From physicsworld: “Parallel-universe search focuses on neutrons”

May 10, 2016
Edwin Cartlidge

No braner: there is no evidence that ILL neutrons venture into an adjacent universe. No image credit.

The first results* from a detector designed to look for evidence of particles reaching us from a parallel universe have been unveiled by physicists in France and Belgium. Although they drew a blank, the researchers say that their experiment provides a simple, low-cost way of testing theories beyond the Standard Model of particle physics, and that the detector could be made significantly more sensitive in the future.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

A number of quantum theories of gravity predict the existence of dimensions beyond the three of space and one of time that we are familiar with. Those theories envisage our universe as a 4D surface or “brane” in a higher-dimensional space–time “bulk”, just as a 2D sheet of paper exists as a surface within our normal three spatial dimensions. The bulk could contain multiple branes separated from one another by a certain distance within the higher dimensions.

Physicists have found no empirical evidence for the existence of other branes. However, in 2010, Michaël Sarrazin of the University of Namur in Belgium and Fabrice Petit of the Belgian Ceramic Research Centre put forward a model showing that particles normally trapped within one brane should occasionally be able to tunnel quantum mechanically into an adjacent brane. They said that neutrons should be more affected than charged particles because the tunnelling would be hindered by electromagnetic interactions.

Nearest neighbour

The researchers have now teamed up with physicists at the University of Grenoble in France and others at the University of Namur to put their model to the test. This involved setting up a helium-3 detector a few metres from the nuclear reactor at the Institut Laue-Langevin (ILL) in Grenoble and then recording how many neutrons it intercepted. The idea is that neutrons emitted by the reactor would exist in a quantum superposition of being in our brane and being in an adjacent brane (leaving aside the effect of more distant branes). The neutrons’ wavefunctions would then collapse into one or other of the two states when colliding with nuclei within the heavy-water moderator that surrounds the reactor core.

Most neutrons would end up in our brane, but a small fraction would enter the adjacent one. Those neutrons, so the reasoning goes, would – unlike the neutrons in our brane – escape the reactor, because they would interact extremely weakly with the water and concrete shielding around it. However, because a tiny part of those neutrons’ wavefunction would still exist within our brane even after the initial collapse, they could return to our world by colliding with helium nuclei in the detector. In other words, there would be a small but finite chance that some neutrons emitted by the reactor would disappear into another universe before reappearing in our own – so registering events in the detector.

Sarrazin says that the biggest challenge in carrying out the experiment was minimizing the considerable background flux of neutrons caused by leakage from neighbouring instruments within the reactor hall. He and his colleagues did this by enclosing the detector in a multilayer shield – a 20 cm-thick polyethylene box on the outside to convert fast neutrons into thermal ones and then a boron box on the inside to capture thermal neutrons. This shielding reduced the background by about a factor of a million.

Stringent upper limit

Operating their detector over five days in July last year, Sarrazin and colleagues recorded a small but still significant number of events. The fact that these events could be residual background means they do not constitute evidence for hidden neutrons, say the researchers. But they do allow for a new upper limit on the probability that a neutron enters a parallel universe when colliding with a nucleus – one in two billion, which is about 15,000 times more stringent than a limit the researchers had previously arrived at by studying stored ultra-cold neutrons. This new limit, they say, implies that the distance between branes must be more than 87 times the Planck length (about 1.6 × 10–35 m).

To try and establish whether any of the residual events could indeed be due to hidden neutrons, Sarrazin and colleagues plan to carry out further, and longer, tests at ILL in about a year’s time. Sarrazin points out that because their model doesn’t predict the strength of inter-brane coupling, these tests cannot be used to completely rule out the existence of hidden branes. Conversely, he says, they could provide “clear evidence” in support of branes, which, he adds, could probably not be obtained using the LHC at CERN. “If the brane energy scale corresponds to the Planck energy scale, there is no hope to observe this kind of new physics in a collider,” he says.

Axel Lindner of DESY, who carries out similar “shining-particles-through-a-wall” experiments (but using photons rather than neutrons), supports the latest research. He believes it is “very important” to probe such “crazy” ideas experimentally, given presently limited indications about what might supersede the Standard Model. “It would be highly desirable to clarify whether the detected neutron signals can really be attributed to background or whether there is something else behind it,” he says.

The research is described in Physics Letters B.

*Science paper:
Search for passing-through-walls neutrons constrains hidden braneworlds

See the full article here .

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From physicsworld.com: “Are wormholes or ‘gravastars’ mimicking gravitational-wave signals from black holes?”

physicsworld.com

Apr 29, 2016
Tushna Commissariat

Into a wormhole: characteristic modes are light-ring potential wells. No image credit

Earlier this year, researchers working on the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) made the first ever detection of gravitational waves.

Caltech/MIT Advanced aLigo detector in Livingston, Louisiana

The waves are believed to have been created by the merger of two binary black holes, in an event dubbed GW150914.

