Scientists are designing a next-generation experiment to map the Big Bang’s relic afterglow.
In the first few hundred thousand years after the Big Bang, the temperature of the universe was too high for stable atoms to form. The universe was filled with a hot, dense plasma of protons and electrons, with the primordial photons bouncing off the protons and electrons like light scatters off water droplets in fog.
As the universe expanded this primordial “fog” cooled until, 380,000 years after the Big Bang, it was finally cool enough for the electrons and protons to combine to form the first stable hydrogen atoms. At this point the fog lifted and the photons escaped, free-streaming into the now-transparent universe. These photons now make up what we call the Cosmic Microwave Background.
In 1964, astronomers Robert Wilson and Arno Penzias discovered that we can still detect this background, called the CMB for short. Since then, scientists have employed a mix of space-, balloon-, and ground-based experiments to crack the secrets contained in the universe’s oldest light.
“Everyone in the CMB community shares a common vision.”
Ground-based experiments designed to study the CMB have gotten larger and more sophisticated over time. Now, nearly 200 scientists who have up until this point worked on different competing CMB experiments have joined forces to propose a 4th generation experiment, the largest ground-based one yet, called CMB-S4.
“Everyone in the CMB community shares a common vision,” says Abigail Vieregg, a scientist at the University of Chicago and CMB-S4 collaborator. “We want to find out what happened at some tiny fraction of a second after the Big Bang.
“CMB-S4 will be the first experiment that is big and bold enough that it requires everybody in the community to join together.”
Over the past few decades, the cameras of CMB experiments have evolved from containing just hundreds of sensors to containing tens of thousands. CMB-S4, however, is aiming for half a million, which will provide unprecedented sensitivity—detecting nanokelvin fluctuations in the CMB, which has a measured average temperature of around 2.713 Kelvin—and position scientists to learn more from the CMB than ever before.
The US Department of Energy, along with the National Science Foundation supports planning for CMB-S4. DOE approved Critical Decision 0 “Mission Need” for CMB-S4 in July 2019. Within the next few months, DOE expects to announce a process to select a lead laboratory to carry out the potential DOE roles and responsibilities.
The CMB permeates all of the universe, carrying the imprint of early cosmological history. Scientists can use it to trace cosmic evolution to the first moments of the universe.
Measuring the polarization of the CMB enables scientists to detect gravitational waves produced in a fraction of a second after the Big Bang and learn more about cosmic inflation, the rapid early expansion of the universe. Tiny temperature fluctuations in the CMB allow them to map its density, which can help them to uncover the structural evolution of the universe and the seeds of the first stars and galaxies. Through the CMB, scientists confirmed the age of the universe and the presence of neutrinos.
“The CMB is a large part of the foundation of our modern picture of cosmology and what we understand about the universe today,” says collaborator Zeeshan Ahmed, a scientist at SLAC National Accelerator Laboratory. “The cool thing about it is it captures information about stuff that happened before its release, and it acts like a backlight for the rest of time. As it streams along, it carries signatures of interactions that happen along the way.”
What sets CMB-S4 apart is both its unprecedented sensitivity and the area of sky it will be able to cover from the ground.
The experiment will employ both small and large telescopes in two of the highest and driest deserts on Earth—the South Pole and the high Chilean Atacama plateau—which helps minimize atmospheric disturbances that obscure the data. Because both sites provide different advantages, combining them will enable the experiment to take the best of both worlds.
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