Planck reveals first stars were born late

New maps from ESA's Planck satellite uncover the "polarised" light from the early Universe across the entire sky, revealing that the first stars formed much later than previously thought. They also include information about our own Milky Way, showing that the contribution from dust in our galaxy is much more widespread than previously thought and revealing complex structures in the Galactic magnetic field. Researchers at the Max Planck Institute for Astrophysics developed important software components for Planck and take part in the scientific interpretation of the mission data; in particular, they contributed to the analysis of the signals from galaxy clusters, from gravitational lensing of the CMB, from Galactic dust and magnetic fields, as well as from primordial magnetic fields.

Fig. 1: A visualisation of the polarisation of the Cosmic Microwave Background, or CMB, as detected by ESA's Planck satellite over the entire sky. A small fraction of the CMB is polarised - it vibrates in a preferred direction. In this image, the colour scale represents temperature differences in the CMB, while the texture indicates the direction of the polarised light. The patterns seen in the texture are characteristic of "E-mode" polarisation, which is the dominant type for the CMB.
Credits: ESA and the Planck Collaboration

Fig. 2: Detailed view of the polarisation map from Fig. 1, across a small patch of the sky measuring 5º across.
Credits: ESA and the Planck Collaboration

Fig. 3: The interaction between interstellar dust in the Milky Way and the structure of our Galaxy's magnetic field, as detected by ESA’s Planck satellite over the entire sky.
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.
Interstellar clouds of gas and dust are 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 "polarised" - it vibrates in a preferred direction – and, as such, could be caught by the polarisation-sensitive detectors on Planck. Scientists at MPA and in the Planck collaboration are using the polarised 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.
Credits: ESA and the Planck Collaboration

The history of our Universe began 13.8 billion years ago and for researchers trying to understand its evolution, one major source of information is the Cosmic Microwave Background, or CMB. This fossil light is resulting from a time when the Universe was hot and dense, only 380 000 years after the Big Bang. 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 numerous scientific papers over the past two years, confirming the standard cosmological picture of our Universe with ever greater accuracy (see e.g. linkPfeil.gifhere).

"The detailed map of CMB temperature structures is one of the key scientific results of the 21st century," explains Simon White, director at the Max Planck Institute for Astrophysics and Co-Investigator of Planck. "It is a high-fidelity image of the boundary of our visible Universe, showing us its detailed structure when it was 40,000 times younger than today and giving us our best indication of what happened at even earlier times."

"But there is more: the CMB carries additional clues about our cosmic history that are encoded in its 'polarisation'," 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."

Light is polarised when it vibrates in a preferred direction, something that may arise as a result of photons - the particles of light - bouncing off other particles, such as electrons. This is exactly what happened when the CMB originated in the early Universe. Planck's polarisation data provide an independent way to measure cosmological parameters and thus confirm the details of the standard cosmological picture determined from CMB temperature fluctuations.

However, as the CMB light travelled through space and time it was also influenced by the first stars and the polarisation data now indicates that these started to shine about 550 million years after the Big Bang, ending the "Dark Ages". This is more than 100 million years later than previously thought but actually helps to resolve a problem: Previous studies of the CMB polarisation seemed to point towards an earlier dawn of the first stars, while very deep images of the sky indicated that the earliest known galaxies in the Universe (forming perhaps 300-400 million years after the Big Bang) would not have been powerful enough to succeed at ending the Dark Ages within 450 million years. The new evidence from Planck significantly reduces the problem, indicating that the earliest stars and galaxies alone might have been enough.

But the first stars are definitely not the whole story. With the new Planck data released today, scientists are also studying the polarisation of foreground emission from gas and dust in the Milky Way to analyse the structure of the Galactic magnetic field.

"With its nine frequency channels, Planck is uniquely suited to disentangle the cosmological signal from foreground emission - but we have to be very careful in analysing the data," explains Torsten Enßlin, leader of the Planck technical team at the Max Planck Institute for Astrophysics. "Our results show that the polarised emission from dust in our Milky Way is significant over the entire sky, dashing earlier hopes that some areas might be clean enough to offer an uncontaminated view of the early universe. The polarised emission beautifully traces Galactic magnetic fields and provides unprecedented insights into the complex weather phenomena of our Milky Way."

The new data have also enabled important insights into the early cosmos and the nature of 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 currently looking for a signature of gravitational waves triggered by inflation and later imprinted on the polarisation of the CMB.

Earlier claims of a direct detection had to be revised in light of Planck’s maps of dust polarisation, as linkPfeilExtern.gifreported last week. Combining the newest Planck data with the latest results from other experiments, the limits on the amount of primordial gravitational waves have been pushed down, producing upper limits that already exclude some models for inflation.

Notes for Editors

A series of scientific papers describing the new results was published on 5 February. They can be downloaded linkPfeilExtern.gifhere.

The new results from Planck are based on the complete surveys of the complete 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.

More about Planck

Launched in 2009, Planck was designed to map the sky in nine frequencies using two state-of-the-art instruments: the Low Frequency Instrument, which includes three frequency bands in the range 30–70 GHz, and the High Frequency Instrument, which includes six frequency bands in the range 100–857 GHz.

HFI completed its survey in January 2012, while LFI continued to make science observations until 3 October 2013, before being switched off on 19 October 2013. Seven of Planck's nine frequency channels were equipped with polarisation-sensitive detectors.

The Planck Scientific Collaboration consists of all the scientists who have contributed to the development of the mission, and who participate in the scientific exploitation of the data during the proprietary period.

These scientists are members of one or more of four consortia: the LFI Consortium, the HFI Consortium, the DK-Planck Consortium, and ESA’s Planck Science Office. The two European-led Planck Data Processing Centres are located in Paris, France and Trieste, Italy.

The LFI consortium is led by N. Mandolesi, Università degli Studi di Ferrara, Italy (deputy PI: M. Bersanelli, Università degli Studi di Milano, Italy), and was responsible for the development and operation of LFI. The HFI consortium is led by J.L. Puget, Institut d'Astrophysique Spatiale in Orsay, France (deputy PI: F. Bouchet, Institut d'Astrophysique de Paris, France), and was responsible for the development and operation of HFI.

For further information, please contact:

Prof. Simon White
Max-Planck-Institut für Astrophysik
Tel: +49 89 30000-2211

Dr. Torsten Enßlin
Max-Planck-Institut für Astrophysik
Tel.: +49 89 30000-2243

Dr. Hannelore Hämmerle
Press Officer
Max-Planck-Institut für Astrophysik
Tel. +49 89 30000-3980


Full ESA Press Release: linkPfeilExtern.gifPlanck reveals first stars were born late
Scientific Papers describing the linkPfeilExtern.gifnew results.