Scientists propose a groundbreaking approach to investigate dark matter and the formation of early galaxies using the 21-cm forest probe, a previously theoretical concept. This innovative method has become feasible with the ongoing Square Kilometre Array project and the recent discovery of high-redshift radio-loud quasars. By simultaneously exploring these realms, researchers aim to unlock valuable insights into the thermal history of the universe and the properties of dark matter.
In the quest to comprehend the first galaxies in the universe, humans possess an unwavering curiosity. The nature of dark matter, which plays a crucial role in their formation, remains one of the most significant challenges faced by fundamental physics. However, understanding the properties of dark matter, such as whether it is cold or warm, and its subsequent impact on the formation of the first galaxies poses a formidable obstacle.
Now, a collaborative research team from China’s Northeastern University and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has proposed an innovative probe to shed light on both the nature of dark matter and the early development of galaxies simultaneously.
Published in the journal Nature Astronomy on July 6, the team’s study presents an exciting opportunity to explore the mysteries of dark matter and the early universe using the 21-cm forest probe.
Detecting small-scale structures, devoid of any previous star formation, to understand dark matter often proves challenging, especially during the cosmic dawn. Fortunately, atomic hydrogen gas surrounding these dark, compact structures from the cosmic dawn generates 21-cm absorption lines along the line of sight between Earth and high-redshift radio sources. This collection of absorption lines is collectively referred to as the 21-cm forest.
The 21-cm forest probe, a theoretical concept proposed over two decades ago, aims to examine gas temperatures and potentially unveil dark matter properties during the cosmic dawn. However, scientists have yet to implement the probe due to various obstacles, including extremely faint signals, the difficulty of identifying high-redshift background sources, and the interplay between the mass of dark matter particles and the heating effect, preventing the probe from effectively constraining either the particle mass or the heating effect originating from the first galaxies.
Recent discoveries of several high-redshift radio-loud quasars and the commencement of construction on the Square Kilometre Array (SKA), the world’s largest radio telescope project, suggest that the utilization of the 21-cm forest probe will soon become a reality.
Inspired by power spectrum analyses commonly employed in cosmological investigations, the researchers from NAOC realized that the distinctive scale-dependences of signals caused by the warm dark matter effect and the heating effect could be statistically extracted to discern key features that differentiate the two effects.
In this study, the researchers propose an innovative statistical solution to tackle the challenges of weak signals and the interplay problem by measuring the one-dimensional (1-D) power spectrum of the 21-cm forest. The amplitude and shape of the 1-D power spectrum reveal scale-dependence in the signals, rendering the 21-cm forest probe an effective and viable means of simultaneously examining dark matter properties and the thermal history of the universe.
“By measuring the one-dimensional power spectrum of the 21-cm forest, we can not only make the probe feasible by enhancing its sensitivity but also provide a method to distinguish the effects of warm dark matter models and the early heating process,” explained XU Yidong, the corresponding author of the study. “We will be able to achieve two goals with one approach!”
In scenarios where cosmic heating remains moderate, the low-frequency array of SKA Phase 1 will effectively constrain both the mass of dark matter particles and gas temperature. In cases where cosmic heating is more substantial, leveraging multiple background radio sources during SKA Phase 2 will enhance the probe’s detection capabilities.
The 21-cm forest presents a viable means of constraining dark matter in redshift ranges beyond the reach of other observations. By measuring the heating level, the 21-cm forest offers a way to delimit the spectral properties of the first galaxies and the first black holes, shedding light on the nature of the earliest luminous objects in the universe. Utilizing the 21-cm forest probe will serve as an indispensable avenue for advancing our comprehension of the early universe and unraveling the mysteries surrounding dark matter and the first galaxies.
As the application of the 21-cm forest probe relies heavily on observations of high-redshift background radio sources, the next step will involve identifying more radio-bright sources during the cosmic dawn, such as radio-loud quasars and gamma-ray burst afterglows, which can be explored further during the SKA era.
Reference: “The 21-cm forest as a simultaneous probe of dark matter and cosmic heating history” by Yue Shao, Yidong Xu, Yougang Wang, Wenxiu Yang, Ran Li, Xin Zhang and Xuelei Chen, 6 July 2023, Nature Astronomy.
Frequently Asked Questions (FAQs) about cosmology, dark matter
What is the 21-cm forest probe and how does it relate to cosmology?
The 21-cm forest probe is a concept that aims to explore the thermal history of the universe and properties of dark matter. It involves detecting 21-cm absorption lines caused by atomic hydrogen gas around dark, small structures from the cosmic dawn. By studying these lines, researchers can gain insights into the early formation of galaxies and potentially constrain the nature of dark matter.
Why has the 21-cm forest probe been considered theoretical until now?
The 21-cm forest probe faced several challenges that prevented its practical implementation. These challenges included weak signals, difficulty in identifying high-redshift background sources, and the interplay between the mass of dark matter particles and the heating effect. Overcoming these obstacles was necessary to make the probe feasible and unlock its potential in studying dark matter and early galaxies.
How has recent progress made the 21-cm forest probe viable?
The discovery of high-redshift radio-loud quasars and the ongoing Square Kilometre Array (SKA) project have significantly contributed to making the 21-cm forest probe feasible. These developments provide valuable resources and observational capabilities necessary for detecting and studying the 21-cm absorption lines. With these advancements, researchers can now utilize the probe to gain insights into dark matter properties and the formation of early galaxies.
What insights can the 21-cm forest probe provide?
The 21-cm forest probe offers a unique opportunity to simultaneously study dark matter and the thermal history of the universe. By measuring the one-dimensional power spectrum of the 21-cm forest, researchers can extract key features that differentiate the effects of warm dark matter models and the early heating process. This enables them to constrain the properties of dark matter particles, understand the spectral properties of the first galaxies and black holes, and gain a better understanding of the early Universe.
What are the next steps in utilizing the 21-cm forest probe?
The next steps involve identifying more radio-bright sources at the cosmic dawn, such as radio-loud quasars and gamma-ray burst afterglows, that can be observed and followed up during the SKA era. These sources will enhance the detection capabilities of the probe and allow for further exploration of the early Universe, providing valuable data to advance our understanding of dark matter, early galaxies, and the thermal history of the universe.
More about cosmology, dark matter
- Nature Astronomy: “The 21-cm forest as a simultaneous probe of dark matter and cosmic heating history”