nearly 400,000 years After the Big Bang, the primordial plasma of the early universe cooled enough for the first atoms to coalesce, creating space for the embedded radiation to soar freely. Radiation (CMB) continues to flow through the sky in all directions, broadcasting snapshots of the early universe, picked up by specialized telescopes and even revealed in still images from old CRT televisions.
After discovering CMB radiation in 1965, scientists meticulously mapped its subtle temperature changes to show exactly what the universe would be like when it was just bubbling plasma. We are now reusing CMB data to catalog large-scale structures that have evolved over billions of years as the universe matures.
“That light has been through most of the history of the universe, and by seeing how it changed, we can learn about different eras,” said a cosmologist at SLAC’s National Accelerator Laboratory. Kimmy Wu said.
Over the course of its nearly 14 billion year journey, light from the CMB has been stretched, squeezed, and distorted by all matter on its way. Cosmologists are beginning to look beyond primary variations in CMB light to secondary imprints left by interactions with galaxies and other cosmic structures. From these signals, they have a clearer picture of the distribution of both ordinary matter (anything made up of atomic parts) and the mysterious dark matter. These insights, in turn, help solve long-standing mysteries of the universe and raise some new ones.
“We recognize that the CMB does not just tell us about the nascent state of the universe. It also tells us about the galaxy itself,” said SLAC cosmologist Emmanuel Shahn. . “And it turned out to be really powerful.”
universe of shadows
Standard optical surveys that track the light emitted by stars miss most of the underlying mass of galaxies. This is because most of the total mass of matter in the universe is invisible to telescopes, either as clumps of dark matter or as diffusely ionized gas that bridges galaxies. However, both dark matter and diffuse gas leave a detectable imprint on the magnification and color of incoming CMB light.
“The universe is really a theater of shadows where the galaxy is the protagonist and the CMB is the backlight,” Shahn said.
Many shadow players are now being bailed out.
When light particles, or photons, from the CMB scatter from electrons in the intergalactic gas, they hit higher energies. Moreover, when these galaxies are in motion relative to the expanding universe, the CMB photons acquire her second energy shift, either up or down, depending on the relative motion of the clusters.
This pair of effects, known respectively as the thermal effect and the kinematic Sunyaev-Zeldovich (SZ) effect, were first theorized in the late 1960s and have improved their detection accuracy over the past decade. Together, the SZ effect leaves characteristic features that can be extracted from CMB images, allowing scientists to map the position and temperature of all normal matter in the universe.
Finally, a third effect, known as weak gravitational lensing, distorts the path of CMB light as it travels near large objects, distorting the CMB as if viewed through the bottom of a wine glass. Unlike the SZ effect, the lens effect is sensitive to all substances (dark or otherwise).
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 track star formation.
