New Details Of The Big Bang: First Stars Formed Much Later Than Thought

Thanks to the expansion of the Universe, we see this light today covering the whole sky at microwave wavelengths.

Between 2009 and 2013, Planck surveyed the sky to study this ancient light in unprecedented detail. Tiny differences in the background’s temperature trace regions of slightly different density in the early cosmos, representing the seeds of all future structure, the stars and galaxies of today.

Scientists from the Planck collaboration have published the results from the analysis of these data in a large number of scientific papers over the past two years, confirming the standard cosmological picture of our Universe with ever greater accuracy.

“But there is more: the CMB carries additional clues about our cosmic history that are encoded in its ‘polarization’,” explains Jan Tauber, ESA’s Planck project scientist.

“Planck has measured this signal for the first time at high resolution over the entire sky, producing the unique maps released today.” 

“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,” says François Bouchet of the Institut d’Astrophysique de Paris, France.

“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.”

Planck’s polarization data confirm the details of the standard cosmological picture determined from its measurement of the CMB temperature fluctuations, but add an important new answer to a fundamental question: when were the first stars born?

CMB polarisation: zoom

“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,” explains Marco Bersanelli of Università degli Studi di Milano, Italy.

“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.

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

This key phase in the history of the cosmos is known as the ‘epoch of reionisation’.

Credit:  ESA and the Planck Collaboration

The newly liberated electrons were once again able to collide with the light from the CMB, albeit much less frequently now that the Universe had significantly expanded. Nevertheless, just as they had 380 000 years after the Big Bang, these encounters between electrons and photons left a tell-tale imprint on the polarisation of the CMB.

“From our measurements of the most distant galaxies and quasars, we know that the process of reionisation was complete by the time that the Universe was about 900 million years old,” says George Efstathiou of the University of Cambridge, UK.

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

Planck’s new results are critical, because previous studies of the CMB polarization seemed to point towards an earlier dawn of the first stars, placing the beginning of reionization about 450 million years after the Big Bang.

This posed a problem. Very deep images of the sky from the NASA–ESA Hubble Space Telescope have provided a census of the earliest known galaxies in the Universe, which started forming perhaps 300–400 million years after the Big Bang.

However, these would not have been powerful enough to succeed at ending the Dark Ages within 450 million years.

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

The new evidence from Planck significantly reduces the problem, indicating that reionization started later than previously believed, and that the earliest stars and galaxies alone might have been enough to drive it.

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.

Planck scanned the sky to detect the most ancient light in the history of the Universe – the cosmic microwave background. It also detected significant foreground emission from diffuse material in our Galaxy which, although a nuisance for cosmological studies, is extremely important for studying the birth of stars and other phenomena in the Milky Way.

Among the foreground sources at the wavelengths probed by Planck is cosmic dust, a minor but crucial component of the interstellar medium that pervades the Galaxy. Mainly gas, it is the raw material for stars to form.

Interstellar clouds of gas and dust are also threaded by the Galaxy’s magnetic field, and dust grains tend to align their longest axis at right angles to the direction of the field. As a result, the light emitted by dust grains is partly ‘polarized’ – it vibrates in a preferred direction – and, as such, could be caught by the popularization-sensitive detectors on Planck.

Scientists in the Planck collaboration are using the polarized emission of interstellar dust to reconstruct the Galaxy’s magnetic field and study its role in the build-up of structure in the Milky Way, leading to star formation.

In this image, the color scale represents the total intensity of dust emission, revealing the structure of interstellar clouds in the Milky Way. The texture is based on measurements of the direction of the polarized light emitted by the dust, which in turn indicates the orientation of the magnetic field. 

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 analyse the structure of the Galactic magnetic field.

The data have also enabled new important insights into the early cosmos and its components, including the intriguing dark matter and the elusive neutrinos, as described in papers also released today.

The Planck data have delved into the even earlier history of the cosmos, all the way to inflation – the brief era of accelerated expansion that the Universe underwent when it was a tiny fraction of a second old. As the ultimate probe of this epoch, astronomers are looking for a signature of gravitational waves triggered by inflation and later imprinted on the polarization of the CMB.

No direct detection of this signal has yet been achieved, as reported last week. However, when combining the newest all-sky Planck data with those latest results, the limits on the amount of primordial gravitational waves are pushed even further down to achieve the best upper limits yet.

Darker regions correspond to stronger polarized emission, and the striations indicate the direction of the magnetic field projected on the plane of the sky. The dark band running horizontally across the center corresponds to the Galactic Plane. Here, the polarization reveals a regular pattern on large angular scales, which is due to the magnetic field lines being predominantly parallel to the plane of the Milky Way. The data also reveal variations of the polarization direction within nearby clouds of gas and dust. This can be seen in the tangled features above and below the plane, where the local magnetic field is particularly disorganized.

The image is a Mollweide projection of the full celestial sphere, with the plane of the Galaxy aligned with the horizontal axis of the oval. Certain areas in the image, mostly at high Galactic latitude, have been masked out. The overall intensity in these regions is low, complicating the separation of foreground and cosmic microwave background (CMB) components. Further data analysis will improve this by the time of the full data release in late 2014.
“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,” says Jan Tauber.

“This is an incredibly rich data set and the harvest of discoveries has just begun.”

Contacts and sources:

A series of scientific papers describing the new results was published on 5 February and it can be downloaded here.

The new results from Planck are based on the complete surveys of the entire sky, performed between 2009 and 2013. New data, including temperature maps of the CMB at all nine frequencies observed by Planck and polarisation maps at four frequencies (30, 44, 70 and 353 GHz), are also released today.

The three principal scientific leaders of the Planck mission, Nazzareno Mandolesi, Jean-Loup Puget and Jan Tauber, were recently awarded the 2015 EPS Edison Volta Prize for “directing the development of the Planck payload and the analysis of its data, resulting in the refinement of our knowledge of the temperature fluctuations in the Cosmic Microwave Background as a vastly improved tool for doing precision cosmology at unprecedented levels of accuracy, and consolidating our understanding of the very early universe.”