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

Black holes merging Swinburne Astronomy Productions

Now, however, new theoretical work done by an international team of researchers suggests that other hypothetical exotic stellar objects – such as wormholes or “gravastars” – could produce a very similar gravitational-wave signal. While it is theoretically possible to differentiate between the different sources, it is impossible to tell whether GW150914 had a more exotic origin than merging black holes because the signal was not strong enough to be resolved.

The researchers point out that, in the future, the detection of stronger gravitational-wave signals could reveal more information about their sources – especially once the sensitivity of aLIGO is increased to its ultimate design level. In addition, future space-based detectors, such as the European Space Agency’s Evolved Laser Interferometer Space Antenna (eLISA), could reveal tiny discrepancies between detected and predicted signals, if they exist.

ESA/LISA Pathfinder

Ringing frequencies

Einstein’s general theory of relativity provides a very clear theoretical framework for the type of gravitational-wave signal that would be produced during the collision and subsequent merger of massive, compact bodies, such as black holes. Gravitational waves are produced constantly before, during and just after a merger. The waves’ frequencies will vary, telling us when the black holes’ orbit begins to reduce and they begin their slow inward collapse, or “inspiral”. The smaller the initial distance between the two, the more radiation is emitted as the black holes plunge into one another. This produces a characteristic “chirp” waveform, wherein the frequency and the amplitude of the waves increase until they peak at the merger.

But such a cataclysmic merger initially gives birth to a highly distorted black hole, which rids itself of its deformity almost instantly, by ringing like a bell and producing further gravitational radiation. The system quickly loses energy and the strength of the waves decays exponentially to form a “ringdown” signal, all of which was picked up by aLIGO for GW150914.

Ringing chirp: the waveform of event GW150914. No image credit.

The chirp and the ringdown signal are of immense interest as these carry crucial information about the mass and spin of both the initial black holes, and of the newly formed one. “This ringdown phase is very important: just as a Stradivarius violin vibrates in a characteristic way, so too do black holes. Thus, by studying carefully how it rings, you hope to know the black hole itself,” says physicist Vitor Cardoso from the University of Lisbon, Portugal.

These vibrational modes of a nascent black hole – known as quasinormal modes – must be detected within the signal, to be absolutely certain that the gravitational waves have arisen from coalescing black holes. Our current understanding suggests that these vibrational modes are inherently linked to a black hole’s key feature – its event horizon, or the boundary past which nothing, not even light, can escape from its gravitational pull.

Light rings

But new simulations and analysis – carried out by Cardoso together with team members Paolo Pani and Edgardo Franzin – have shown that a virtually indistinguishable ringdown signal can be produced by a “black-hole mimicker”, thereby potentially allowing us to detect these exotic objects. These mimickers are hypothetical objects that could be as compact as black holes but do not have an event horizon. They could be gravastars – celestial objects whose interior is made of dark energy – or wormholes – a tunnel through space–time connecting two distant regions of the universe.

These exotic objects possess “light rings”, which are yet another artefact of general relativity – a circular photon orbit which is predicted to exist around very compact objects. “A light ring is very different from an event horizon, because signals can escape from regions within the light ring – although they would be highly red-shifted – whereas nothing can escape from the event horizon,” explains Cardoso. All compact objects would in theory possess a light ring. Indeed back holes have one that is associated with the border of their silhouette. These are the so-called “black-hole shadows” that lie just outside of their event horizon. On the other hand, neutron stars, while very compact, are not compact enough to develop a light ring.

Cardoso and colleagues looked into objects with only light rings and found that “if an object is compact enough to possess a light ring, then the ringdown would be almost identical to that of a black hole. The more compact the object, the more similar the ringdown”. Indeed, the team’s simulations showed that the ringdown signal is mostly associated with the light ring. It is the light ring itself that is vibrating, not the event horizon.
Mimicking wormholes

The team’s simulations calculated this explicitly for a wormhole, but Pani told physicsworld.com that the “same result is valid for gravastars and, as we claim, for all ultracompact black-hole mimickers”. But the researchers’ analysis also showed that these mimickers eventually leave an imprint in the gravitational-wave signal in the form of “echoes”, which are reflections of the waves from the surface of these objects. “These echoes may take a long time to reach our detectors, so it is important to scrutinize the data even long after the main pulse has arrived,” says Cardoso. More precisely, the mimicker signal will ultimately deviate from that predicted for a black hole, but only at late times.

LIGO scientist Amber Stuver, who is based at the LIGO Livingston Observatory in Louisiana, US, is “thrilled” by the possibility that aLIGO may have detected an exotic object, but she confirms that “there is nothing in our observations that is inconsistent with this being a normal stellar mass black-hole system possessing an event horizon. Until we have evidence otherwise, we can’t claim that this was anything but a stellar mass black hole binary merger.” She tells physicsworld.com that “advanced detectors such as aLIGO, aVirgo, and KAGRA will need to increase their sensitivity” to pick up such signals. She also points out that the GW150914 event “was detected with aLIGO at about 30% of its design sensitivity. The potential is real that, if these exotic horizonless objects are out there mimicking black holes, we may very well find them in the near future”.

