Skip to main content

Reset your password

Please enter the e-mail address you used to register to reset your password

Note: The verification e-mail to change your password should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder.

If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org

Registration complete

Thank you for registering with Physics World
If you'd like to change your details at any time, please visit My account

Close
Home » Page 1665

Cosmic radiation strings physicists along

“If you’re religious, it’s like seeing God.” So said George Smoot of Lawrence Berkeley National Laboratory in 1992 when he announced the discovery of tiny ripples in the radiation left over from the Big Bang. Smoot (who shared the 2006 Nobel Prize for Physics for the discovery) was referring to the possibility that fluctuations in the temperature of the cosmic microwave background as measured by the COBE satellite revealed the seeds of galaxies — and thus of stars, planets and ultimately life itself.

Since then, searching for patterns in this relic radiation has become a popular activity because it provides a snapshot of the universe when it was just 380,000 years old. Furthermore, COBE’s successor — the Wilkinson Microwave Anisotropy Probe (WMAP) — has increased the resolution of this snapshot enormously, opening a window onto the universe at even earlier times.

So far a mysterious cold spot some billion light-years across has turned up in the WMAP data, not to mention a series of hot and cold patches dubbed “an axis of evil” because it defies conventional explanation. But physicists in the UK and Switzerland now claim to have seen tentative signs of something slightly more tangible: “cosmic strings” that could be the relics of cosmic phase transitions that took place in the first trillionths of a second after the Big Bang.

Cosmological defects

The cosmic microwave background (CMB) is best understood via inflation, a period of rapid expansion that took place 10–35 seconds after the Big Bang. Inflation is thought to have blown up initial quantum fluctuations in space–time to cosmological scales, leading to the density perturbations and thus the hot and cold patches in the CMB seen today. Crucially, inflation also explains why the universe is so isotropic on the largest scales (the CMB is the same to 1 part in 105 in all directions). But the theory is somewhat ad hoc, and physicists would prefer to embed inflation in a more fundamental framework — such as grand unified theory (GUT), which unifies the electromagnetic, strong and weak forces, or superstring theory, which incorporates gravity as well as the other three forces.

Cosmic strings — massive 1D objects that can be billions of light-years long — arise naturally in such frameworks. They are predicted to form during phase transitions as the early universe expanded and cooled, and are analogous to crystal defects that form in ice when water is cooled below freezing. Such cosmic phase changes are associated with broken symmetries of nature, for example when the electroweak force (which is described by the Standard model of particle physics) separated out into the electromagnetic and weak forces that we experience today. Indeed, the breaking of such a symmetry, which would have formed a more complex cosmic defect called “texture”, has already been proposed to explain the giant cold spot in the WMAP data.

Detecting cosmic strings would therefore provide a direct link to the physics of the first few moments of the universe, bridging the ultimate gap between large and small. “A conclusive discovery would be momentous,” says Craig Hogan of the University of Washington.

String simulations

Neil Bevis and colleagues at the University of Sussex and Martin Kunz at the University of Geneva have now simulated the CMB on a computer using models with and without cosmic strings (Phys. Rev. Lett. 100 021301). Because cosmic strings would be present long after the initial quantum fluctuations, they would contribute an additional independent source of CMB perturbations to those from inflation. Comparing the models to the WMAP data, the team found that a model in which 11% of the contribution came from cosmic strings and the rest from inflation was preferred over one that contains no strings at all.

However, the agreement between the data and the model that contained strings (which were of the sort associated with GUTs rather than with superstring theory) corresponded to just two standard deviations. Roughly speaking, this means that if the researchers had datasets from 100 WMAP missions then they would find such “evidence” in about five of them purely due to random variations. “The authors have certainly not claimed a detection of cosmic strings,” says Tom Kibble of Imperial College in London, who pioneered the concept of cosmic strings in the late 1970s.

Limit on string tension

Pedro Ferreira of Oxford University says that although the statistical significance is small, it is still an interesting result because it provides an upper limit on the string tension. “In the context of GUTs, this tells you when unification might have happened or, more specifically, below which energy,” he says.

