensitive to all matter, dark or not.
Taken together, these effects allow cosmologists to separate ordinary matter from dark matter. Scientists can then overlay these maps with images from galaxy surveys to measure cosmic distances and even star formation trail.