B S Sathyaprakash from Cardiff University in the UK, who is also a part of the LIGO team, agrees with the theorists’ work, saying that “Our signal is consistent with both the formation of a black hole and a horizonless object – we just can’t tell.” He further explains that, although Einstein’s equations predict how slightly deformed black holes vibrate, our understanding is incomplete when their deformation is large. “That’s why we need a signal in which the post-merger oscillations of the merged object are large, and this can happen if we detect even more massive objects than GW150914, or if GW150914 is at least two to four times closer.” Then, it would be possible to distinguish the signals, he says.

Cardoso acknowledges that “black-hole mimickers are very exotic objects and by far black holes remain the most natural hypothesis”. But he adds: “It is important to understand whether these exotic objects can be formed (for example in a stellar collapse) and if they are stable. Most importantly, we only focused on the ringdown part, but it is equally relevant to explain the entire gravitational-wave signal, including the inspiral and the merger phases. This would require performing numerical simulations with supercomputers to understand whether this picture is viable or not. We are currently working on this.”

The research* is published in Physical Review Letters.

*Science paper:
Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?

See the full article here .

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From physicsworld.com: “Einstein meets the dark sector in a new numerical code that simulates the universe”

physicsworld.com

Mar 10, 2016
Keith Cooper

A powerful numerical code that uses [Albert] Einstein’s general theory of relativity to describe how large-scale structures form in the universe has been created by physicists in Switzerland and South Africa. The program promises to help researchers to better incorporate dark matter and dark energy into huge computer simulations of how the universe has evolved over time.

At the largest length scales, the dynamics of the universe are dominated by gravity. The force binds galaxies together into giant clusters and, in turn, holds these clusters tight within the grasp of immense haloes of dark matter. The cold dark matter (CDM) model assumes that dark matter comprises slow-moving particles. This means that non-relativistic Newtonian physics should be sufficient to describe the effects of gravity on the assembly of large-scale structure in the universe.

Universe map 2MASS Extended Source Catalog XSC

However, if dark matter moves at speeds approaching that of light, the Newtonian description breaks down and Einstein’s general theory of relativity must be incorporated into the simulation – something that has proven difficult to do.

Upcoming galaxy surveys, such as those to be performed by the Large Synoptic Survey Telescope in Chile or the European Space Agency’s Euclid mission, will observe the universe on a wider scale and to a higher level of precision than ever before.

The LSST, camera built at SLAC, and the building in Chile which will house the telescope

ESA/Euclid spacecraft

Computer simulations based on Newtonian assumptions may not be able to reproduce this level of precision, making observational results difficult to interpret. More importantly, we don’t know enough about what dark matter and dark energy are, to be able to conclusively say which treatment of gravity is most appropriate for them.

Evolving geometry

Now, Julian Adamek of the Observatoire de Paris and colleagues have developed a numerical code called “gevolution”, which provides a framework for introducing the effects of general relativity into complex simulations of the cosmos. “We wanted to provide a tool that describes the evolution of the geometry of space–time,” Adamek told physicsworld.com.

General relativity describes gravity as the warp created in space–time by the mass of an object. This gives the cosmos a complex geometry, rather than the linear space described by Newtonian gravity. The gevolution code is able to compute the Friedmann–Lemaítre–Robertson–Walker metric that solves Einstein’s field equations to describe [spacetime’s] complex geometry and how particles move through that geometry. The downside is that it sucks up a lot of resources: 115,000 central-processing-unit (CPU) hours compared to 25,000 CPU hours for a similarly sized Newtonian simulation.

Other uncertainties

Not everyone is convinced that the code is urgently required, and Joachim Harnois-Déraps of the Institute for Astronomy at the Royal Observatory in Edinburgh points out that there are other challenges facing physicists running cosmological simulations. “There are many places where things could go wrong in simulations.”

Harnois-Déraps cites inaccuracies in modelling the nonlinear clustering of matter in the universe, as well as feedback from supermassive black holes in active galaxies blowing matter out from galaxies and redistributing it. A recent study led by Markus Haider of the University of Innsbruck in Austria, for example, showed that jets from black holes could be sufficient to blow gas all the way into the voids within the cosmic web of matter that spans the universe.

“Central and shining”

“In my opinion, the bulk of our effort should instead go into improving our knowledge about these dominant sources of uncertainty,” says Harnois-Déraps who, despite his scepticism, hails gevolution as a great achievement in coding. “If suddenly a scenario arises where general relativity is needed, the gevolution numerical code would be central and shining.”