I would be delighted if there were a confirmed detection of cosmic strings. But we need a lot more evidence before getting too excited! Tom Kibble,Imperial College

However, Kibble also points out that because cosmic strings are expected to emit gravitational waves, such limits potentially conflict with more sensitive ones derived from pulsars because the very regular orbits of pulsars places an upper limit on the background intensity of gravitational waves. “I would be delighted if there were a confirmed detection of cosmic strings,” he says. “But we need a lot more evidence before getting too excited!”

Hopes for UK funding-cut reversal dashed

The UK government has launched a review into physics funding in the wake of a damaging £80m shortfall in the budget of Science and Technology Facilities Council (STFC) that emerged last month. The review had been promised after the STFC said the cuts will force it to end UK participation in the International Linear Collider (ILC), withdraw from the Gemini telescopes in Hawaii and Chile, and axe funding for solar-terrestrial physics.

However, physicists’ hopes that the review will lead to the cuts being reversed took a knock yesterday when Keith Mason, chief executive of the STFC, told the House of Commons Select Committee on Innovation, Universities and Skills that “the review is a valuable exercise but it was never intended to address the current situation.”

Priorities for investment

Research Councils UK (RCUK) — the umbrella body for the UK’s seven main science-funding agencies — has now agreed to the terms of reference for the review. To be led by the physicist Bill Wakeham who is vice-chancellor of Southampton University, the review will “consider the priorities for investment across physics as a whole”; “identify the contribution physics makes to other areas of research;” and “examine the provision of physics-based facilities and means of sustaining their operation”.

Wakeham will be assisted by nine other panel members. Although the names have not yet been finalized, they will include three members from the UK and one from both the US and Europe. The other four panel members will be taken from nominations made by the Institute of Physics, the Royal Society, the Royal Astronomical Society and the Royal Academy of Engineering.

Deep lack of trust

Physicists in the UK, who were quick to express outrage when the potential impact of the funding shortfall first emerged, have now started a campaign to have the cuts reversed. Last week, for example, 64 young researchers in solar-system science wrote a joint letter to Mason, expressing their “bewilderment and development of a deep lack of trust in, and respect for, STFC management”. The scientists also complained that the cuts — the impact of which is outlined in the STFC’s “delivery plan” — will destroy the government’s hopes of getting more students into physics.

It now seems clear that major mistakes have indeed been made in determining the [comprehensive-spending review] settlement for the new STFC John Dainton, Liverpool University

Particle physicists also claim to have obtained a series of documents under the Freedom of Information Act that show that the STFC had warned the government as far back as last July of the potential impact of the funding shortfall , which saw its budget rise by an average of just 4.5% a year to £652m by 2010/11.

“It now seems clear that major mistakes have indeed been made in determining the [comprehensive-spending review] settlement for the new STFC,” says John Dainton, a particle physicist at Liverpool University.

The Wakeham review aims to report its findings by autumn this year and then make recommendations to John Denham, Secretary of State for innovation, Universities and Skills.

Graphene breaks speed record

An international team of physicists has discovered that electrons move much more easily through graphene than any other known material. Their findings strengthen the belief of some researchers that graphene — which is a 2D sheet of carbon just one atom thick and a semiconductor — might be the best material for making electronic devices of the future.

Intrinsic electron mobility is a measure of how easily electrons move in a substance and boosting mobility is one way of making semiconductor devices smaller and run faster.

Now, André Geim of Manchester University and colleagues in Russia, the Netherlands and the US have found that the intrinsic mobility of graphene was around 200,000 cm2/Vs (Phys. Rev. Lett. 100 016602). This value is more than 100 times higher than that of silicon and over 20 times higher than gallium arsenide (1500 and 8500 cm2/Vs repectively).

“Our result is also counterintuitive,” Geim told physicsworld.com. “Scientists know that the thinner you make a material, the dirtier and less conductive it becomes. Graphene, which is a 2D sheet of carbon just one atom thick, is ultimately thin but has a higher electronic quality than any known material.”

Our work thus singles graphene out as the best possible material for electronic applications André Geim, Manchester University

Graphene is normally not perfectly flat but instead has a corrugated appearance and these corrugations vibrate as the graphene warms up. By measuring mobility as a function of temperature, the team concluded that scattering from these vibrations reduced the mobility of electrons at higher temperatures.

