A feature of light absorption by the hydrogen surrounding early galaxies could be used as a new probe into the mysteries of dark matter and how it affected the evolution of the universe during the cosmic dark ages.
Scientists have long theorized that dark matter, a mysterious substance that makes up about 85 percent of the matter in the universe, played a huge role in the formation of the first galaxies. But since dark matter doesn’t interact with light (unlike the “normal” matter that makes up stars, planets, and us), its nature remains unknown. This means that the precise role it played when galaxies started forming remains a gap in cosmological models.
To investigate this conundrum, scientists from Northeastern University in China and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) have suggested a new probe to shed light on both the nature of dark matter and the early formation of galaxies.
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One possible way to study the particles that make up dark matter and their mass has been to study the small-scale structures in the universe. The problem arises when trying to do this for a time called the “cosmic dawn,” about 380 million years after the Big Bang, a time when the first stars were just being born. There were thus few viable light sources to illuminate this ancient era for astronomers.
But there were atoms during this era, in the form of a gas of the lightest chemical element, hydrogen. Like all elements, hydrogen absorbs light at characteristic wavelengths, leaving its imprint on the light that passes through it.
Searching for dark matter in a cosmic forest
Atomic hydrogen gas in and around the small-scale structures that existed during the cosmic dawn, which ended about a billion years after the Big Bang, creates characteristic absorption lines at 21 centimeters, in the radio range of the electromagnetic spectrum. These are collectively called the 21-cm forest, which has been proposed as a potential probe of the temperature of gas and dark matter during cosmic dawn for over 20 years.
This has remained only a theoretical concept, however, due to the fact that light from this era traveled approximately 13.4 billion years to reach us. Along the way, it lost energy and its wavelength lengthened and its frequency was lowered, moving it along the electromagnetic spectrum towards the red region and further into the infrared.
The more distant the light source, the more extreme this “redshift” process. With starlight absent, using the 21-cm forest as a dark matter probe requires noisy radio sources such as quasars to be visible at cosmic dawn and then at a high redshift. But the signals from such radio sources in this era are weak, and these high redshift background sources are difficult to identify.
However, this situation may be about to change. Not only have several high-redshift radio-noisy quasars been discovered recently, but the world’s largest radio telescope, the Square Kilometer Array (SKA), began construction in Australia and South Africa in December 2022 and will soon open its sensitive radio eye on the universe. This suggests that the detection and use of the 21cm forest may soon be feasible.
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The team behind the new study thinks that measuring the 21cm forest’s energy distribution, or its “power spectrum,” will make it a plausible probe for simultaneously measuring the properties of dark matter and the thermal history of the universe.
This could help researchers distinguish between a cold dark matter model of the universe — one with massive dark matter particles moving slowly relative to the speed of light — and a hot dark matter model, with lighter dark matter particles. and faster.
“By measuring the one-dimensional power spectrum of the 21-cm 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.” said National Astronomical Observatory researcher Yidong Xu, corresponding author of the new study. “We will be able to kill two birds with one stone!”
As long as cosmic warming has not been too extreme during the cosmic dawn, the low-frequency capabilities of SKA phase 1 operations should mean that scientists can limit the mass of dark matter particles and the temperature of the gas. If cosmic warming is too high, the second phase of SKA will see the instrument expanded, leading to the use of more background radio sources offering the same constraints.
Since the potential use of the 21 cm forest as a dark matter probe is related to observations of background high-redshift radio sources, the next step in this research is the identification of more radio-bright sources during dawn cosmic spectrum, including stronger radio sources quasars and the afterglow of gamma-ray bursts.
These sources can then be followed once SKA begins observing the universe in 2027, thus allowing astronomers to shed more light on the mysteries of both dark matter and the first galaxies.
The team’s research is presented in the July 6 edition of the journal Nature Astronomy.
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