Indeed, Adamek views the gevolution code as a tool, ready and waiting should it be required. Newtonian physics works surprisingly well for the current standard model of cold dark matter and dark energy as the cosmological constant. However, should dark matter prove to have relativistic properties, or if dark energy is a dynamic, changing field rather than a constant, then Newtonian approximations will have to make way for the more precise predictions of general relativity.

“The Newtonian approach works well in some cases,” says Adamek, “But there might be other situations where we’re better off using the correct gravitational field.”

The research is described in Nature Physics.

See the full article here .

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From physicsworld.com: “How LIGO will change our view of the universe”

physicsworld.com

Feb 19, 2016
Tushna Commissariat

Gravitational waves, Werner Benger, Zuse-Institut Berlin and Max-Planck-Institut für Gravitationsphysik

Results and data from the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) collaboration – which revealed last week that it had observed a gravitational wave for the first time – are already providing astronomers and cosmologists the world over with previously unknown information about our universe. While the current results have posed intriguing questions for astronomers regarding binary black-hole systems, gravitational-wave astronomy will also revolutionize our understanding of the universe during its infancy, according to cosmologist and Perimeter Institute director Neil Turok.

Many scientists, such as LIGO veteran Kip Thorne, have pointed out that the collaboration’s results have opened a new window onto the universe. Each time that this has happened in the past, unexpected phenomena have come to light – for example, the advent of radio astronomy revealed the universe’s most luminous objects in the form of quasars and pulsars.

NRAO/ Very Large Array

Pristine objects

Turok told physicsworld.com that black holes – some of the most prolific producers of these ripples – are some of the simplest objects in the universe. He points out that when it comes to these “perfectly pristine objects”, there are “not too many parameters that need to be determined” because a black hole’s dynamics are mainly determined by its mass. Turok also points out that gravitational waves will provide even deeper insights, as they involve the fundamental force of gravity, which itself is still something of a puzzle.

Indeed, for Turok, this is what is most exciting about aLIGO’s discovery, which he says “may mark a bit of a transition as gravitational-wave observatories become the high-energy colliders of the future as we probe gravity and other extremely basic physics”. Gravitational waves can go to a time/place that, currently, we have very little information about – the early universe, which is opaque to all electromagnetic radiation.

Looking back in time

Thankfully, gravitational waves can travel freely through the hot plasma of the early universe and could be used “to look back to a trillionth of a second after the Big Bang”, according to Turok. For him, the discovery is very timely, as he is currently working with colleagues on a new theoretical proposal for “shockwaves” produced a millionth of a second after the Big Bang, which would have been present across all scales in the early universe. If these shockwaves exist, they would have an effect on the measured density variation that is seen in the cosmic microwave background, and could only be detected by gravitational radiation. Once they have a more complete theoretical description, Turok is convinced that LIGO and its successors such as the LISA Pathfinder and other space-based experiments could pick up the shockwave signal, if it exists.

ESA\LISA Pathfinder

Ultimately, Turok is delighted by LIGO’s discovery, and although he says that it is “much more important than any prize”, he is sure that it will win not only a Nobel prize, but also a slew of others, such as the Breakthrough prize.

A preprint of Turok’s paper on shockwaves is available on the arXiv server.

See the full article here .

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From physicsworld.com: “The world of physics in 2016”

physicsworld.com

Dec 17, 2015
Matin Durrani

As another year draws to a close, it’s time for me to peer into my crystal ball and predict the key events in physics that could take place in 2016. I always find it simpler and easier to say what’s coming up in “big science” – dominated as it is by massive projects in particle physics, astronomy and cosmology that are planned years in advance. And next year is no exception.

Fresh direction: CERN’s new boss Fabiola Gianotti

So let’s start at CERN, where physicists at the Large Hadron Collider (LHC) will spend 2016 continuing to smash protons together at an energy of 13 TeV as part of “Run II”, which began last year.

LHC at CERN

Fabiola Gianotti, who takes the reins from Rolf-Dieter Heuer next month as CERN’s 15th director-general, will be keen to ensure the lab gathers as many top-quality data as possible, even if the LHC’s unlikely to reach its planned collision energy of 14 TeV or get “new physics” beyond the Standard Model in 2016.

Rolf-Dieter Heuer

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

Indeed, a presentation at CERN just before Christmas of the first Run II data from the ATLAS and CMS experiments already appears to limit the possibility of “supersymmetric” particles to yet higher energies.

ATLAS and a Higgs event in ATLAS.

CMS and a Higgs event in CMS

Standard Model of Supersymmetry

Up in space, NASA’s Juno mission is set to enter the orbit of Jupiter on 4 July, handily timed for a watching US public. After a five-year journey, Juno will be the first craft to visit Jupiter since Galileo in 1995.

NASA/Juno

NASA/Galileo 1

The Japanese Space Agency (JAXA) is set for a busy year, too. Its Akatsuki spacecraft entered orbit around Venus last month, and mission scientists expect to receive its first data in April.