Its high intrinsic mobility (or electronic quality as it also known) means that graphene is the only material where electrons at room temperature can move thousands of interatomic distances without scattering. Geim said that although he knew that electrons in graphene could move long distances (longer than in conventional semiconductors), he did not expect that the material could outperform carbon nanotubes in this respect — or indeed the current record holder indium antimonide. “Our work thus singles graphene out as the best possible material for electronic applications,” he stated.

The researchers say that the intrinsic mobility could go even higher if the impurities in graphene were cleaned up — and if the corrugations could be removed. However, Geim stresses that mobilities any higher than around 20,000 cm2/Vs are not really needed for applications, like transistors, for example. However, cleaner samples with higher mobilities would come in very handy for fundamental research.

The high mobility value also means that graphene could be used to make devices that operate in the the terahertz region of the electromagnetic spectrum — something that is very difficult today. Terahertz radiation is important for a range of applications, including security and defence, medicine, astronomy and biological research. Terahertz radiation penetrates many materials (except metals) and so can be used to “see” through clothing and packages at airports, for instance.

Geim believes that graphene-based devices like chemical sensors and terahertz sources and detectors could be made within three to five years. However, two big challenges still need to be overcome before applications like graphene logic circuits see the light of day. First, high-quality wafers of the material need to be made and second, its on-off ratios are presently too low. “I have no idea when or even if these two hurdles could be circumvented,” he said.

String theorists and astrophysicist share Crafoord Prize

The theoretical physicist Edward Witten, mathematician Maxim Kontsevich and astrophysicist Rashid Alievich Sunyaev have been awarded the 2008 Crafoord Prize. Witten and Kontsevich share one half of the prize for their work on the mathematics of string theory, while Sunyaev gets the other half for his theoretical work on the cosmic background radiation and black holes.

The prize is worth a total of $500,000 and is awarded by the Royal Swedish Academy of Sciences in fields that are not covered by the Nobel prizes.

Witten, 56, is at the Institute for Advanced Study in Princeton, New Jersey, and is widely regarded as the leading figure in the development of string theory. String theory implies that “elementary particles” such as electrons and photons are just manifestations of a more fundamental layer of nature described by 1D strings 10–35 m in length. Originally formulated to describe the strong force acting on quarks and gluons, string theory soon became a potential “theory of everything” that could unify gravity with the other three forces in nature.

Maxim Kontsevich
Maxim Kontsevich

Representing particles as strings requires 10 or 11 dimensions, which makes it very difficult to do practical calculations such as what would happen when two electrons collide. Witten tackled this challenge by adapting the mathematics of particle physics to create new methods for performing calculations within string theory.

Witten’s approach was largely based on intuition and it was Kontsevich who was able to show that these new methods were mathematically sound. Kontsevich, 43, works at the Institut des Hautes Études Scientifiques outside of Paris

However, despite the mathematical rigour of string theory, it remains a highly contentious issue amongst some physicists — mostly because it has been impossible to test experimentally.

Rashid Alievich Sunyaev
Rashid Alievich Sunyaev

Sunyaev, 64, has joint appointments with the Space Research Institute of the Russian Academy of Sciences in Moscow and the Max Planck Institute for Astrophysics in Garching, Germany. In his work on the formation of black holes, Sunyaev was able to explain why the matter swirling into these extremely dense objects forms a thin disk that is an intense source of radiation. His insights into how to identify black holes from the radiation their disks emit have allowed astronomers to locate black holes.

Sunyaev was also cited for his work on the cosmic microwave background (CMB) — radiation that was born when the universe was about 380,000 years old and can still be detected today. This radiation is not uniform and Sunyaev has made important contributions to the understanding of how massive sound waves that propagated through the early universe gave rise to temperature fluctuations in the CMB. He also developed, with the late Yakov Zel’dovich, a theory that describes how photons in the CMB change energy when they scatter off the hot gas contained within clusters of galaxies. Now known as the Sunyaev-Zel’dovich effect, it allows astronomers to detect galaxy clusters extremely far away from Earth.

The prize will be presented in Stockholm on 23 April.

Canada replaces nuclear-safety chief

The physicist Michael Binder has taken over as president of the Canadian Nuclear Safety Commission (CNSC). A senior civil servant, Binder replaces Linda Keen, who was fired on Tuesday by the Canadian government for her role in last year’s extended shutdown of the Canadian reactor that produces more than half the world’s supply of medical isotopes. Keen remains a member of the CNSC, which oversees the safety of Canada’s nuclear reactors.

