The Universe was pitch black for 550m years

After the Big Bang there was no light in the Universe for 550 million years, which was 100 million years later than previously thought, the European Space Agency (ESA) has revealed.

New maps made from ESA’s Planck satellite data show the “polarized” light from the Universe’s early days (astronomically speaking) across the entire sky.

Scientists try to trace the Universe’s 13.8 billion-year history by studying the comets, asteroids, planets and other objects in our Solar System, and gathering light emanated from distant stars, galaxies and the matter spread between them.

A major source of data astronomers used to better understand this history of the Universe is the Cosmic Microwave Background (CMB), the thermal radiation left over from the Big Bang, or as ESA puts it “the fossil light resulting from a time when the Universe was hot and dense, only 380 000 years after the Big Bang.”

Polarization of the CMB

A visualization of the polarization of the Cosmic Microwave Background as detected by ESA’s Planck satellite over the entire sky. (Image: ESA and the Planck Collaboration)

Thanks to the Universe’s expansion, we are able to see this light today covering the whole sky at microwave wavelengths.

Planck surveyed the sky from 2009 to 2013 to study this ancient light in unprecedented detail. Minute differences in the temperature of the background trace regions of marginally different density in the early Universe, representing the seeds of all future stars and galaxies.


Planck scientists have published the results from their analysis of these data in several scientific papers over the past two years, confirming the standard cosmological picture of the Universe in ever greater accuracy.

ESA’s Planck project scientists Jan Tauber said:

“But there is more: the CMB carries additional clues about our cosmic history that are encoded in its ‘polarization’. Planck has measured this signal for the first time at high resolution over the entire sky, producing the unique maps released today.”

Light is polarized when it vibrates in a certain direction, something that may occur when particles of light (photons) bounce off other particles. This is precisely what occurred when the CMB began in the early Universe.

It all started off as a hot, dense soup

At first, photons were trapped in a dense, hot soup of particles that, within a few seconds after the Big Bang, consisted mainly of neutrinos, protons and electrons.

Due to the very high density, photons and electrons collided with one another so frequently that light was not able to travel very far before bumping into another electron, making the young Universe very “foggy”.

Gradually, as the cosmos expanded and cooled, the particles (including photons) grew father apart, and collisions occurred less frequently.

This had two outcomes: 1. protons and electrons were finally able to combine and form neutral atoms without incoming photons tearing them apart again, and 2. photons had enough space to travel, being no longer trapped in the cosmic fog.

Once it was free of the cosmic fog, the light was set on its cosmic journey that would take it all the way to today, where Planck and other telescopes detect it as the CMB. However, the light also retains a memory of its last collision with the electrons, captured in its polarization.

François Bouchet, of the Institut d’Astrophysique de Paris in France, said:

“The polarisation of the CMB also shows minuscule fluctuations from one place to another across the sky: like the temperature fluctuations, these reflect the state of the cosmos at the time when light and matter parted company.”

“This provides a powerful tool to estimate in a new and independent way parameters such as the age of the Universe, its rate of expansion and its essential composition of normal matter, dark matter and dark energy.”

When did the birth of stars begin?

Planck’s polarization data also provided another important piece of information that scientists were after – when did the first stars appear?

Marco Bersanelli, of Università degli Studi di Milano in Italy, said:

“After the CMB was released, the Universe was still very different from the one we live in today, and it took a long time until the first stars were able to form.”

“Planck’s observations of the CMB polarization now tell us that these ‘Dark Ages’ ended some 550 million years after the Big Bang – more than 100 million years later than previously thought.”

“While these 100 million years may seem negligible compared to the Universe’s age of almost 14 billion years, they make a significant difference when it comes to the formation of the first stars.”

The epoch of reionization

As the first stars began to emit light the Dark Ages ended, and as their light interacted with gas in the Universe, more and more atoms were turned back into electrons and protons, their constituent particles.

This key phase in the Universe’s history is known as the “epoch of reionization”.

The newly-freed electrons could again collide with light from the CMB, but much less frequently now because the Universe had expanded. These encounters between photons and electrons, which had also occurred 380,000 years after the Big Bang, left a tell-tale imprint on the polarization of the CMB.

George Efstathiou, of the University of Cambridge in England, said:

“From our measurements of the most distant galaxies and quasars, we know that the process of reionization was complete by the time that the Universe was about 900 million years old.”

“But, at the moment, it is only with the CMB data that we can learn when this process began.”

Previous studies of the CMB polarization pointed to the birth of stars occurring 450 million years after the Big Bang. Planck’s new results show that this occurred 100 million years later.

The Planck’s team’s findings posed a problem. Very deep images from the NASA–ESA Hubble Space Telescope indicated that the earliest-known galaxies in the Universe started perhaps 300 to 400 million years after the Big Bang.

But these would not have been powerful enough to succeed in ending the Dark Ages within 450 million years.

Professor Efstathiou said:

“In that case, we would have needed additional, more exotic sources of energy to explain the history of reionization.”

Planck’s new evidence significantly reduces the problem. It indicates that reionization began later than previously believed, and that the earliest galaxies and stars alone may not have been enough to drive it.

The European Space Agency wrote:

“This later end of the Dark Ages also implies that it might be easier to detect the very first generation of galaxies with the next generation of observatories, including the James Webb Space Telescope.”

“But the first stars are definitely not the limit. With the new Planck data released today, scientists are also studying the polarization of foreground emission from gas and dust in the Milky Way to analyze the structure of the Galactic magnetic field.”

The data have given scientists important insights into the early Universe and its components, including dark matter and neutrinos.

Jan Tauber said:

“These are only a few highlights from the scrutiny of Planck’s observations of the CMB polarisation, which is revealing the sky and the Universe in a brand new way. This is an incredibly rich data set and the harvest of discoveries has just begun.”

Several scientific papers describing the new results were published on February 5th, 2015.