JAXA/AKATSUKI

JAXA also plans to launch the ASTRO-H X-ray telescope into low Earth orbit this year, to study everything from the large-scale structure of the universe,

2MASS LSS chart large-scale structure of the universe

to the distribution of dark matter in galaxy clusters.

Meanwhile, the European Space Agency will release the first data early next year from its Gaia mission, which seeks to create a 3D catalogue of about a billion astronomical objects.

ESA/Gaia

March will see the European Space Agency’s Lisa Pathfinder craft begin work to test the technology for a future space-based gravitational-wave observatory.

ESA/Pathfinder 1

Another tantalizing prospect for 2016 will be the Event Horizon Telescope imaging a black hole for the first time.

Event Horizon telescope and map

Astroparticle physicists, meanwhile, are set to start work in 2016 on a $14m upgrade to the Pierre Auger Observatory – the world’s largest cosmic-ray observatory – in Argentina.

Pierre Auger Observatory

The AugerPrime upgrade will involve installing scintillation detectors alongside the 1660 existing water Cherenkov detectors, allowing researchers to more efficiently separate the electrons and muons that are created in the cascade of secondary particles created when a comic ray hits the Earth’s atmosphere. This, in turn, should make it easier to identify cosmic rays that are high-energy protons.

Ups and downs

All is not entirely rosy in astronomy, though. Hawaii’s Supreme court recently ruled that the construction permit for the $1.4bn Thirty Meter Telescope (TMT) on top of Mauna Kea mountain is invalid.

The more than proposed TMT

The ruling will force the telescope’s backers to restart the entire permit process, delaying the project and adding further uncertainty. Construction of the TMT has already been on hold since last April following protests by native Hawaiians, who see its construction on Mauna Kea as desecration of their spiritual and cultural pinnacle.

In nuclear physics, the ITER tokomak fusion reactor, which is being built in Cadarache in southern France, faces another turbulent year.

ITER tokamak

After last November’s ITER council meeting, rumours surfaced that the project’s completion could slip by six years, from 2019 to 2025. The council will now carry out its own review to see if there is scope for tightening the timeline and cutting costs, with a new plan, or “baseline”, due out in June. On a related note, the Wendelstein 7-X stellerator in Greifswald, Germany, which switched on last week, is set to be put through its paces next year as researchers test this type of fusion device.

Wendelstein 7-X stellerator

Quantum frontiers

Predicting what will happen across the rest of physics and in physics-based industry is harder, where progress is vital but fragmented across myriad groups, sectors and businesses. My tip is seeing “Li-Fi” – a light-based alternative to radio-frequency Wi-Fi – gaining commercial traction. Work on graphene and other 2D materials will continue, with the focus on layering a few 2D materials to make novel “designer” heterostructures using, say, graphene layers as electrodes and boron nitride as insulators.

Graphene. The ideal crystalline structure of graphene is a hexagonal grid.

Applications of physics are crucial, and it is thanks to them – and through the advocacy of organizations like the Institute of Physics (IOP), which publishes Physics World – that science funding in the UK survived cuts in the country’s recent Comprehensive Spending Review. There will be further positive developments for UK science in 2016, with the opening of the massive new £650m Francis Crick Institute in London. Named after the co-discoverer of the structure of DNA, the institute will be the country’s flagship biomedical-science lab, with as many as a fifth of the 1250 staff being physicists, chemists, mathematicians and engineers. Remember that biosciences and the environment dominate Altmetric’s list of the top 100 most popular scientific papers of 2015, as judged by how much they were shared and discussed in mainstream and social media.

Future tech: quantum physics will continue to throw up surprises

The beauty of physics, however, is that even the most esoteric research can unleash unforeseen benefits – as the winners of the Physics World 2015 Breakthrough of the Year will concur. We picked Jian-Wei Pan and Chaoyang Lu of the University of Science and Technology of China in Hefei, for being the first to achieve the simultaneous quantum teleportation of two inherent properties of a fundamental particle – the photon. The researchers are already talking about applications, such as “long-distance quantum communications that provide unbreakable security, ultrafast quantum computers and quantum networks”. We can also look forward to further developments in 2016 from the UK’s ambitious £270m National Quantum Technologies Programme, which seeks to stimulate applications of quantum physics.

Speaking of which, surely 2016 will be the year when Anton Zeilinger – the doyen of quantum communication, computation and information – will finally win a long-overdue Nobel Prize for Physics? I’ve backed the Austrian quantum guru for Nobel glory for a long time, and 2016 has to be his year, possibly with Alain Aspect and John Clauser for their Bell’s inequality experiments. The Nobel Committee for Physics take note.

See the full article here .