Keen was sacked following a very public disagreement with Gary Lunn, Minister of Natural Resources in Canada’s Conservative minority government, who is responsible for the CNSC. Friction between the two arose last month after the CNSC apparently refused to allow the National Research Universal (NRU) reactor in Chalk River, Ontario to resume operations following a scheduled shutdown for maintenance. The CNSC had been concerned that the reactor’s owner, Atomic Energy of Canada Limited (AECL), had not made certain modifications to the reactor’s emergency power system.

Medical treatment delayed

NRU produces North America’s entire supply of molybdenum-99 — from which the radioactive isotopes technetium-99 and iodine-131 are made — and the shutdown caused delays to the diagnosis and treatment of thousands of seriously ill patients. In a surprise move, the Canadian Parliament overruled the CNSC, and ordered AECL to restart the reactor without completing the modifications.

If we had not acted, people would have died Gary Lunn, Minister of Natural Resources

Lunn defended his firing of Keen and his overall handling of the situation yesterday in an appearance before the House of Commons Natural Resources Committee in Ottawa. “The extended shutdown of the reactor threatened a national and international health crisis,” he said. “If we had not acted, people would have died.” He added that “She has lost the confidence of the government.”

Linda Keen
Linda Keen

After saying that she would appear at the hearing, Keen chose not to do so. However, she had made her views known last week in a written exchange with the minister.

Lunn told Keen that the issue has “cast doubt on whether you possess the fundamental good judgment required by the incumbent of the office of President of the Commission, and whether you are duly executing the requirements of the office”.

Keen responded that “Any objective assessment of the facts will reveal that the allegations contained in your letter are entirely without merit”. She pointed out that “the CNSC did not order or force AECL to shut down, or extend the shutdown…[T]he decision was made by AECL’s senior management.” Keen added that Lunn’s requirement that “we, an independent, quasi-judicial administrative tribunal, answer you about this case [is an example] of improper interference with both the institutional independence of the CNSC and with the administration of justice.”

‘Blatant political interference’

Opposition MPs took up that point in yesterday’s committee hearing. Omar Alghabra of the Liberal Party accused Lunn of “blatant political interference.” Claude DeBellefeuille of the Bloc Québécois added that Lunn had “shaken the confidence that people should have in this independent watchdog for nuclear safety.”

For many in the medical physics community, the decision involved balancing potential problems that might stem from malfunction of the reactor against difficulties created by the loss of medical isotopes. “It is always difficult to justify the government getting involved in the actions of the regulator,” said Alexander McEwan, president of the [US-based] Society for Nuclear Medicine and director of the imaging department at the Cross Cancer Institute in Edmonton, Alberta. “But the other side is: How do you weigh the risk and benefit?” He added: “We’re delighted that the NRU is back.”

McEwan emphasized the role of organizations such as the Society for Nuclear Medicine in such debates. “Any professional medical society has to ensure that the public, the regulator, and the government are appropriately educated in the benefits and risks of radiation at every level of the production chain,” he said. “We’ve been trying to do that for ten years, and will continue to do so.”

‘Cloak of silence’ design is unveiled

Two independent teams of researchers have come up with a recipe for making special materials that could completely cloak an object from sound. Although the “acoustic metamaterials” have yet to be made, a third team is now trying to create a real cloak.

These metamaterials promise to guide sound waves around an enshrouded object as if the object wasn’t there. As well as being used to conceal submarines from detection by sonar, such metamaterials could be used to improve the acoustics in concert halls.

Electromagnetic cloaks

Over the past few years physicists have shown that it should be possible to create materials that can be used to cloak objects from electromagnetic radiation such as microwaves or light. Indeed, in some very special situations, such “invisibility cloaks” have actually been built.

Sound propagates as waves and physicists had suspected that it should be possible to create similar “acoustic cloaks”. For example, it is relatively easy to show mathematically that sound waves impinging on a cylindrically-shaped acoustic cloak behave in much the same way as electromagnetic waves on a cylindrically-shaped invisibility cloak.

