The First Stars In The Universe Formed Later Than We Thought

A visual representation of the polarization of the CMB that was used to estimate when the epoch of reionization started. ESA/Planck Collaboration

When we look at the universe we see it full of glittering stars, but it has not always been this way. Stars took a long time to form after the Big Bang, and according to the latest research they took even longer than we previously thought.

Astronomers using ESA’s Planck satellite used the light from the Big Bang itself, known as the Cosmic Microwave Background (CMB), to estimate that only a tiny fraction of stars was present between 300 and 500 million years after the Big Bang.

The Planck collaboration is probing the mysterious epoch of reionization, a phase of the universe when the hydrogen formed after the Big Bang was once again split into protons and electrons by the powerful light of the first stars.

According to the paper, published in Astronomy & Astrophysics, only 10 percent of the universe was ionized 500 million years after its birth. Combining this with other observations, it tells us that process was very quick, with half the universe being reionized by 700 million years and completely reionized by 900 million years. The scientists also confirmed the first stars were plentiful enough to do this on their own.

"We have also confirmed that no other agents are needed, besides the first stars, to reionize the universe," said Matthieu Tristram, a Planck Collaboration scientist at the Linear Accelerator Laboratory (LAL) in Orsay, France, in a statement.

To estimate the amount of non-ionized gas in the very distant universe, the team exploited a property of the CMB. Its light is polarized, meaning that it vibrates in a specific direction. This polarization is given by the photons bouncing off free-floating electrons, something that happened often before the CMB was released and after the end of reionization.

"It is in the tiny fluctuations of the CMB polarization that we can see the influence of the reionization process and deduce when it began," explains Jan Tauber, Planck project scientist at ESA.

"The CMB can tell us when the epoch of reionization started and, in turn, when the first stars formed in the universe."

The first stars are believed to be very massive, living short bright lives before exploding in spectacular supernovae. Although we have found hints of these first stars, we have yet to directly observe them. The James Webb Space Telescope, which will launch in 2018, will hopefully glimpse some of these mysterious objects.

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