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From physicsworld.com: “Astronomers gaze upon the oldest stars in the galaxy”

physicsworld.com

Nov 13, 2015
Tushna Commissariat

Dark heart: the dusty heart of the Milky Way galaxy

The oldest stars in our Milky Way galaxy have been discovered by an international team of researchers. These ancient stars could contain vital clues about how the first stars in the early universe died, and their discovery marks the first time that extremely metal-poor stars have been observed in the central region of the galaxy. The location of the stars suggests that they formed when the Milky Way underwent rapid chemical changes during the first 1–2 billion years of the universe.

After the Big Bang, only elements such as hydrogen, helium and some trace amounts of lithium existed in the universe. Heavier elements such as oxygen, nitrogen, carbon and iron – referred to as “metals” by astronomers – were forged in the extremely high-pressure centres of the first massive stars, which are predicted to have formed within 200 million years after the Big Bang. The metals were scattered across the cosmos when these first stars, known as “population III” stars, quickly burned out and exploded in supernovae. These explosions seeded the universe with the metals to form “population II” stars, which are still “metal-poor” compared with “population I” stars like the Sun.

Not the stars we are looking for?

A true first population-III star has not yet been discovered, although the best evidence for them was found earlier this year in an extremely bright and distant galaxy in the early universe. Astronomers believe that old metal-poor stars would have formed in the central regions or the “bulges” of galaxies, where the effects of gravity were the strongest. The Milky Way bulge underwent a rapid chemical enrichment in the early universe, and this should have created a host of metal-poor stars – indeed, we should find them there even today. However, metal-poor stars have only been found in the outer regions or the “halo” of the Milky Way and not at its centre.

Now, Louise Howes of the Australian National University in Canberra and an international team have used the SkyMapper telescope to identify nearly 500 extremely metal-poor stars in the Milky Way bulge.

SkyMapper telescope

The team also confirmed that most of these old stars are in tight orbits around the galactic centre, rather than being halo stars passing through the bulge. The researchers also found that the chemical compositions of these stars are, for the most part, similar to typical halo stars of the same metal content (or metallicity). However, some unexpected differences exist when it comes to the amount of carbon in such stars.

Stars with a low metal content look slightly bluer than others, so the team could sift through the millions of stars at the centre and whittle the observations down to 14,000 promising candidates. From those, the researchers identified 500 stars that had less than 100th the amount of iron in the Sun, making it the first extensive catalogue of metal-poor stars in the bulge. Of these, Howse and colleagues focused on 23 candidates that were the most metal-poor, and from these data, they homed in on nine stars with a metal content less than 1000th of the amount seen in the Sun. This includes one star with an iron abundance 10,000 times lower than that of the Sun – now the record-breaker for the most metal-poor star in the centre of the galaxy.

To and fro

To ensure that these stars were truly old – and not those that had formed much later in other parts of the galaxy that were not as dense and are now merely passing through the centre – the researchers used precise measurements and computer simulations to plot the stars’ movement in the sky. This allowed them to predict where the stars came from and where they were moving to. The team found that while some stars were indeed just passing through, seven of them were formed in the bulge and had remained there since.

“These pristine stars are among the oldest surviving stars in the universe, and certainly the oldest stars we have ever seen,” says Howes. “These stars formed before the Milky Way, and the galaxy formed around them.” While it is currently not possible to directly determine the ages of these ancient stars, the researchers say that it could be inferred from data collected by the extended Kepler mission or its successors.

The team’s discovery also challenges current theories about the environment of the early universe from which these stars formed. “The stars have surprisingly low levels of carbon, iron and other heavy elements, which suggests the first stars might not have exploded as normal supernovae,” says Howes. “Perhaps they ended their lives as hypernovae – poorly understood explosions of probably rapidly rotating stars, producing 10 times as much energy as normal supernovae.” If true, such hypernovae would be one of the most energetic things in the universe, and very different from the kinds of stellar explosions that we see today.

The research is published in Nature.

See the full article here .

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From physicsworld.com: “Gran Sasso steps up the hunt for missing particles”

physicsworld.com

Nov 11, 2015
Edwin Cartlidge

XENON1T

Physicists working at the Gran Sasso National Laboratory in central Italy, located 1400 m under the mountain of the same name, are soon to start taking data from two new experiments.

Gran Sasso

Each facility will target a different kind of missing matter: one will search for dark matter while the other will try and detect absent neutrinos to prove that neutrinos are their own antiparticle.

Dark flash

The hunt for dark matter – the mysterious substance believed to make up about 80% of all matter in the universe but not yet detected directly – will be carried out using XENON1T. This experiment, which was inaugurated at an event at Gran Sasso today, consists of 3.5 tonnes of liquid xenon. It is designed to measure very faint flashes of light that are given off whenever particles from the dark matter halo of the Milky Way collide with the xenon nuclei. The xenon will be stored at a temperature of about –100 °C in a cryostat and surrounded by a tank containing some 700 tonnes of purified water to minimize background radioactivity.