However, cylindrical cloaks only work for waves travelling perpendicular to the axis of the cylinder — which is why they are called “2D” cloaks. A true “3D” cloak that would work for waves coming from all directions would have to be spherical in shape. Until recently, some physicists had argued that 3D acoustic cloaks were a mathematical impossibility — but now, two independent teams of researchers have shown that it should be possible

Density and compressibility

One team, led by Steven Cummer at Duke University in the US, used the mathematics describing how sound scatters from materials to come up with a recipe that defines the density and compressibility of a spherical shell that would act as an acoustic cloak (Phys. Rev. Lett. 100 024301). These properties would have to vary across the thickness of the shell and crucially, would have to appear to be different for sound waves moving in different directions. No natural materials have such properties, and therefore such a cloak would have to be made from a “metamaterial” that contains manmade structures.

The second team — Huanyang Chen and Che Ting Chan of the Hong Kong University of Science and Technology — managed to show that the equations describing a spherical acoustic cloak were indeed the same as those describing a spherical electromagnetic cloak (Appl. Phys. Lett. 91 183518). This led them to the same recipe proposed by Cummer and colleagues.

Matrix of cylindrical rods

The considerable challenge of building an acoustic cloak has been taken up by José Sánchez-Dehesa and colleagues at the Polytechnic University of Valencia in Spain. The team has calculated that a cylindrical cloak can be made by surrounding the region to be cloaked with a matrix of cylindrical rods (New Journal of Physics 10 023004). By choosing rods with the right elastic properties and by varying the radius and spacing of the rods, Sánchez-Dehesa believes that cloaking can be achieved over a wide range of acoustic frequencies. The team are now looking for an appropriate material for the rods.

Sánchez-Dehesa told physicsworld.com that the theoretical breakthroughs in the US and Hong Kong mean that it should be possible to build a spherical cloak by surrounding a region with a matrix of spheres in a manner consistent with the recipe proposed by Cummer, Chen and Chan.

In Sánchez-Dehesa’s metamaterial, the voids between the rods or spheres would simply contain air, which means that it be used to cloak sound waves in air. Such a metamaterial could be used, for example, to coat surfaces in concert halls in order to guide sound away from problem areas. If the voids were filled with a material with the same acoustic properties as water, the metamaterials could in principle be used to cloak submarines from sonar detection systems.

Pope calls off university visit

The protests of nearly 70 scientists, including former CERN director general Luciano Maiani, have forced Pope Benedict XVI to cancel tomorrow’s visit to La Sapienza University in Rome. The scientists, who expressed their objections in a joint letter to the university’s rector earlier this week, deemed the visit would be “incongruous” with the Pope’s previous support of the persecution of Galileo in the 17th century.

The Pope had intended to visit La Sapienza, which was founded in the 14th century by Pope Boniface VIII, to address the university at the start of the new academic year. However, the Vatican have had to abandon the event amid mounting protests from staff. Students had also been separately organizing an “anti-clerical week”. “Following the well known incidents of recent days…it was felt necessary to cancel the occurrence,” the Vatican said in a formal announcement yesterday.

Marcello Cini, a particle physicist at La Sapienza, was the first member of staff to write a letter of dissent to the rector Renato Guarini, who had organized the visit. Many of Cini’s colleagues quickly chose to back his protest by signing another letter highlighting the Pope’s contentious views on the trial of Galileo. They recalled a speech made in 1990 by the Pope, then known by his baptismal name Joseph Ratzinger, in which he quoted the Austrian science philosopher Paul Feyerabend: “In the age of Galileo the Church showed to be more faithful to reason than Galileo himself. The trial against Galileo was reasonable and just.”

The joint letter reads: “These are words that offend and humiliate us as scientists who are loyal to reason and as teachers who have dedicated our lives to the advance and dissemination of knowledge.” The letter was signed by 67 of the university’s scientists including Luciano Maiani, theorist Giorgio Parisi and Andrea Frova, author of a study into the Church’s persecution of Galileo. According to Frova, however, ten times more staff agreed with its contents but didn’t sign because they had executive duties.

The views have not been supported by Italian politicians. “No voice should be stifled in our country, least of all the Pope’s,” Romano Prodi, Italy’s prime minister, has said. Meanwhile, the mayor of Rome has described the withdrawal of the invitation as “paradoxical”.

