Early universe much brighter than predicted

Observations from NASA’s Spitzer Space Telescope have revealed that some galaxies within the early universe were much brighter than cosmologists had predicted.

The finding, published in the journal Monthly Notices of the Royal Astronomical Society and available in full on the preprint server arXiv, offers clues to one of the most poorly understood stages in the early development of the universe, known as the Epoch of Reionisation.

The period began about 300,000 years after the Big Bang and was complete 700,000 years later.

Prior to that, the universe comprised mostly neutral hydrogen gas, which soon began to coalesce to form the first stars. 

These would have been very different to the vast bulk of stars that began to form in earnest by the time the universe was one billion years old. In particular, they would have consisted of hydrogen and helium atoms only. 

A second generation of young stars incorporated small amounts of “heavy” elements, such as carbon, oxygen and nitrogen, but were still very element-poor compared to later ones.

Very early stars nevertheless emitted radiation across a spectrum. Radiation with long wavelengths, such as radio waves and visible light, was able to traverse the vast interstellar medium full of neutral hydrogen atoms very easily.

Shorter wavelength radiation, however, such as ultraviolet light, X-rays and gamma rays, had a tougher time of it, and smashed into the hydrogen atoms. They did this with such force that they stripped the atoms of their electrons – thus ionising them.

This, in some manner yet to be fully understood, catalysed the development of the universe into the bright, star-studded entity it is today. 

One of the central parts of the mystery, for cosmologists, is finding a source big enough to produce enough short wavelength radiation to ionise, in a relatively short period of time, the entire thing. 

Modern stars don’t release vast amounts of ionising radiation, so it’s likely that their distant antecedents didn’t, either. Quasars have been suggested, but, ultimately, no one really knows.

“It’s one of the biggest open questions in observational cosmology,” says Stephane De Barros, from the University of Geneva in Switzerland, and lead author of the latest study.  

“We know it happened, but what caused it? These new findings could be a big clue.”

The findings in question arise from Spitzer’s observations of 135 distant galaxies in two areas of the sky. The telescope imaged both regions for more than 200 hours, capturing light that had been emitted 13 billion years ago.

The results were then combined with archival data gathered by the Hubble Space Telescope.

To the surprise of De Barros and his colleagues, the images revealed that the young stars were significantly brighter than anticipated. The brightness was not restricted to one or two galaxies – which might have then been classified as anomalies – but was present in all of them.

Analysis revealed that the ancient galaxies were full of young, massive stars – second generation bodies comprising primarily hydrogen and helium, but with small amounts of heavier elements.

The brightness was confined to two specific wavelengths of infrared light – produced by ionising radiation interacting with hydrogen and oxygen gases within the galaxies.

The full significance of the findings is yet to be determined, and may not be known for some time. Nevertheless, they take cosmologists one step closer to determining the mechanics of a critical phase in the growth of the universe.

“These results by Spitzer are certainly another step in solving the mystery of cosmic reionization,” says co-author Pascal Oesch. 

“We now know that the physical conditions in these early galaxies were very different than in typical galaxies today. It will be the job of the James Webb Space Telescope to work out the detailed reasons why.”