Run by an international collaboration of 120 students and scientists from more than 2 institutions, XENON1T is expected to be about 100 times more sensitive than its 160 kg predecessor experiment and around 40 times better than the world’s current leading dark-matter detector – the 370 kg Large Underground Xenon experiment in South Dakota, US.

LUX

Due to start taking data by the end of March next year, XENON1T will either detect dark matter or place severe constraints on the properties of theoretically-favoured weakly interacting massive particles (WIMPs), says collaboration spokesperson Elena Aprile of Columbia University in New York.

Dark heart

The other new experiment at Gran Sasso is the Cryogenic Underground Observatory for Rare Events (CUORE), which will look for an extremely rare nuclear process known as neutrinoless double beta decay.

CUORE

That decay, if it exists, would involve two neutrons in certain nuclei decaying simultaneously into two protons while emitting two electrons but no antineutrinos (unlike normal beta decay), implying that the neutrino is its own antiparticle. Due to turn on early next year, CUORE will measure the energy spectrum of electrons emitted by 741 kg of tellurium dioxide surrounded by radioactively inert lead blocks recovered from a Roman ship that sank 2000 years ago.

Meanwhile, towards the end of 2016 another group of scientists at Gran Sasso should take delivery of about a kilogram of cerium oxide powder, which they will place several metres below the Borexino neutrino detector.

Borexino

The Short Distance Neutrino Oscillations with BoreXino (SOX) experiment will look for a sinusoidal-like variation in the number of interactions generated within the detector by neutrinos from the radioactive cerium. SOX leader Marco Pallavicini of the University of Genoa says that such a variation would be a sure sign of “sterile” neutrinos – hypothetical particles outside the Standard Model of particle physics that would “oscillate” into ordinary neutrinos but would not interact with any other kind of matter.

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From physicsworld: “Why do physicists do experiments deep underground?”

physicsworld.com

Sep 23, 2015

To those who crave natural daylight, the idea of spending large chunks of time deep underground may seem like hell. But to particle physicists, this subterranean lifestyle is a price worth paying for the excellent radiation shielding provided by the overlying rock.

Art McDonald, who was a long-standing director of the Sudbury Neutrino Observatory (SNO), explains how these underground science labs are designed to detect the interesting particles that can make it through the layers of overlying rock.

An example is the neutrinos produced in the core of the Sun, whose properties can help to verify solar dynamics models. “We also make the surrounding areas really clean, avoiding the radioactivity contained in any mine dust that would potentially get into our experiments,” McDonald adds.

SNO has now expanded into SNOLAB, which covers a more diverse range of research. This includes the search for dark-matter particles and the hunt for a rare form of decay called neutrinoless double beta decay – a process that could help explain why the universe has significantly more matter than antimatter.

To find out more about subterranean physics, check out this feature article from the May 2015 issue of Physics World that looks at how deep underground laboratories of the world are no longer the scientific realm of astroparticle physics alone.

Access the video explaining Why Physicists Do Experiments Deep Underground in the full article.

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From IOP PhysicsWorld: “Physicists isolate neutrinos from Earth’s mantle for first time”

physicsworld.com

Aug 14, 2015
Hamish Johnston

Seeing the light: some of Borexino’s many light detectors

The first confirmed sightings of antineutrinos produced by radioactive decay in the Earth’s mantle have been made by researchers at the Borexino detector in Italy. While such “geoneutrinos” have been detected before, it is the first time that physicists can say with confidence that about half of the antineutrinos they measured came from the Earth’s mantle, with the rest coming from the crust. The Borexino team has also been able to make a new calculation of how much heat is produced in the Earth by radioactive decay, finding it to be greater than previously thought. The researchers say that in the future, the experiment should be able to measure the quantities of radioactive elements in the mantle as well.

According to the bulk silicate Earth model (BSE) model, most of the radioactive uranium, thorium and potassium in our planet’s interior lies in the crust and mantle. Accounting for about 84% of our planet’s total volume, the mantle is the large rocky layer sandwiched between the crust and the Earth’s core. Heat flows from the interior of the Earth into space at a rate of about 47 TW, but one of the big mysteries of geophysics is how much of this heat is left over from when the Earth formed, and how much comes from the radioactive decay chains of uranium-238, thorium-232 and potassium-40.

Peering deep underground

One way to settle the question is to measure the antineutrinos produced by these decay chains. These tiny particles travel easily through the Earth, which means that detectors located near the surface could give geophysicists a way of measuring the abundance of radioactive elements deep within the Earth – and thus the heat produced deep underground.

Back in 2005 physicists working on the KamLAND neutrino detector in Japan announced that they had detected 22 geoneutrinos, while Borexino, which has been running since 2007, reported in 2010 that it had seen 10 such particles.

