Planck reveals late birth of first stars

Cosmic satellite delivers detailed maps of cosmic microwave background

February 09, 2015

New maps from ESA’s Planck satellite show the entire sky in the ‘polarised’ light from the early Universe, revealing that the first stars formed approx. 100 million years later than previously thought. They also shed new light onto our own Milky Way, whose dust reveals spectacular views of Galactic magnetic fields. Researchers at the Max Planck Institute for Astrophysics have developed important software components for Planck and were intensively involved in evaluation the scientific interpretation of the mission data.

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.

“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 magnetic fields and provides unprecedented insights into the complex weather phenomena of our Galaxy.” 

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 reported 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.


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