Cosmology: The oldest cosmic light
Nature 518, 7538 (2015). doi:10.1038/518170a
Authors: David Spergel & Brian Keating
The cosmic microwave background is a faint glow of light left over from the Big Bang. It fills the entire sky and records the Universe's early history. Two independent experts outline what we know about this ancient light, both theoretically and observationally.
Star formation: Sibling rivalry begins at birth
Nature 518, 7538 (2015). doi:10.1038/518173a
Authors: Kaitlin M. Kratter
High-resolution astronomical observations of a nearby molecular gas cloud have revealed a quadruplet of stars in the act of formation. The system is arguably the youngest multiple star system detected so far. See Letter p.213
The formation of a quadruple star system with wide separation
Nature 518, 7538 (2015). doi:10.1038/nature14166
Authors: Jaime E. Pineda, Stella S. R. Offner, Richard J. Parker, Héctor G. Arce, Alyssa A. Goodman, Paola Caselli, Gary A. Fuller, Tyler L. Bourke & Stuartt A. Corder
The initial multiplicity of stellar systems is highly uncertain. A number of mechanisms have been proposed to explain the origin of binary and multiple star systems, including core fragmentation, disk fragmentation and stellar capture. Observations show that protostellar and pre-main-sequence multiplicity is higher than the multiplicity found in field stars, which suggests that dynamical interactions occur early, splitting up multiple systems and modifying the initial stellar separations. Without direct, high-resolution observations of forming systems, however, it is difficult to determine the true initial multiplicity and the dominant binary formation mechanism. Here we report observations of a wide-separation (greater than 1,000 astronomical units) quadruple system composed of a young protostar and three gravitationally bound dense gas condensations. These condensations are the result of fragmentation of dense gas filaments, and each condensation is expected to form a star on a timescale of 40,000 years. We determine that the closest pair will form a bound binary, while the quadruple stellar system itself is bound but unstable on timescales of 500,000 years (comparable to the lifetime of the embedded protostellar phase). These observations suggest that filament fragmentation on length scales of about 5,000 astronomical units offers a viable pathway to the formation of multiple systems.
Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years
Nature 518, 7538 (2015). doi:10.1038/nature14159
Authors: Rita Schulz, Martin Hilchenbach, Yves Langevin, Jochen Kissel, Johan Silen, Christelle Briois, Cecile Engrand, Klaus Hornung, Donia Baklouti, Anaïs Bardyn, Hervé Cottin, Henning Fischer, Nicolas Fray, Marie Godard, Harry Lehto, Léna Le Roy, Sihane Merouane, François-Régis Orthous-Daunay, John Paquette, Jouni Rynö, Sandra Siljeström, Oliver Stenzel, Laurent Thirkell, Kurt Varmuza & Boris Zaprudin
Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer. The presence of water-ice grains in periodic comets is now well established. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. Micrometre-sized interplanetary dust particles collected in the Earth’s stratosphere and certain micrometeorites are assumed to be of cometary origin. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. At its previous recurrence (orbital period 6.5 years), the comet’s dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun.
New maps of ‘polarised’ light in the young Universe have revealed that the first stars formed 100 million years later than earlier estimates. The new images of cosmic background radiation, based on data released today from the European Space Agency’s Planck satellite, have shown that the process of reionisation, which ended the ‘Dark Ages’ as the earliest stars formed, started 550 million years after the Big Bang.
The history of our Universe is a 13.8 billion-year tale that scientists endeavour to read by studying the planets, asteroids, comets and other objects in our Solar System, and gathering light emitted by distant stars, galaxies and the matter spread between them.
A major source of information used to piece together this story is the Cosmic Microwave Background, or CMB, the fossil light 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, the Planck satellite 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.
The imaging is based on data from the Planck satellite, and was developed by the Planck collaboration, which includes the Cambridge Planck Analysis Centre at the University's Kavli Institute for Cosmology, Imperial College London and the University of Oxford at the London Planck Analysis Centre.
“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. This is exactly what happened when the CMB originated in the early Universe.
Initially, photons were trapped in a hot, dense soup of particles that, by the time the Universe was a few seconds old, consisted mainly of electrons, protons and neutrinos. Owing to the high density, electrons and photons collided with one another so frequently that light could not travel any significant distant before bumping into another electron, making the early Universe extremely ‘foggy’.
Slowly but surely, as the cosmos expanded and cooled, photons and the other particles grew farther apart, and collisions became less frequent. This had two consequences: electrons and protons could finally combine and form neutral atoms without them being torn apart again by an incoming photon, and photons had enough room to travel, being no longer trapped in the cosmic fog.
The new Planck data fixes the date of the end of these ‘Dark Ages’ to roughly 550 million years after the Big Bang, more than 100 million years later than previously determined by earlier polarisation observations from the NASA WMAP satellite (Planck’s predecessor), and has helped resolve a problem for observers of the early Universe.
The Dark Ages lasted until the formation of the first stars and galaxies, specifically the formation of very large stars with extremely hot surfaces, which resulted in the energetic UV-radiation that began the process of reionisation of the neutral hydrogen throughout the Universe.
Very deep images of the sky from the NASA/ESA Hubble Space Telescope have provided a census of the earliest known galaxies, which started forming perhaps 300–400 million years after the Big Bang.
The problem is that with a date for the end of the Dark Ages set at 450 million years after the Big Bang, astronomers can estimate that UV-radiation from such a source would have proved insufficient. “In that case, we would have needed additional, more exotic sources of energy to explain the history of reionisation,” said Professor George Efstathiou, Director of the Kavli Institute of Cosmology.
The additional margin of 100 million years provided by Planck removes this need as stars and galaxies would have had the time to supply the energetic radiation required to bring the Dark Ages to a close and begin the Epoch of reionisation that would last for a further 400 million years.
Although the joint investigation between Planck and BICEP2, searching for the imprinted signature on the polarisation of the CMB of gravitational waves triggered by inflation, found no direct detection of this signal, it crucially placed strong upper-limits on the amount of primordial gravitational waves.
Searching for this signal remains a major focus of ongoing and planned CMB experiments. “The results of the joint analysis demonstrate the power of combining CMB B-mode polarisation observations with measurements at higher frequency from Planck to clean Galactic dust,” said Dr Anthony Challinor of the Kavli Institute for Cosmology.
Inset image: Polarised emission from Milky Way dust Credit: ESA and the Planck Collaboration
New maps from the 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.We would have needed additional, more exotic sources of energy to explain the history of reionisationGeorge EfstathiouESA and the Planck CollaborationPolarisation of the Cosmic Microwave Background
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Firing off a string of snapshots like a sports photographer at a NASCAR race, NASA's Hubble Space Telescope captured a rare look at three of Jupiter's largest moons zipping across the banded face of the gas-giant planet: Europa, Callisto, and Io. Jupiter's four largest moons can commonly be seen transiting the face of the giant planet and casting shadows onto its cloud tops. However, seeing three moons transiting the face of Jupiter at the same time is rare, occurring only once or twice a decade. Missing from the sequence, taken on January 24, 2015, is the moon Ganymede that was too far from Jupiter in angular separation to be part of the conjunction.