Supermassive Dark Matter Stars Could Be Hidden in the Early Universe

We might be observing the earliest indications of peculiar stars that harness dark matter. These dark stars could provide explanations for some of the universe’s most enigmatic entities, and offer insights into the actual nature of dark matter itself.

Standard stars are birthed when a gas cloud collapses, leading to a core dense enough to initiate nuclear fusion. This fusion generates significant heat and energy, radiating into the surrounding gas and plasma.

Dark stars could have emerged in a similar fashion during the universe’s infancy, a period of higher density which also saw a notably concentrated presence of dark matter. If a gas cloud collapsing into a star contains substantial dark matter, it may begin to collide and dissipate prior to nuclear fusion, generating enough energy to illuminate the dark star and halt further collapse.

The process leading to the formation of dark stars is relatively straightforward, and currently, a team led by Katherine Freese from the University of Texas at Austin is exploring its potential outcome.

In an ordinary large star, once the hydrogen and helium are depleted, it continues fusing heavier elements until it runs out of energy and collapses into a black hole. The more mass the star contains, the quicker this transition occurs.

However, the same is not true for dark stars. “By incorporating dark matter into a star roughly the mass of the Sun, and sustaining it through dark matter decay rather than nuclear means, you can continuously nourish the star. Provided it receives enough dark matter, it won’t undergo the nuclear transformations that lead to complications,” explains George Fuller, a collaborator with Freese at the University of California, San Diego.

Despite this, general relativity imposes a limit on how long dark matter can preserve these unusual giants. Albert Einstein’s theory suggests that an object’s gravitational field does not increase linearly with mass; instead, gravity intensifies the gravitational force. Ultimately, an object may reach a mass at which it becomes unstable, with minor variations overpowering its gravitational pull and resulting in a collapse into a black hole. Researchers estimate this threshold for a dark star is between 1,000 and 10 million times the Sun’s mass.

This mass range makes supermassive dark stars prime candidates for addressing one of the early universe’s profound mysteries: the existence of supermassive black holes. These giants were spotted relatively early in the universe’s history, but their rapid formation remains a puzzle. One prevailing theory posits that they didn’t arise from typical stars, but rather from some colossal “seed.”

“If a black hole weighs 100 solar masses, how could it possibly grow to a billion solar masses in just a few hundred million years? This is implausible if black holes were formed solely from standard stars,” asserts Freese. “Conversely, this situation changes significantly if the origin is a relatively large seed.” Such faint stars could serve as those seeds.

Yet, the enigmas of the early universe extend beyond supermassive black holes that dark stars could elucidate. The James Webb Space Telescope (JWST) has unveiled two other unforeseen object types, referred to as the little red dot and the blue monster, both appearing at substantial distances. The immediate hypothesis for these is that they are compact galaxies.

However, like supermassive black holes, these objects exist too far away and too early in universal history for simple formation explanations. Based on observations, Freese and her associates propose that both the little red dot and the blue monster may represent individual, immensely massive dark stars.

If they indeed are dark stars, they would display particular clues in their light. This aspect pertains to specific wavelengths that dark stars should ostensibly absorb. Normal stars and galaxies dense with them are too hot to capture that light.

Freese and colleagues have found possible indicators of this absorption in initial JWST observations of several distant entities; however, the data is too inconclusive to confirm its existence. “Currently, of all our candidates, two could potentially fit the spectrum: a solitary supermassive dark star or an entire galaxy of regular stars,” Freese notes. “Examining this dip in the spectrum, we’re convinced it points to a dark star rather than a conventional star-filled galaxy. But for now, we only possess a faint hint.”

While it remains uncertain if we have definitively detected a dark star, this development marks progress. “It isn’t a definitive finding, but it certainly fuels motivation for ongoing inquiries, and some aspects of what JWST has been examining seem to align with that direction,” remarks Dan Hooper from the University of Wisconsin-Madison.

Establishing whether these entities are genuinely dark stars necessitates numerous more observations, ideally with enhanced sensitivity; however, it remains ambiguous whether JWST can achieve the level of detail required for such distant galaxies or dark stars.

“Confirming the existence of dark stars would be a remarkable breakthrough,” emphasizes Volodymyr Takistov from the High Energy Accelerator Research Organization in Japan. This could facilitate new observational avenues into foundational physics. This is particularly true if dark stars are recognized as seeds for supermassive black holes. Freese, Fuller, and their team deduced that the mass at which a black hole collapses correlates with the mass of the dark matter particle annihilating at its center, implying that supermassive black holes could serve as metrics to evaluate or at least restrict dark matter properties. Of course, validating the existence of dark stars is the first priority. “Even if these entities exist, their occurrence is rare,” Hooper states. “They are uncommon, yet significant.”

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