Population III0.1 Flash Reionization at Z=8.8, with Τ=0.041, Relieves CMB and Cosmological Tensions

The early universe underwent a period of reionization, transforming from a neutral state to one filled with ionised gas, and recent research investigates a particularly rapid, transient phase of this process known as “The Flash”. Jonathan C. Tan from Chalmers University of Technology and the University of Virginia, alongside Eiichiro Komatsu from the Max-Planck-Institut für Astrophysik, Ludwig-Maximilians-Universität München, and The University of Tokyo Institutes for Advanced Study, and their colleagues, explore how this “Flash” reionization, driven by the first supermassive stars, impacts measurements of the cosmic microwave background. Their work demonstrates that this early burst of ionisation significantly alters the expected signal, potentially resolving discrepancies between current cosmological models and observations, and offering a compelling explanation for anomalies such as the preference for negative neutrino masses and the possibility of dynamic dark energy. By modelling the effects of this “Flash”, the team reveals a way to reconcile theoretical predictions with the latest data from observations of the early universe, providing new insights into the conditions that prevailed shortly after the Big Bang.

Mapping Neutral Hydrogen During Reionization

Scientists are investigating the early universe, specifically the period of reionization and the 21cm signal, to understand how the first stars and galaxies formed. Reionization refers to the time after the Big Bang when radiation from these first structures ionized neutral hydrogen that filled the universe. The 21cm signal, a radio wave emitted by neutral hydrogen, provides a way to map the distribution of this gas during reionization, offering insights into the formation of the earliest cosmic structures. Researchers are using this signal to probe the conditions of the early universe and understand the nature of the first light sources.

Several observational projects contribute to this research, including the EDGES experiment and SARAS3. The Hydrogen Epoch of Reionization Array (HERA) is a radio telescope designed to map the large-scale distribution of neutral hydrogen. Future experiments like the Simons Observatory, LiteBIRD, and the Dark Energy Spectroscopic Instrument (DESI) will also provide valuable data, complementing existing data from the Planck and Wilkinson Microwave Anisotropy Probe (WMAP) missions used to constrain cosmological parameters. This research relies on the standard cosmological model (Lambda-CDM) and explores how parameters like dark matter density, baryon density, the Hubble constant, and the mass of neutrinos affect the 21cm signal and reionization.

Scientists are investigating how these parameters can be constrained by observations of the 21cm signal and other cosmological probes. Challenges include the EDGES anomaly and the contamination of the 21cm signal by radio emission from other sources, necessitating accurate calibration of radio telescopes. Researchers employ techniques including analyzing the average 21cm signal, measuring fluctuations in the signal (power spectrum), and using higher-order statistics (bispectrum). Computer simulations and machine learning are also used to model the evolution of the universe and separate the 21cm signal from noise. Future 21cm experiments, such as HERA, and the combination of 21cm observations with other cosmological probes, hold the potential to deepen our understanding of the early universe.

Early Reionization and CMB Optical Depth

Recent work explores a theory for supermassive black hole formation, predicting an early, brief period of cosmic reionization, termed “The Flash”, powered by exceptionally massive stars. Scientists determined that this initial phase was followed by a period of neutrality before galaxies reionized the intergalactic gas at later times. Calculations show that The Flash contributes to the overall Thomson scattering optical depth, with a value of approximately 0. 03, resulting in a total optical depth of 0. 09 when combined with later reionization.

This value is larger than previous estimates derived from observations of the cosmic microwave background, but addresses tensions within the standard cosmological model, specifically regarding neutrino masses and the nature of dark energy. Researchers computed power spectra for models of The Flash, discovering that its high redshift dramatically reduces its impact on large-scale features of the cosmic microwave background compared to standard reionization models with the same optical depth, while boosting power at smaller scales, providing a unique signature for this early reionization scenario. The team parameterized the reionization history with two components: a standard model describing reionization at lower redshifts and a model for The Flash based on supermassive Population III stars forming at redshifts of 20 and 25. Calculations show that for a typical case, the timescale for ionization to reach its peak value is approximately 30 million years, implying that these early stars began shining at redshifts of 23 and 30. These findings demonstrate a pathway to reconcile cosmic microwave background observations with alternative cosmological models and provide a framework for future investigations into the earliest stages of cosmic structure formation.

Early Reionization Resolves Cosmological Tensions

This research demonstrates that a specific model of early reionization, driven by the first generation of massive stars, offers a compelling solution to existing tensions within the standard cosmological model. By proposing a transient, intense period of ionization, dubbed “The Flash”, at very high redshifts, the team shows how to simultaneously increase the overall Thomson scattering optical depth and better align theoretical predictions with recent observations of the cosmic microwave background’s polarization. Crucially, this model predicts a distinct shape for the polarization power spectrum, differing from traditional late-phase reionization scenarios, and potentially resolving discrepancies related to neutrino masses and the nature of dark energy. The team’s calculations reveal that “The Flash” significantly reduces the contribution to certain wavelengths while boosting power at others, creating a unique spectral signature.

This allows for a higher value of the optical depth, potentially alleviating cosmological tensions, while remaining consistent with current low-polarization CMB observations. The authors suggest that current constraints on reionization history, derived from patchy kinematic Sunyaev-Zel’dovich effect measurements, which assume a monotonic ionization fraction, need revision for the Pop III scenario. Future observations, particularly with instruments like LiteBIRD and the Simons Observatory, are expected to provide the necessary precision to distinguish between different Pop III models and further test this innovative reionization history, potentially through signatures in the 21-cm emission from the early universe.