The Big Bang’s backlight reveals invisible cosmic structures

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About 400,000 years After the Big Bang, the primordial plasma of the infant universe cooled enough for the first atoms to fuse, making room for radiation to be released. That light—the cosmic microwave background (CMB)—continues to drift across the sky in all directions, capturing the early universe with special telescopes and even showing up on static on old cathode-ray televisions.

Since the discovery of CMB radiation in 1965, scientists have carefully mapped the tiny temperature variations that reveal the exact state of the cosmos when it was plasma. Now they are reusing the CMB data to map the large-scale structures that built up over billions of years as the universe grew.

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

During its journey of nearly 14 billion years, the light from the CMB has been stretched, compressed and distorted by all matter along its path. Cosmologists are beginning to see secondary imprints left by interactions with galaxies and other cosmic structures beyond the primary fluctuations in the CMB’s light. From these signals you are getting a clear view of the distribution of both ordinary matter – everything made up of atomic parts – and the mysterious dark matter. In turn, those insights are helping to solve some long-standing cosmological mysteries and create some new ones.

“We’re realizing that the CMB doesn’t just tell us about the early conditions of the universe. It also tells us about the galaxies themselves,” said SLAC cosmologist Emmanuel Schaan. “And that would be very powerful.”

A universe of shadows

Standard optical surveys that track the light emitted by stars scan most of the mass at the base of galaxies. That’s because most of the universe’s total matter content is invisible to telescopes—out of sight, like dark matter or the ionized gas that binds galaxies together. But both the dark matter and the scattered gas leave recognizable traces on the brightness and color of the incoming CMB light.

“The universe is really a shadow theater where the galaxies are the protagonists and the CMB is the background light,” Schaan said.

Many of the shadow players are now coming into relief.

When light particles, or photons, from the CMB scatter electrons in the gas between galaxies, they are compressed to higher energies. Additionally, if those galaxies are moving relative to the expanding universe, the CMB photons will experience a secondary energy shift, either up or down, depending on the cluster’s relative motion.

This pair of effects, known as the thermal and kinematic Sunyav-Zeldovich (SZ) effects, respectively, was first identified in the late 1960s and has been observed with increasing accuracy over the past decade. Together, the SZ effects leave a characteristic signature that can be teased out of CMB images, allowing scientists to reveal the location and temperature of all ordinary objects in the universe.

Finally, a third effect known as weak gravitational lensing causes the CMB light to bend its path as it travels toward massive objects, distorting the CMB as if viewed through a wine glass. According to SZ results, glasses are sensitive to all matter—dark or otherwise.

Together, these effects allow cosmologists to distinguish ordinary matter from dark matter. Scientists can then overlay these maps with everything from galaxy surveys to measuring cosmic distances and even measuring star formation.

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