KamLAND

Both detectors have since spotted more geoneutrinos and, taken together, their measurements suggest that about one half of the heat flowing out of the Earth is generated by radioactive decay, although there is large uncertainty in this value.

Italian adventure

The Borexino detector is made up of 300 tonnes of an organic liquid, and is located deep beneath a mountain at Italy’s Gran Sasso National Laboratory to shield the experiment from unwanted cosmic rays that would otherwise drown out the neutrino signal.

Gran Sasso National Laboratory

Whenever electrons in the liquid are struck by an antineutrino, they recoil and create a flash of light. In the latest work, Borexino physicists have analysed a total of 77 detector events, with the team calculating – using data from the International Atomic Energy Agency – that about 53 of these antineutrinos were produced by nuclear reactors.

The remaining 24 geoneutrinos could have come from either the Earth’s crust or its core. However, scientists have a pretty good idea of how much uranium and thorium are in the crust, allowing the Borexino physicists to say that half of these geoneutrinos were produced in the mantle and the other half in the crust. Furthermore, the physicists can say with 98% confidence that they have detected mantle neutrinos – a much greater level of confidence than achieved in previous studies.

The team also calculated the heat generated by radioactive decay in the Earth and found it to be in the 23–36 TW range. This is larger than estimates based on assumptions about the amount of radioactive elements in the Earth, which are in the 12–30 TW range, and also larger than an estimate based on previous antineutrino measurements.

The Borexino team also tried to work out what proportion of the geoneutrinos came from the uranium decay chain and what proportion from the thorium chain. Potassium decays were not considered because they are not expected to make a significant contribution to the numbers detected. The data suggest that the currently accepted ratio of thorium to uranium in the Earth is correct, but that the uncertainty in the Borexino values is very large. More data, the Borexino physicists say, should let them make more precise measurements of the contributions of uranium and thorium to the heating of the Earth.

See the full article here.

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From physicsworld: “How to efficiently capture carbon dioxide out of thin air”

physicsworld.com

Apr 16, 2015
Tamela Maciel

Captive gas: prototype carbon-collection system

A novel synthetic material that is a thousand times more efficient than trees at capturing carbon dioxide from the atmosphere was presented by Klaus Lackner, director of Arizona State University’s new Center for Negative Carbon Emissions, at a meeting of the American Physical Society in Maryland last Sunday. According to Lackner, the amount of carbon dioxide in the atmosphere has reached the point where simply reducing emissions will not be enough to tackle climate change. Referring to recent environmental reports, Lackner emphasized the need for prolonged periods of carbon capture and storage – also known as “negative carbon emission”.

Trees and other biological matter are natural sinks of carbon dioxide but they do not trap it permanently and the amount of land required is prohibitive. “There is no practical solution that doesn’t include large periods of negative emission,” says Lackner, adding that “we need means that are faster than just growing a tree.” During the past few years, Lackner and his colleagues have developed a synthetic membrane that can capture carbon dioxide from the air passing through it. The membrane consists of an “ion-exchange” resin – positive anions in the resin attract carbon dioxide, with a maximum load of one carbon-dioxide molecule for every positive charge. This process is moisture sensitive, such that the resin absorbs carbon dioxide in dry air and releases it again in humid air. As a result, this material works best in warm, dry climates.

Show and tell

Lackner plans to install corrugated collecting panels incorporating the membrane material on the roof of the Center for Negative Carbon Emissions this summer. The researchers hope that this public installation will demonstrate the economic feasibility and efficiency of a new technology that can address the issue of climate change, and help shift the debate from reduced carbon emissions to negative carbon emissions.

To keep costs low, the first step – capturing the carbon from the air – is free. “We made it cheap by being passive. We can’t afford to be blowing air around,” says Lackner. The resin itself is readily available and can be mass-produced, because it is already widely used to soften and purify water. The collectors trap between 10 and 50% of the total carbon dioxide that passes through. Compared with the amount of carbon dioxide that a typical tree collects during the course of its lifetime, these panels are a thousand times more efficient.

Able membrane: panels of carbon-capture resin

“I believe we have reached a point where it is really paramount for substantive public research and development of direct air capture,” says Lackner. “The Center for Negative Carbon Emissions cannot do it alone.”
Post trappings

Lackner estimates that about a hundred-million shipping-container-sized collectors would be needed to deal with the world’s current level of carbon emissions. As these collectors would typically become saturated within an hour, Lackner envisions a possible “ski-lift” approach where saturated panels are taken away to a humid environment to release their carbon dioxide and then recycled back to the dry air for more carbon capture.

The question also remains of what to do with the carbon dioxide once it is trapped. Burying it is one option, which is something Lackner says is likely, given the sheer quantity of carbon that must be captured. His centre is also testing ways to recycle the carbon dioxide and sell it to industries that could use it to make products such as fire extinguishers, fizzy drinks and carbon-dioxide-enhanced greenhouses, and even synthetic fuel oil.

See the full article here.

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