Approximately 200 million years after the Big Bang, the universe was a very different place.
For starters, there was no starlight as there were no stars. This period was known descriptively as the “Dark Ages.” As there were no stars, only clouds of the most basic elements persisted, fogging up the cosmos.
Although it’s believed the first stars (known as “Population III stars”) were sparked when hydrogen and helium gases cooled enough to clump together, collapsing under gravity and initiating nuclear fusion in the star cores (thus generating heavier elements), there’s another possibility.
Around the time of early star formation it is thought there was an abundance of dark matter. Although it’s not entirely clear what dark matter actually is, we know from various observations that it’s out there in vast quantities. Dark matter makes up the majority of the mass of our universe and during these early days, dark matter may have fueled the earliest stars.
This may sound a little strange, but it gets even stranger than that.
According to new research headed by Katherine Freese of the University of Michigan, Ann Arbor, 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.
Supermassive black holes are the behemoths of the universe. With a mass of a million suns, these monsters can be found living in the centers of galaxies, devouring any stars that stray too close.
Although we’ve come a long way in detecting and understanding the physics behind supermassive black holes, it’s far from clear as to how they evolved.
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.
As dark matter is its own anti-particle, when dark matter particles collide, they annihilate and release energy. (Normal matter needs “anti-matter” to annihilate, a commodity that is, fortunately, very rare in nature.) Inside a hypothetical dark star, the dark matter particles were forced together, colliding and annihilating. This produced an outward force, maintaining the dark star against gravitational collapse and making it shine.
The “dark star” name actually comes from the theory that these stellar bodies were fueled by dark matter, not because they were actually “dark.” In fact, they would have been the very bright first stars in our universe (before Population III stars), easily outshining our sun.
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. Whereas the early Population III stars had a limit on how massive they could become (as they grew too big, they’d get too hot, preventing any more matter from accreting), 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:
The black holes formed after this gargantuan collapse became the modern-day supermassive black holes that we find hidden in the center of galaxies.
But wait. Although this theory sounds very exciting, there is no observational evidence that dark stars even existed, let alone helped to spawn supermassive black holes.
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 in 2014.
Space telescopes like the JWST and its predecessor, Hubble, have the ability to look for the most distant objects. Hubble’s distance record is 13.1 billion light years after it spotted a gamma ray burst in the act in 2009. As the light from that gamma ray burst took 13.1 billion years to hit Hubble’s lens, the light was actually the most ancient light we have ever seen, produced by an exploding star only a few hundred million years after the Big Bang.
JWST will push our observing capability even further, possibly spotting the light created by these old mysterious dark stars. Although it won’t be an easy task (very long exposure times would be needed to collect the faint light from these distant bodies), Freese’s team will know what to look for, potentially discovering the link between these supermassive dark stars and the supermassive black holes we find in the cosmos today.
Publication: Supermassive Dark Stars: Detectable in JWST, Freese et al., 2010, arXiv:1002.2233v1 [astro-ph.CO]