Thanks to observatories like the venerable Hubble Space Telescope (HST) and its next-generation cousin, the James Webb Space Telescope (JWST), astronomers are finally getting the chance to study galaxies that existed just one billion years after the Big Bang. This period is known as "Cosmic Dawn" because it was during this period that the first stars formed and came together to create the first galaxies in the Universe. The study of these galaxies has revealed some surprising and fascinating things that are allowing astronomers to learn how large-scale structures in the Universe came to be and how they've evolved since.
For the longest time, it was thought that this cosmological period could only be seen by space telescopes, as they don't have to deal with interference from Earth's atmosphere. With advanced technologies ranging from adaptive optics (AO) and coronagraphs to interferometry and spectrometers, ground-based telescopes are pushing the boundaries of what astronomers can see. In recent news, an international team of astronomers using the Cosmology Large Angular Scale Surveyor (CLASS) announced the first-ever detection of radiation from the cosmic microwave background (CMB) interacting with the first stars in the Universe. These findings shed light on one of the least understood periods in cosmological history.
The study that details their findings, which recently appeared in The Astrophysical Journal, was led by Yunyang Li — an observational cosmologist from the Kavli Institute for Cosmological Physics (University of Chicago) and The William H. Miller III Department of Physics and Astronomy at Johns Hopkins University (JHU). He was joined by many JHU colleagues, as well as astrophysicists from the National Institute of Standards and Technology, the Argonne National Laboratory, the Los Alamos National Laboratory, the Harvard-Smithsonian Center for Astrophysics, the Massachusetts Institute of Technology (MIT), the NASA Goddard Space Flight Center, and many prestigious universities.
Studying the Cosmic Dawn Epoch has always been extremely challenging for astronomers, regardless of whether space-based or ground-based telescopes were used. During this period, the Universe was shrouded in neutral hydrogen, and the only photons visible to modern instruments were from the relic radiation left over from the Big Bang (the CMB) and those released from the ionization of neutral hydrogen atoms. The latter were caused by the first stars that formed during Cosmic Dawn, which released tremendous amounts of ultraviolet (UV) energy that reionized the hydrogen clouds, releasing free electrons that scattered and collided with other particles. This event effectively ended the Cosmic Dark Ages and led to the Universe becoming "transparent" (i.e., visible to modern instruments).
While cosmic microwaves have very long wavelengths (measuring just millimeters) and are very faint, polarized microwave light (which happens when light interacts with another object and scatters) is about a million times fainter. Moreover, there's the issue of atmospheric interference and noise caused by Earth-based radio transmissions, including satellites, radar, and microwave transmissions. As a result, extremely sensitive equipment is needed to measure polarized microwaves, even under ideal conditions. Previously, only space-based telescopes like NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the ESA's Planck mission could detect this light.
This led to the National Science Foundation's (NSF) CLASS Observatory, which operates in the Parque Astronómico Atacama in northern Chile, a telescope uniquely designed to detect signatures of the first stars in the relic radiation of the Big Bang. "People thought this couldn't be done from the ground," said Tobias Marriage, the leader of the CLASS project and a professor of physics and astronomy at JHU. "Astronomy is a technology-limited field, and microwave signals from the Cosmic Dawn are famously difficult to measure. Ground-based observations face additional challenges compared to space. Overcoming those obstacles makes this measurement a significant achievement."
For their study, the research team tested the probability that a photon from the Big Bang encountered one of the freed electrons released by the ionized gas that then skittered off course. By comparing the CLASS telescope data with data obtained by the Planck and WMAP space missions, the researchers identified interference and narrowed the signal down to polarized microwave light. "When light hits the hood of your car and you see a glare, that's polarization. To see clearly, you can put on polarized glasses to take away glare," said Li. "Using the new common signal, we can determine how much of what we're seeing is cosmic glare from light bouncing off the hood of the Cosmic Dawn, so to speak."
The work builds on previous research (published last year) in which the team used the CLASS telescopes to map 75% of the night sky. These new results solidify the CLASS team's approach and greatly improve the measurement of the CMB's polarization signal, which is leading to a clearer picture of the early Universe. Said Charles Bennett, a Bloomberg Distinguished Professor at Johns Hopkins who led the WMAP space mission:
Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research. For us, the Universe is like a physics lab. Better measurements of the Universe help to refine our understanding of dark matter and neutrinos, abundant but elusive particles that fill the Universe. By analyzing additional CLASS data going forward, we hope to reach the highest possible precision that's achievable.
Further Reading: JHU, The Astrophysical Journal