The Rate of Starbirth is Crashing in the "Modern" Universe

The study found that the rate of starbirth in the "modern" universe has been declining, and it suggests that this decline is not due to a lack of available fuel. The research team used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Dark Energy Spectroscopic Instrument (DESI) to measure cosmic neutral hydrogen (HI) over the past 4.5 billion years. They observed a sample of 2.5 million galaxies, revealing that the star formation rate was about 2.5 times higher 4.5 billion years ago compared to today. The neutral atomic hydrogen density was about 1.4 times its current level, which is puzzling because it doesn't explain the current decline in star formation.

• The study indicates that the decline in star formation is not due to a lack of available fuel, but rather due to changes in the baryon cycle. This cycle involves the exchange of normal matter between galaxies and the intergalactic environment. As the gas supply from the larger cosmic web declines, it affects the efficiency of converting neutral hydrogen into molecular gas (H2), which is needed for star formation. The overall neutral hydrogen supply remains stable, but the molecular gas gradually disappears and isn't replenished, leading to a drop in star formation rates.

• The findings suggest that the changes seen in the past 4.5 billion years may not be occurring in the reservoir of neutral atomic hydrogen. Instead, the decline in star formation is linked to the baryon cycle, which affects the conversion of neutral hydrogen into molecular gas. This implies that the process of converting gas into stars is more complex and influenced by factors beyond just the availability of hydrogen.

• The study highlights the importance of understanding the conditions that affect star formation. While knowing the total amount of hydrogen in the Universe is important, this study provides new insight into the factors that influence star birth. Future research should look at conditions in the Universe before 7.2 billion years ago, and include deeper surveys across various wavelengths and frequencies to better understand the evolution of galaxies.

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