Dark matter not only had a role to play in fueling early stars, it may have created "dark stars" so massive that they went on to spawn supermassive black holes found at the cores of the 500 billion galaxies estimated to populate the universe.
This is an intruiging theory developed by Katherine Freese, Associate Director of the University of Michigan Center for Theoretical Physics, where she has been working to identify the dark matter and dark energy that permeate the universe as well as to build a successful model for the early universe immediately after the Big Bang. Freese has shown that most of the mass in galaxies does not consist of ordinary stellar material, and has proposed ways to look for alternatives such as supersymmetric particles. There is a great deal of excitement about possible detections of these particles.
The first phase of stellar evolution in the universe may have been powered by dark matter heating rather than the current popular theory of nuclear fusion. The power source was the annihilation of WIMPS (Weakly Interacting Massive Particles), which are their own antiparticles and may be discovered by ongoing searches at CERN's Large Hadron Collider.
As the universe evolved, the dark matter fuel is exhausted and the dark star becomes a heavy main sequence star, which eventually collapses to form massive black holes that may have provided the seeds for the supermassive black holes now found at the enters of all galaxies, devouring any stars that stray too close.
Some theories suggest they appeared straight from the primordial soup immediately after the Big Bang (13.75 billion years ago); other theories suggest they formed over long periods of time, sucking in (or "accreting") mass by swallowing stars and gas. But there isn't a definitive answer, and this is where dark stars come in.
"Dark Matter" is the invisible, undetectable and utterly transparent mystery matter which apparently has to exist all over the universe for current cosmological theories to not be totally off the mark. Inventing a magic omnipresence to explain the way things are might sound suspiciously religious, but with scientists like Stephen Hawking pursuing proof it might not be nothing.
The "dark star" hypothesis proposes that the very first stars, formed when the universe was far smaller than it is now, had a greater dark matter density to play with. Of all the incredibly odd properties dark matter has, one of the most interesting is how it acts as its own antiparticle - if two dark particles hit they'll explode into pure energy. Why doesn't this cause the entire universe to explode? Because dark particles are thought to be WIMPs, Weakly Interacting Massive Particles - they find it very hard to even interact with each other, never mind anything else.
Dark stars forming in a very dense dark matter region would thus be powered by antimatter annihilation, which converts 100% of the available mass into energy. Compare this to the wimpy hydrogen fusion which powers the Sun and all life on Earth: a mere 0.7% of the available mass energy, and that's the glorious ideal which we're working towards and dreaming of harnessing. These dark stars could thus reach sizes millions of times larger than our Sun, and despite their name they'd emit visible light.
You have to admit it's a fantastic idea: for something almost terminally lacking direct evidence, what more could a dark matterologist dream of than an entire star made of the stuff, glowing brightly (and observably!) and orders of magnitude bigger than even our own Sun!
The scientists say such stars could still exist in the far reaches of the universe (albeit with their light doppler-shifted into infrared), or evidence of their passing could be marked by the distribution of supernova-formed elements throughout the universe.
As pointed out by Freese's team, dark stars would have had a surface temperature of less than 10,000 Kelvin. Also, they would have started out as very large, puffy stars, extending over 2000 times the size of our sun. They were composed of mainly normal matter, but 0.1% of the dark star's mass was dark matter fuel.
If there was plenty of dark matter surrounding dark stars, they could have enough fuel to be sustained for millions (or possibly billions) of years.
The most interesting thing about these dark stars is that there is no limit on how massive they could become. So long as there was a large "halo" of dark matter to fuel it, normal matter would be pulled into the star, making it grow. The dark stars just kept on growing, devouring dark matter and fattening up on normal matter.
According to this research, Freese's team modeled the growth of these dark stars until they created supermassive dark stars (dark stars over 100,000 times more massive than the sun), leading to a fascinating conclusion: Once the supermassive dark stars run out of dark matter fuel, they contract and heat up. The core reaches 108K and fusion begins. As fusion-powered stars they don't last very long before collapsing to black holes. The black holes formed after this collapse became the modern-day supermassive black holes that we find at the core of all (or most) galaxies.
NASA's James Webb Space Telescope (JWST) might be able to spot the biggest dark stars on the edge of the observable universe after it's launched, providing Freese and her team with observational evidence that dark stars even existed, let alone help to spawn supermassive black holes.
The Daily galaxy via arxiv.org
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