Breakthrough Cosmology: The Role of 21-cm Forest Probes in Deciphering Dark Matter

Explore dark matter and the first galaxies at the same time as the 21cm forest. This approach can help constrain the properties of dark matter and provide insight into the thermal history of the universe. Credit: NAOC & NEU

Simultaneously exploring dark matter and the first luminous galaxies: a 21-cm forestry probe could reveal the secrets of the early universe.

A joint research team has proposed a new way to study dark matter and the formation of the first galaxies using the 21cm Forest Probe, a concept previously thought to be theoretical. The ongoing Square Kilometer Array project and the recent discovery of high redshift radio loud quasars have made this method viable. This new approach could unlock insights into the thermal history of the universe and the properties of dark matter.

The mystery of the first galaxies of the universe is an indomitable impulse of the human being. Their formation is dominated by the nature of dark matter, which is also one of the most important problems faced by fundamental physics. However, understanding the nature of dark matter, such as whether it is cold or hot, and its subsequent effect on the formation of the first galaxy is a huge challenge.

Now, a joint research team from Northeastern University (China) and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has proposed using a new probe to try to shed light on the nature of dark matter and the early formation of galaxies at the same time .

The team’s study was published in the journal Nature astronomy on July 6th.

The simulated 21cm forest 1-D power spectra. Credit: Shao et al.

One way to understand dark matter is to try to measure the mass of dark matter particles through cosmological observations of small-scale structures. But detecting small-scale structures where no star formation has ever occurred is difficult, especially during the cosmic dawn. Fortunately, the atomic hydrogen gas in and around these small dark structures of the cosmic dawn creates 21-cm absorption lines along the lines of sight between the Earth and high-redshift radio point sources. These absorption lines are known collectively as the 21cm forest.

The 21-cm Forest Probe is a theoretical concept that has been proposed for more than 20 years to probe gas temperatures and potentially dark matter properties during the cosmic dawn. So far, scientists have not attempted to actually use the spacecraft due to numerous challenges, including extremely weak signals, the difficulty in identifying high redshift background sources, and degeneracy between the mass of dark matter particles and the spacecraft. heating effect, which would prevent the probe from constraining particle mass or the heating effect of early galaxies.

Recently, however, several high-redshift radio quasars have been discovered. In addition, construction began last December on the Square Kilometer Array (SKA), an international initiative to build the world’s largest radio telescope. Both of these developments suggest that using the 21cm forest probe will soon be feasible.

Inspired by power spectrum analyzes widely used in cosmological probes, the NAOC researchers realized that the distinctive scale dependencies of the signals caused by the hot dark matter effect and the warming effect, respectively, could be used to statistically extract the key features to distinguish the two effects.

In this study, the researchers proposed a novel statistical solution to simultaneously solve the weak signal problem and the degeneracy problem by measuring the one-dimensional (1-D) power spectrum of the 21-cm forest. The dependence on the scale of the detected signal on the width and shape of the 1-D power spectrum makes the 21-cm Forest probe a viable and effective means of simultaneously measuring dark matter properties and the thermal history of the Universe.

By measuring the one-dimensional power spectrum of the 21cm forest, we can not only make the probe actually feasible by increasing the sensitivity, but also provide a way to distinguish the effects of hot dark matter patterns and the initial heating process, XU Yidong said , corresponding author of the study. We can kill two birds with one stone!

In scenarios where cosmic warming is not too severe, SKA Phase 1’s low-frequency array will be able to effectively limit both the mass of dark matter particles and the temperature of the gas. In cases where cosmic warming is more significant, using more background radio sources during SKA Phase 2 will enable robust detection capabilities.

The 21-cm forest offers a viable means of limiting dark matter to redshift ranges beyond the reach of other observations. By measuring the level of warming, the 21-cm forest provides a way to constrain the spectral properties of the first galaxies and black holes, thus shedding light on the nature of the first luminous objects in the Universe. The use of the 21-cm Forest Probe will serve as an indispensable avenue for advancing our understanding of the early Universe and peering into the mysteries of both dark matter and early galaxies.

Since the application of the 21 cm Forest probe is closely related to observations of high redshift background radio sources, the next step will also involve the identification of more radio-luminous sources at cosmic dawn (such as loud radio volume and gamma-ray bursts afterglows) which can be followed in the SKA era.

Reference: The 21cm Forest as a Simultaneous Probe of Dark Matter and Cosmic Warming History by Yue Shao, Yidong Xu, Yougang Wang, Wenxiu Yang, Ran Li, Xin Zhang, and Xuelei Chen, July 6, 2023, Nature astronomy.
DOI: 10.1038/s41550-023-02024-7