For decades, scientists have been puzzled by the missing mass in the universe. While we know that about 85% of the universe is made up of dark matter, the remaining 15% is made up of normal (baryonic) matter, including stars, gas, and galaxies. However, much of this normal matter has been unaccounted for, especially in the form of hydrogen gas. In a breakthrough study published in Science Advances, a team of researchers has confirmed the existence of this missing matter in the form of ionized hydrogen gas, which had been undetectable using traditional methods.
Astronomers have long theorized that this hydrogen gas existed in the galactic halos surrounding galaxies, as well as in cosmic filaments that link galaxies. The newly discovered gas could help solve a major mystery of baryonic matter distribution, which has remained a puzzle since the early days of cosmology. This elusive matter is crucial in understanding galaxy formation and cosmic evolution.
The Role of Ionized Hydrogen in the Universe
The ionized hydrogen that has been discovered is part of a vast network known as the warm-hot intergalactic medium (WHIM). This gas exists between galaxies, connecting them in a massive cosmic web. The newly found hydrogen gas is part of this filamentary structure stretching across the universe, too diffuse and cold to be detected using traditional optical telescopes. Scientists were able to identify it using the kinematic Sunyaev-Zel’dovich effect, which involves the scattering of cosmic microwave background radiation by electrons in the ionized gas.
To detect the faint signal of this gas, the team stacked images of over seven million galaxies within 8 billion light-years of Earth, measuring subtle variations in the cosmic microwave background. This provided the first direct evidence of the missing hydrogen gas, offering a significant clue to understanding the large-scale structure of the universe.
“We think that, once we go farther away from the galaxy, we recover all of the missing gas,” said Boryana Hadzhiyska, a Miller postdoctoral fellow at the University of California, Berkeley. She is the first author of the study and explained that a more detailed analysis with simulations would further refine these findings. “To be more accurate, we have to do a careful analysis with simulations, which we haven’t done. We want to do a careful job.”
This revelation is pivotal because it helps resolve the missing baryons problem. For years, astronomers struggled to account for the half of normal matter that didn’t seem to appear in stars or galaxies. This discovery of hidden hydrogen gas might be the key to unlocking the missing piece of the puzzle.
The Role of Cosmic Feedback and Active Galactic Nuclei (AGN)
The study also suggests that active galactic nuclei (AGNs), which are the supermassive black holes at the centers of galaxies, may play a more active role in expelling gas than previously understood. Traditionally, astronomers believed that AGNs were only active during the early stages of a galaxy’s life when they were consuming vast amounts of matter. However, the new findings suggest that AGNs might cycle on and off throughout a galaxy’s lifetime, contributing to the expulsion of gas in a more continuous manner than previously thought.
“One problem we don’t understand is about AGNs, and one of the hypotheses is that they turn on and off occasionally in what is called a duty cycle,” said Hadzhiyska. This new information could have significant implications for how astronomers model galaxy evolution, suggesting that the feedback mechanisms responsible for gas expulsion might be much more dynamic than previously thought.
These discoveries also challenge the assumption that gas only follows the distribution of dark matter in galaxies. By incorporating this new gas distribution into current cosmological models, scientists will need to rework their understanding of galactic evolution and the formation of stars.
Advancing Cosmology with New Measurements
The ability to measure this previously elusive gas has vast implications for cosmological research. Not only does it provide insight into the missing baryons, but it also allows scientists to investigate fundamental questions about the early universe, including the formation of galaxies and stars. The new findings suggest that understanding the location of this gas is essential for solving many of the remaining questions in cosmology.
“The measurements are certainly consistent with finding all of the gas,” said Simone Ferraro, a senior scientist at Lawrence Berkeley National Laboratory and UC Berkeley. He highlighted how the cosmic microwave background (CMB), which acts as the “backlight” for all cosmic observations, plays a crucial role in tracking this gas. “The cosmic microwave background is in the back of everything we see in the universe. It’s the edge of the observable universe. So you can use that as a backlight to see where the gas is.”
These findings not only help resolve a long-standing issue in astronomy but also open new avenues for future research, particularly in understanding cosmic feedback and refining models of galaxy formation.
Implications for Future Studies
The discovery of the missing hydrogen gas is just one step toward understanding the full story of the universe’s evolution. Researchers are now looking to further refine these measurements using advanced simulations to understand the precise role this gas plays in cosmic structures. The ongoing collaboration between scientists across the globe promises to shed light on many of the fundamental mysteries that still persist about the universe’s formation.
As we continue to explore the vastness of the universe, this finding offers a clearer picture of the baryonic mass that has eluded scientists for decades, ultimately helping us understand the building blocks of the cosmos.
