Imagine taking a journey back to the early days of the universe, specifically, the start of the Epoch of Reionization (EoR). This is when the first stars and galaxies formed, and their energy separated the protons and electrons of the dense, dark primordial hydrogen gas that comprised the universe. Unfortunately, scientists can’t create time machines to take us back to the EoR, but they can collect terabytes of data and create simulations to gain insights into the early universe.
Srinivasan Raghunathan, a postdoctoral fellow at the Center for AstroPhysical Surveys (CAPS) at the University of Illinois, Urbana-Champaign, led a multi-institutional research team that aimed to detect the kinetic Sunyaev-Zel’dovich effect from cosmic microwave background temperature maps using data obtained from the South Pole Telescope and the SPIRE instrument onboard the Herschel Space Observatory. The results were published in Physical Review Letters.
The team used high-resolution simulations of the early universe, performed on the Illinois Campus Cluster at NCSA, to validate their measurements. Although they haven’t yet detected the signal, their research sets an upper limit on when the EoR likely ended, providing a parameter for scientists to work from as they continue to investigate the early universe and the EoR.
Probing the Early Universe: Unveiling the Epoch of Reionization
The universe as we know it today is vastly different from its early stages, where darkness prevailed before the first stars and galaxies began to shine. The epoch of reionization, a period when the universe transitioned from a neutral state to an ionized one, remains shrouded in mystery. Scientists have been working tirelessly to uncover the secrets of this era, and recent research has brought us closer to understanding this pivotal moment in cosmic history.
The Quest for the kSZ Signal
A team of researchers, led by postdoctoral fellow Srinivasan Raghunathan from the NCSA Center for AstroPhysical Surveys, has been searching for a faint signal in the cosmic microwave background (CMB) radiation. This signal, known as the kinetic Sunyaev-Zel’dovich (kSZ) effect, is expected to provide valuable insights into the epoch of reionization.
The kSZ effect is a result of the scattering of CMB photons by free electrons in the ionized gas surrounding galaxies. As galaxies began to form and reionization occurred across the universe, this scattering would have given a slight boost in energy (measured in temperature) to the CMB. The researchers used data from the South Pole Telescope, measured in frequencies of 90, 150, and 220 GHz in the submillimeter band, to search for this non-Gaussian, or patchy, nature of the kSZ effect.
High-Resolution Simulations: A Key to Unlocking the Mystery
To assist in their hunt for the kSZ signal, the researchers developed high-resolution computer simulations of the universe. These simulations, run on the Illinois Campus Cluster at NCSA, mimic the data and help filter out extraneous signals and the effects of gravitational lensing from the CMB.
“We have several terabytes of data, and we want to validate what we are measuring,” Raghunathan explained. “So, we build simulations of the early universe. Lots and lots of simulations.”
These simulations allow researchers to “see” how early signals in the universe interact with each other and how they make their way into the three frequencies being measured.
A Significant Step Forward
Although the kSZ effect from the EoR has not yet been detected in the data, the research team’s findings are a significant step forward in probing the early universe. The fact that they didn’t detect the signal tells us that it can’t be above a certain amplitude, ruling out models of reionization like early and slow models.
“This analysis is a key first step towards such future analyses,” Raghunathan said.
The Future of Cosmic Microwave Background Research
As new, high-resolution, low-noise datasets covering larger spans of the universe become available, scientists will continue to investigate the early universe and the EoR. The ongoing South Pole Telescope survey will significantly reduce noise in the data, making it better suited for analysis. The next-generation ground-based cosmic microwave background experiment called CMB-S4 will collect survey data from telescopes at the South Pole and in Chile, providing even more available data covering a wider swath of the sky.
“In the near future, we are hoping to detect this signal,” Raghunathan said. “We know it’s there and that reionization happened. Now we want to know why and how the entire process happened.”
The quest for understanding the epoch of reionization is an ongoing journey, with scientists pushing the boundaries of what we know about the universe. As new data and analysis techniques become available, we will continue to unravel the mysteries of this pivotal moment in cosmic history.
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