Haiman in Nature, “Cosmology: A smoother end to the dark ages”

“Independent lines of evidence suggest that the first stars, which ended the cosmic dark ages (lasting for 100 million years), came in pairs rather than singly. The appearance of the first stars marked a significant milestone, separating the history of the Universe into two stages. The first stage is well understood: dark matter, primordial ionized plasma and radiation formed a nearly uniform mixture, expanding and cooling continuously with cosmic time. In the second stage, stars lit up and started wreaking havoc. The radiation of the stars penetrated the neutral cosmic plasma, once again ionizing and heating it, and modifying the formation of the subsequent generations of stars. At the time of the transition between the two stages, the 100-million-year-old Universe may have resembled Swiss cheese: cold and neutral background gas was filled with numerous, roughly spherical, hot, ionized holes surrounding the sites where the earliest stars had lit up. There is, however, another possibility, in which energetic X-ray radiation — not normally associated with stars — was present during the transition. The evidence that the first stars may have formed in pairs makes the latter hypothesis more likely. This could change the prevailing view that the early Universe had a Swiss-cheese-like appearance.” Extracts from Zoltán Haiman, “Cosmology: A smoother end to the dark ages,” Nature 472: 47–48, 07 April 2011, based on recent evidence by I. F. Mirabel, M. Dijkstra, P. Laurent, A. Loeb and J. R. Pritchard, “Stellar black holes at the dawn of the universe,” Astronomy & Astrophysics 528: A149, published online 18 March 2011 [ArXiv preprint].

“Why would the companionship of the first stars matter for the rest of the Universe? As Mirabel and colleagues argue, a natural outcome of the latter hypothesis is for one member of a pair of massive stars to implode, leaving behind a black hole that remains gravitationally bound to its massive partner. The black hole could then pull material off the surface of its partner, and swallow it up. While devouring its partner, the black hole would return a fraction of the ingested energy in the form of copious amounts of X-rays. In fact, these ‘micro-quasars’ seem to be more common in smaller galaxies, as well as in galaxies whose chemical composition is closer to that of the pristine plasma of hydrogen and helium in the early Universe. They could have produced sufficient X-rays to significantly change the prevailing Swiss-cheese scenario.”

“If micro-quasars were indeed as common, and as efficient producers of X-ray radiation, as Mirabel and colleagues argue, they may well have dominated X-ray production in the transition epoch, when the first stars started to shine in the Universe. They would then have been responsible for ending the dark ages in a smooth fashion. The hardest X-ray photons (those with energies above a few kiloelectronvolts) would be reaching Earth now, forming a feeble X-ray background. Existing measurements place an upper limit on the present-day value of this background that is consistent with this hypothesis. The possibility of X-ray production by binary stars should prompt further theoretical modelling of the population of such binaries, including their abundance, radiation output and spectra, as well as modelling of the possible observable signatures they left behind.”

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