KShortly after the Big Bang, it got dark again. When the hot matter had increasingly cooled down in the course of expansion, it still took around a hundred million years before the first stars and galaxies formed and their light illuminated the universe again. With today’s technology, it is not possible to image individual stars from the early days of the universe, but some brightly radiating galaxy cores have been observed. These are called quasars. Their luminosity comes from hot masses of gas pouring into a supermassive black hole of millions to billions of solar masses. The gas in a so-called accretion disk heats up extremely, then radiates and sends a bright greeting across the universe shortly before it finally disappears.
The following applies: The more massive such a galactic gorge is, the hotter and brighter the inflowing matter becomes and the better such distant objects can be identified at all. However, it is considered an open question of cosmology where such heavyweights in the young universe come from. Because black holes are commonly formed by the collapse of very heavy stars when they have used up their fuel and then collapse under their gravitational pressure. But the heaviest known stars are only around 200 solar masses. Such stars shine so intensely that they blow away all the gas in their surroundings.
If such a star becomes a black hole with about 100 solar masses, then this also has to grow for a long time and suck in gigantic streams of material before it reaches the weight class of giant holes weighing billions of suns. However, the inflowing matter also radiates very brightly and thus limits the further inflow of matter. So far, cosmological simulations have not been able to explain where the 200 quasars known to have existed less than a billion years after the Big Bang came from.
Turbulent gas flows in the heart of young galaxies
Some time ago, the idea arose that the riddle of the heavyweight quasars could be solved if the black holes were significantly heavier at birth than the typical ten to 100 solar masses. This would be possible, for example, if the progenitor stars had been extremely large and massive, with over a thousand solar masses. Such – hitherto hypothetical – giant stars could only have formed under the conditions of the early universe. It has not yet been possible to simulate this satisfactorily.
However, according to a study published in the journal “Nature”, the solution to the riddle could lie in turbulent gas flows in the heart of young galaxies. An international team led by astrophysicist Dan Whalen from the University of Portsmouth has used detailed supercomputer simulations to recreate the conditions in the central regions of young galaxies in the first few hundred million years after the Big Bang. The enormously strong turbulence in the violent currents of cold gases effectively suppressed the formation of new stars. This allowed extraordinarily massive gas clouds to accumulate, which eventually collapsed into gigantic stars. “In our simulations, we see giant stars from 31,000 to 40,000 solar masses,” says Whalen. That is more than 100 times the mass of today’s heaviest stars.
According to the simulation, these massive stars only achieved a very short lifetime and then imploded into correspondingly large black holes. In addition, these black holes are located in the central area of ​​inflowing gas masses in the center of a galaxy. This could explain the early formation of the observed quasars in the young universe. Unfortunately, the James Webb Space Telescope will be able to observe many of these quasars, but hardly any of their progenitor stars – although these should shine tens of millions of times brighter than our sun. However, if pairs of such giant stars had formed in proximity, a merger of their black holes with future gravitational wave detectors should be possible in a good ten years. That would be a clear indication of such – by far the heaviest – stars that would belong to the class of red or blue hypergiants.