A new assessment of the Cosmic Microwave Background shows that the oldest stars ignited 150 million years later than previously thought. It's a realization that's forcing cosmologists to rethink the 13.8 billion year history of the Universe.

Top image: Polarization of the Cosmic Microwave Background. (ESA)

To study the early stages of the Universe, cosmologists scan the Cosmic Microwave Background (CMB), an "afterglow" of a time — some 380,000 years after the Big Bang — when the Universe was still intensely hot and dense. Because the Universe is expanding, this light blankets our entire field of vision at microwave wavelengths. Minute variances in the CMB's temperature identify regions of slightly different density in the early Universe.


In addition to this, Planck scientists have been analyzing readings of the CMB's polarization, which is another way to study the light. From 2009 to 2013, Planck measured this signal at high resolution over the entire sky.

Above, you can see the polarization of the CMB on a small patch of sky. In this image, the color scale represents temperature differences, while the texture indicates the direction of the polarized light. These differences correspond to regions of slightly different densities, which can predict future structure, such as stars and galaxies. (ESA)

Results now show that a period known as the "cosmic dark ages" lasted longer than assumed. From New Scientist:

After the CMB was released, the universe was dominated by a fog of opaque hydrogen gas. It stayed dark for hundreds of millions of years until gravity clumped matter together into the first stars and galaxies, which produced enough radiation to ionise the hydrogen and make it transparent.

Astronomers are still short on details about how this lighter period, known as reionisation, began and ended. The Hubble Space Telescope has spotted very old galaxies from the middle and end of reionisation, thought to be about 1 billion years after the big bang, and Planck's predecessor, WMAP, pegged the start of the era to about 420 million years after the big bang.

But simulations suggested that wouldn't give gravity enough time to work its magic and produce stars. "The measurements we had indicated that formation of stars was much earlier than our understanding would allow," says [project scientist Jan] Tauber.

Consequently, cosmologists have had to push back their estimates as to when the first stars were able to form.


"Planck's observations of the CMB polarisation 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," noted team scientist Marco Bersanelli in an ESA statement. "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 results are changing our understanding of how certain key events transpired during the earliest stages of the Universe's history. Encouragingly, the later end of the Dark Age means it might be easier to detect the very first generation of galaxies using the next generation of observatories, including the James Webb Space Telescope.


The new Planck result can be found in a series of new papers posted to the ESA website.

[ ESA | New Scientist ]