Making our Sun’s darker cousin
Formed not long after the Big Bang, these brilliant members of the early universe aren’t your conventional stars
The most massive of the dark stars would have bypassed the fusion stage altogether, collapsing straight into a black hole. These black holes were so massive that they offer a solution to a problem that had previously puzzled scientists. Supermassive black holes, which can be billions of solar masses, exist at the centre of every galaxy, and are known to have existed only a billion years after the Big Bang. However, an ordinary star collapsing into a black hole would need more than a few hundred million years to gobble up enough material to become a supermassive black hole. “Ordinary stars cannot do it, because ordinary stars are too small,” explains Katherine Freese from the University of Texas at Austin. “Dark stars, on the other hand, can grow to become a million times as massive as the Sun, and then when they run out of fuel they collapse into million-solar-mass black holes, the perfect seeds for monstrous supermassive black holes.”
The supermassive WIMP-powered dark stars could only have formed in the minihalos of the
early universe, when the density of dark matter was much higher than it is today. Over time, as the universe expanded, everything became more spread out, so there are no longer minihalos capable of birthing supermassive dark stars.
This confines them to the early universe, which also means that they are at a great distance from us here on Earth. Astronomers use the term ‘redshift’ to denote distance in cosmology, as the light from a distant object will get shifted towards the red end of the spectrum, permitted that it’s moving away from us. Dark stars only exist at high redshifts, making them an observing challenge. The infrared Ultra Deep Field images taken by Hubble were used to look for dark stars, but none were found. This doesn’t necessarily mean they don’t exist, as there could be less luminous dark stars lurking beyond Hubble’s vision. The upcoming James Webb Space Telescope (JWST) – due to be launched in October
“When dark stars run out of fuel they collapse into million-solar-mass black holes”
Katherine Freese
2021 – will outdo its predecessor by looking further back in time.
“If dark stars do exist, and are sufficiently massive, numerous and long-lived, then the JWST certainly has a decent chance of confirming their existence at high redshifts,” says Zackrisson. “However, since the distribution of dark star properties hinges on both the properties of the dark matter particles and the cosmological evolution of the dark matter halos that host them, success is by no means guaranteed.”
Even if the JWST can’t detect individual dark stars, it might still be able to detect their overall glow. Just as individual street lights all add up to produce an infuriating yellow glow over cities, light from stars and galaxies accumulates into what is known as the extragalactic background light (EBL). The EBL has already been mapped to a certain extent, but the improved measurements from the JWST will help to sniff out the contributions from dark stars, which hasn’t been manageable before.
While WIMP annihilation can theoretically provide enough fuel to keep a dark star going for billions of years, it is unlikely that any of the dark stars from the early universe are still around today. However, it is possible that a new generation of dark stars could exist where dark matter concentrations are still somewhat high, such as in the centre of galaxies. As there is less dark matter in galactic centres compared to the minihalos of the ancient universe, the new generation of dark stars would be much less massive – only equivalent to that of our Sun – and will never be able to rival the glory days of the first stars.