ESA's Planck satellite has revealed that
the first stars in the Universe started forming later than previous
observations of the Cosmic Microwave Background indicated. This new
analysis also shows that these stars were the only sources needed to
account for reionising atoms in the cosmos, having completed half of
this process when the Universe had reached an age of 700 million years.
Credit: ESA – C. Carreau
With the multitude of stars and galaxies
that populate the present Universe, it's hard to imagine how different
our 13.8 billion year cosmos was when it was only a few seconds old. At
that early phase, it was a hot, dense primordial soup of particles,
mostly electrons, protons, neutrinos, and photons – the particles of light.
In such a dense environment the Universe appeared like an 'opaque'
fog, as light particles could not travel any significant distance before
colliding with electrons.
As the cosmos expanded, the Universe grew cooler and more rarefied
and, after about 380 000 years, finally became 'transparent'. By then,
particle collisions were extremely sporadic and photons could travel
freely across the cosmos.
Credit: ESA
Today, telescopes like Planck can
observe this fossil light across the entire sky as the Cosmic Microwave
Background, or CMB. Its distribution on the sky reveals tiny
fluctuations that contain a wealth of information about the history,
composition and geometry of the Universe.
The release of the CMB happened at the time when electrons and
protons joined to form hydrogen atoms. This is the first moment in the
history of the cosmos when matter was in an electrically neutral state.
After that, a few hundred million years passed before these atoms
could assemble and eventually give rise to the Universe's first
generation of stars.
As these first stars came to life, they filled their surroundings
with light, which subsequently split neutral atoms apart, turning them
back into their constituent particles: electrons and protons.
Scientists refer to this as the 'epoch of reionisation'. It did not take long for most material in the Universe to become completely ionised, and – except in a very few, isolated places – it has been like that ever since.
Scientists refer to this as the 'epoch of reionisation'. It did not take long for most material in the Universe to become completely ionised, and – except in a very few, isolated places – it has been like that ever since.
Observations of very distant galaxies hosting supermassive black
holes indicate that the Universe had been completely reionised by the
time it was about 900 million years old. The starting point of this
process, however, is much harder to determine and has been a hotly
debated topic in recent years.
"The CMB can tell us when the epoch of reionisation started and, in turn, when the first stars formed in the Universe," explains Jan Tauber, Planck project scientist at ESA.
To make this measurement, scientists exploit the fact that a fraction
of the CMB is polarised: part of the light vibrates in a preferred
direction. This results from CMB photons bouncing off electrons –
something that happened very frequently in the primordial soup, before
the CMB was released, and then again later, after reionisation, when
light from the first stars brought free electrons back onto the cosmic
stage.
"It is in the tiny fluctuations of the CMB polarisation that we
can see the influence of the reionisation process and deduce when it
began," adds Tauber.
Credit: ESA and the Planck Collaboration
A first estimate of the epoch of
reionisation came in 2003 from NASA's Wilkinson Microwave Anisotropy
Probe (WMAP), suggesting that this process might have started early in
cosmic history, when the Universe was only a couple of hundred million
years old. This result was problematic, because there is no evidence
that any stars had formed by then, which would mean postulating the
existence of other, exotic sources that could have caused the
reionisation at that time.
This first estimate was soon to be corrected, as subsequent data from
WMAP pushed the starting time to later epochs, indicating that the
Universe had not been significantly reionised until at least some 450
million years into its history.
This eased, but did not completely solve the puzzle: although the
earliest of the first stars have been observed to be present already
when the Universe was 300 to 400 million years old, it remained unclear
whether these stars were the main culprits for reionising fully the
cosmos or whether additional, more exotic sources must have played a
role too.
In 2015, the Planck Collaboration provided new data to tackle the
problem, moving the reionisation epoch even later in cosmic history and
revealing that this process was about half-way through when the Universe
was around 550 million years old. The result was based on Planck's
first all-sky maps of the CMB polarisation, obtained with its
Low-Frequency Instrument (LFI).
Now, a new analysis of data from Planck's other detector, the
High-Frequency Instrument (HFI), which is more sensitive to this
phenomenon than any other so far, shows that reionisation started even
later – much later than any previous data have suggested.
"The highly sensitive measurements from HFI have clearly
demonstrated that reionisation was a very quick process, starting fairly
late in cosmic history and having half-reionised the Universe by the
time it was about 700 million years old," says Jean-Loup Puget from Institut d'Astrophysique Spatiale in Orsay, France, principal investigator of Planck's HFI.
"These results are now helping us to model the beginning of the reionisation phase."
"We have also confirmed that no other agents are needed, besides the first stars, to reionise the Universe," adds Matthieu Tristram, a Planck Collaboration scientist at Laboratoire de l'Accélérateur Linéaire in Orsay, France.
The new study locates the formation of the first stars much later
than previously thought on the cosmic timeline, suggesting that the
first generation of galaxies are well within the observational reach of
future astronomical facilities, and possibly even some current ones.
In fact, it is likely that some of the very first galaxies have
already been detected with long exposures, such as the Hubble Ultra Deep
Field observed with the NASA/ESA Hubble Space Telescope, and it will be
easier than expected to catch many more with future observatories such
as the NASA/ESA/CSA James Webb Space Telescope.
Notes for Editors
'Planck intermediate results. XLVII. Planck constraints on reionization history' and 'Planck
intermediate results. XLVI. Reduction of large-scale systematic effects
in HFI polarization maps and estimation of the reionization optical
depth' by the Planck Collaboration are published in Astronomy and Astrophysics.
Launched in 2009, Planck was designed to map the sky in nine
frequencies using two state-of-the-art instruments: the Low Frequency
Instrument (LFI), which includes three frequency bands in the range
30-70 GHz, and the High Frequency Instrument (HFI), 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 (CNRS/Université Paris-Sud), France (deputy PI: F. Bouchet, Institut d'Astrophysique de Paris (CNRS/UPMC), France), and was responsible for the development and operation of HFI.
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 (CNRS/Université Paris-Sud), France (deputy PI: F. Bouchet, Institut d'Astrophysique de Paris (CNRS/UPMC), France), and was responsible for the development and operation of HFI.
For further informatin, please contact:
Jan Tauber
ESA Planck Project Scientist
Scientific Support Office
Directorate of Science
European Space Agency
Email: jan.tauber@esa.int
Phone: +31-71-565-5342
Jean-Loup Puget
Principal Investigator, High Frequency Instrument
Institut d'Astrophysique Spatiale
Orsay, France
Email: jean-loup.puget@ias.u-psud.fr
Phone: +33-169858665
Matthieu Tristram
CNRS - IN2P3
Laboratoire de l'Accélérateur Linéaire
Université Paris-Sud 11
Orsay, France
Email: tristram@lal.in2p3.fr
Phone: +33-164468388
Source: ESA/PLANCK
