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Scientists are exploring the intriguing possibility of “dark” dark matter haloes β cosmic structures devoid of galaxies. New research investigates whether these dark matter haloes, theorized to be crucial for galaxy formation, could exist independently, without the conventional stellar components. This concept challenges traditional understandings of dark matter and galaxy development, prompting astrophysicists to reconsider the universe’s composition and the distribution of its most mysterious substance.
The Enigma of Galaxy-Less Dark Matter Haloes
The prevailing theory posits a strong connection between galaxies and dark matter. Galaxies are believed to arise from gas and dust accumulating within gravitational “wells” sculpted by clumps of dark matter. As matter congregates in these wells, star birth ignites, gradually fostering galactic growth within a surrounding dark matter halo.
However, Ethan Nadler, a computational astrophysicist at the University of California, San Diego, is delving into the potential for star-free, or “dark,” dark matter haloes. His investigation originated from calculations determining the minimum mass at which a halo would be unable to ignite star formation.
“Every observed galaxy aligns with the presence of a dark matter halo. Yet, it remains unknown if dark matter haloes can exist without birthing stars, essentially if entirely dark, starless haloes populate the cosmos,” Nadler explained to Space.com. “A starless dark matter halo would lack a galaxy at its core.”
Revisiting the Star Formation Threshold
Previous estimates placed the star formation mass limit between 100 million and 1 billion solar masses. This suggested that only haloes near this mass range could remain “dark,” implying a scarcity of such objects. Nadler’s research has now established a lower mass threshold, indicating that “dark” dark matter haloes might be more prevalent than previously considered. Cosmological models, in fact, suggest that less massive versions of these entities are more likely.
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Hydrogen, the most basic element in the universe, is critical for star formation. Nadler clarified that earlier attempts to define the star formation limit only accounted for the cooling of simple hydrogen atoms. By also considering the cooling of hydrogen molecules, composed of two bonded hydrogen atoms, his findings reveal that stars could form in dark matter haloes as small as 1 million solar masses shortly after the Big Bang β significantly smaller than earlier predictions.
Over time, these haloes would have expanded by accumulating more dark matter, eventually reaching masses of 10 million solar masses in the present universe. Even smaller dark matter haloes would have grown through mass accretion, but their star formation would have been inhibited by radiation emanating from other, more successful galaxies.
“From a theoretical perspective, most dark matter models predict a greater abundance of small haloes compared to large ones, making starless haloes potentially numerous,” Nadler stated.
Einstein’s Theory Illuminates Invisible Dark Matter
Dark matter is theorized to outweigh ordinary matter in the universe by a factor of five. Visible matter, encompassing stars, planets, and everyday objects, constitutes a mere 15% of the universe’s matter content.
Despite its abundance, dark matter remains essentially invisible, interacting minimally with light and ordinary matter. However, its gravitational influence is undeniable, providing evidence of its existence. Thus, “dark” dark matter haloes, while lacking galaxies, could still exert gravitational effects.
“Even without hosting galaxies, starless haloes maintain gravitational influence on both dark matter and normal matter,” Nadler noted. “Consequently, the quantity and mass distribution of existing starless haloes are crucial for understanding the universe’s small-scale structure and its gravitational dynamics.”
“Observationally, evidence for haloes below the galaxy formation threshold is still lacking. Detecting these objects is the next major objective.”
Seeking Gravitational Lenses in the Cosmos
Detecting these “dark” dark matter haloes presents considerable challenges due to dark matter’s inherent invisibility. Astronomers typically infer dark matter’s presence by observing its gravitational impact on visible galactic components. The absence of a galaxy within a dark matter halo eliminates this conventional method.
“Direct observation of ‘dark’ dark matter haloes is not feasible; we must deduce their presence through their gravitational influences,” Nadler elucidated. “Strong gravitational lensing presents a promising avenue for this endeavor.”
Gravitational Lensing: A Key to Detection
Gravitational lensing, a phenomenon predicted by Albert Einstein’s general theory of relativity in 1915, arises from the curvature of spacetime by massive objects. This curvature bends the path of light passing nearby, with the degree of bending depending on the proximity to the massive object, or “gravitational lens.” Light from a single source can arrive at different times, leading to signal magnification or the appearance of multiple images of the source.
“Dark” dark matter haloes, despite their invisibility and lack of galaxies, possess mass and can therefore act as gravitational lenses. Unexplained lensing effects, not attributable to visible matter, could indicate the presence of galaxy-less dark matter haloes.
“Gravitational lensing data from the James Webb Space Telescope (JWST) is now sensitive enough to detect haloes around 10 million solar masses, and forthcoming facilities like the Rubin Observatory will uncover thousands of new strong lenses,” Nadler stated optimistically. “I am hopeful for a definitive detection of these objects within this decade.”
Implications for Dark Matter and Cosmology
The confirmation or refutation of “dark” dark matter haloes below Nadler’s star formation mass limit bears significant implications. It could enable scientists to distinguish between various dark matter models, including the standard cosmological model, Lambda Cold Dark Matter (ΞCDM), currently the leading model.
“Their absence would also be profoundly insightful, suggesting that standard ‘cold’ dark matter model predictions regarding the abundance of small haloes are incorrect. This could imply that dark matter behaves differently at sub-galactic scales, as proposed by alternative models like warm, fuzzy, or self-interacting dark matter,” Nadler concluded. “Determining which haloes host galaxies provides crucial insights into star formation processes, a fundamental aspect of galaxy formation theory.”