Cosmic microwaves show reionization happened later than thought

Later reionization fingers first stars as the culprit
By | Published: September 15, 2016
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New analysis of data taken by the European Space Agency’s (ESA) Planck spacecraft of the cosmic microwave background (CMB) has revealed that the “epoch of reionization” occurred much later than previously thought.

The universe was initially ionized just after the Big Bang, explains Jan Tauber, Planck project scientist at ESA and one of the authors of the paper on reionization, which was accepted on July 23 for publication in Astronomy & Astrophysics.

The latest data show that reionization occurred about 700 million years after the Big Bang; the previous estimate was 450 million years.

After 380,000 years, the universe had expanded and cooled enough for electrons and protons to stick together in electrically neutral hydrogen atoms. At that point, photons were free to travel, because they were no longer scattering off of ions. Those freed photons are what we now see as the CMB.

From detailed observations of the CMB, astronomers have known that at some point, the universe became reionized, but when it happened has been a subject of hot debate. The reionized universe is not opaque enough to block the CMB photons from traveling, but it is enough to polarize them to some degree. Istvan Szapudi of the Institute for Astronomy at the University of Hawaii at Manoa likens looking through the reionized universe to looking toward a mountain on a misty morning. Szapudi studies the CMB, but was not involved in Tauber’s work.

The CMB bears the polarization imprint of the ionization history of the universe from the epoch of reionization to today.

“The CMB photons scatter off free electrons, and therefore when the CMB travels through an ionized medium, it is slightly attenuated,” says Tauber. “By measuring this attenuation (which we call the ‘opacity’) we can estimate the ‘distance’ traveled through the medium, which in turn tells us when the medium was ionized. We can determine the opacity with a precision of 10-20 percent, depending on what assumptions you make.”

“The highly sensitive measurements from [Planck’s High-Frequency Instrument (HFI)] have clearly demonstrated that reionization was a very quick process, starting fairly late in cosmic history and having half-reionized the universe by the time it was about 700 million years old,” says Jean-Loup Puget from Institut d’Astrophysique Spatiale in Orsay, France, principal investigator of the HFI and a fellow author of the reionization paper with Tauber.

The previous estimate of 450 million years was based on data gathered by the Wilkinson Microwave Anisotropy Probe (WMAP), which was more accurate than the Cosmic Background Explorer before it. However, Planck has better resolution than WMAP and carried a Low-Frequency Instrument sensitive to three frequency bands in the range 30-70 gigahertz and the HFI, which is sensitive to six frequency bands in the range 100-857 GHz.

What does all this mean? Tauber and his colleagues in ESA’s Planck Collaboration believe pushing back the epoch of reionization shows that the first generation of stars in the universe were the only sources needed to account for reionization. Four hundred and fifty million years post Big Bang, the universe didn’t have enough stars emitting ultraviolet (UV) light to reionize the cosmos, so astronomers were forced to postulate the existence of other, exotic ionizing forces.

The new estimate places reionization at a time when there were enough stars to accomplish the task.

The first stars in the universe would have been excellent ionizers. They were most likely very large — from 300 solar masses up to as much as 1,000 solar masses — and very hot, with surface temperatures as high as 100,000 kelvin. Their light would have been mainly UV and would have been very effective at ionizing the neutral hydrogen and helium gas around them.

Szapudi said he had not yet studied Tauber et al.’s latest paper on reionization, but expressed confidence that the paper had been “vetted by a large collaboration of hundreds of scientists who are expert on the CMB and on Planck itself, so my first instinct would be that this is correct.”

The influence of Planck’s data is still being felt, even though the spacecraft was turned off in October 2013. It operated twice as long as it was designed to and completed eight full-sky surveys instead of the planned two before it ran out of coolant.


Allen Zeyher is a freelance writer in the Chicago area.