Variations in the ‘fogginess’ of the universe identify a milestone in cosmic history



Large differences in the ‘fogginess’ of the early universe were caused by islands of cold gas left behind when the universe heated up after the big bang, according to an international team of astronomers.

These neutral islands of cold gas explain the puzzling observations.

- Martin Haehnelt

The results, reported in the Monthly Notices of the Royal Astronomical Society, have enabled astronomers to zero in on the time when reionisation ended and the universe emerged from a cold and dark state to become what it is today: full of hot and ionised hydrogen gas permeating the space between luminous galaxies.

Hydrogen gas dims light from distant galaxies much like streetlights are dimmed by fog on a winter morning. By observing this dimming in the spectra of a special type of bright galaxies, called quasars, astronomers can study conditions in the early universe.

In the last few years, observations of this specific dimming pattern (called the Lyman-alpha Forest) suggested that the fogginess of the universe varies significantly from one part of the universe to another, but the reason behind these variations was unknown.

“We expected the light from quasars to vary from place to place at most by a factor of two at this time, but it is seen to vary by a factor of about 500,” said lead author Girish Kulkarni, who completed the research while a postdoctoral researcher at the University of Cambridge. “Some hypotheses were put forward for why this is so, but none were satisfactory.”

The new study concludes that these variations result from large regions full of cold hydrogen gas present in the universe when it was just one billion years old, a result which enables researchers to pinpoint when reionisation ended.

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Image: Artist's impression of reionisation period

Credit: Amanda Smith, Institute of Astronomy


Reproduced courtesy of the University of Cambridge

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