Astrophysicists thought they were looking at the oldest galaxies, born just after the Big Bang, but something didn’t add up. They were too bright, too massive. Now, hints are emerging that the sightings might be not galaxies after all, but never-before-seen dark matter stars.
If true, the discovery, made possible by James Webb Space Telescope (JWST), could change our understanding of the Universe's formation.
A team of astrophysicists from the University of Texas at Austin found clues that three distant objects from the past that were initially identified as galaxies in December 2022 by the JWST Advanced Deep Extragalactic Survey (JADES) might actually be “dark stars.”
The mechanism of how dark stars shine would be utterly different from our sun or any star in the known universe. Dark stars could be powered by yet unknown particles of dark matter annihilating each other, and therefore those theoretical objects would be much bigger and brighter than casual stars.
The existence of dark stars also could provide an explanation of how supermassive black holes in the centers of the galaxies formed. If confirmed, dark stars could reveal the nature of dark matter, one of the deepest unsolved problems in physics.
“Discovering a new type of star is pretty interesting all by itself, but discovering it’s dark matter that’s powering this — that would be huge,” said Katherine Freese, director of the Weinberg Institute for Theoretical Physics and the Jeff and Gail Kodosky Endowed Chair in Physics at UT Austin.
Before JWST, we had very limited data on the cosmic dawn era, when the first stars and galaxies formed.
The first observations of JWST introduced an unexpected problem – there seem to be too many too-large galaxies too early in the universe, which contradicts the predictions of the standard cosmology model.
“Since beginning to take data, JWST has discovered a surprising number of extremely bright high redshift galaxy candidates, which are difficult to reconcile with expectations from numerical simulations of the universe in the canonical ΛCDM scenario,” scientists write.
The three mysterious objects, named JADES-GS-z13-0, JADES-GS-z12-0, and JADES-GS-z11-0, were observed at a time range from about 320 million to 400 million years after the Big Bang, making them some of the earliest ever observed objects. The objects seem to be a single-point source of light. In comparison, a galaxy would look more fuzzy. Candidate dark stars were originally identified as galaxies.
“Some of these things are over 30 billion light years away, meaning that their light has been traveling to us for most of the age of the universe. We're looking at the cosmos as it was only around 400 million years after the Big Bang or around 3% or less of its current age,” explained Australian astrophysicist Matt O'Dowd, writer and host of PBS Space Time channel on Youtube.
For a more definitive answer, a higher-quality spectroscopy of the objects will be required.
How would a dark matter star work?
Although dark matter makes up about 25% of the universe, its nature is yet to be discovered.
Dark matter does not emit, absorb, or reflect electromagnetic radiation, such as light, and it doesn’t interact with ordinary matter except gravitationally.
Scientists believe it consists of a new type of elementary particle, and the hunt to detect such particles is on. One of the leading candidates is Weakly Interacting Massive Particles or WIMPs. WIMPs are known to annihilate themselves if collided.
To achieve that interaction, however, massive amounts of gravity would be required.
It’s thought that dark stars could’ve formed in the early stages of the universe when large hydrogen clouds collapsed in the process of star formation. That formed a gravitational well that pulled the dark matter with it. Dark matter particles are their own antiparticles, their annihilation provides a heat source that stops the collapse of the hydrogen clouds and produces a different type of star, a dark star, in thermal and hydrostatic equilibrium.
Dark stars, theoretically, would be made almost entirely of hydrogen and helium, less than 0.1% of the mass comes in the form of dark matter. They remain cool (surface temperatures ∼ 10,000 K), without a central hot core, “puffy” with the size of 10 astronomical units (AU, 1 AU is the distance between Earth and the Sun, 149.6 million km). In dark stars, no nuclear fusion would happen, instead, dark matter annihilations happen throughout their volume.
Supermassive objects would outshine our Milky Way galaxy, but once the dark matter fuel runs out, the dark star dies and collapses into a black hole.
“Dark stars may provide seeds for the supermassive black holes observed throughout the universe and at early times,” scientists write.
The identification of supermassive dark stars would open up the possibility of learning about dark matter based on their observed properties.
Follow-up observations from JWST of the objects’ spectroscopic properties — including dips or excess of light intensity in certain frequency bands — could help confirm whether these candidate objects are indeed dark stars, researchers explain.
“It’s more likely that something within the standard model needs tuning, because proposing something entirely new, as we did, is always less probable,” Freese said. “But if some of these objects that look like early galaxies are actually dark stars, the simulations of galaxy formation agree better with observations.”
Dark stars could theoretically grow to be several million times the mass of our sun and up to 10 billion times as bright as the sun.
“We know there are giant black holes in the centers of most galaxies, and those black holes seem to have grown very quickly in the early Universe. Perhaps dark stars give us a way to produce the seed black holes with a million suns worth of mass that could then grow into the billion solar mass monsters,” O’Dowd wondered.
JWST provides sufficient technology to provide explanations of what the mysterious objects are. However, researchers will need more observation time.
While dark stars are still speculative, according to O’Dowd, we’re guaranteed to learn more about the mysterious epoch when the very first stars, dark or light, lit up the dawn of space-time.
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