The outburst is unusual for a country known for its religious devoutness. But Italian scientists see the Pope’s views on science as a backward turn on his predecessor John Paul II, who conceded that the Church was wrong to attack Galileo for saying that the Earth revolved around the Sun.

Cosmic explosion, but no gravitational waves

Physicists searching for gravitational waves with the LIGO detector in the US have released their first major scientific result. Oddly enough, however, it stems from having detected no gravitational waves at all.

Instead of heralding the much-anticipated first direct detection of these tiny ripples in space–time, the team announced that gravitational waves did not appear to emanate from the source of a gamma-ray burst detected last year. The LIGO team has used this apparent absence of gravitational waves to gain further insight into the origins of the dramatic astrophysical events that produce intense bursts of gamma rays.

“I wish that the first major announcement were a detection of gravitational waves, but this is not the primary goal of our field,” Kip Thorne of Caltech told physicsworld.com. Thorne, who is a long-time member of the LIGO team, also said: “As I see it, that goal is to open up the gravitational wave window onto the universe so that we can explore poorly understood processes. The LIGO non-observation is in that spirit.”

Disturbances in space–time

Gravitational waves are predicted by Einstein’s general theory of relativity, in which gravity arises from the curvature of space–time. The waves are oscillations of space–time that are produced when a mass accelerates. However, despite strong indirect evidence for their existence — in particular from measurements of the rate at which neutron stars in binary systems lose energy and spiral towards one other (a result that earned Russell Hulse and Joe Taylor the 1993 Nobel Prize for Physics) — there is no direct proof. This is partly because their amplitude is so small, with even the most violent astrophysical events disturbing space–time by less than one part in 1022.

LIGO (the Laser Interferometer Gravitational-wave Observatory) is the largest of several facilities designed to detect such disturbances. It comprises two giant interferometers, one located at Hanford, Washington state, and the other at Livingston in Louisiana. By bouncing a laser off mirrors located at the ends of two 4 km-long arms at right angles to one another, any changes in the relative lengths of the arms caused by the passage of a gravity wave would produce a characteristic interference pattern.

Crucially, LIGO’s Hanford interferometer was in “science mode” on 1 February last year, when several space telescopes registered a short burst of gamma rays in the direction of the nearby Andromeda galaxy.

First glimpsed 40 years ago, gamma-ray bursts (GRBs) are among the most energetic and mysterious events in the universe. They come in two broad types: “long”, lasting between 2 s and a few minutes; and “short”, lasting from a few milliseconds to 2 s. In 2003 researchers successfully traced the former to supernovae, but astrophysicists are only beginning to understand the origins of short GRBs.

Colliding black holes

The leading candidate for the majority of short GRBs is the merger of two ultra-dense objects such as neutron stars or black holes — events that should also produce a burst of gravitational waves. However, at a conference on GRBs held in Santa Fe last November, the LIGO team announced that its interferometers had detected no such signature at the time when “GRB070201” went off.

Either the source was not a coalescing binary or there is some exotic situation where the gravitational waves disappear into another dimension Jim Hough, Glasgow University

“We know that coalescing binary have to produce gravitational waves,” says Jim Hough of Glasgow University , who is principle investigator for the UK of the GEO600 gravitational wave detector based in Hannover, Germany. “Therefore, either the source was not a coalescing binary or there is some exotic situation where the gravitational waves disappear into another dimension. The latter seems unlikely, but would be very exciting of course!”

Other causes for the event, such as a “soft gamma ray repeater” (SGR) or a binary merger from much further away, are now the most likely contenders. However, Stan Woosley of the University of California at Santa Cruz — who was one of the first to link long-lived GRBs with supernovae — points out that the merger of neutron stars is excluded only to the 90% level, which is not as tight as astrophysicists would like. “If the event was indeed in Andromeda, it was likely a SGR. The likelihood of two neutron stars merging in this nearby galaxy while we happen to be watching is perhaps one in a million years,” he says. “However, the result is a technological tour de force which illustrates the potential of coordinated gravity wave and gamma-ray observations.”

The result has recently been accepted for publication in the Astrophysical Journal.

Nanowires convert heat to electricity

Silicon — a staple of modern electronics — could soon be used to make low-cost devices that convert waste heat to electricity. That is the bold future outlined by two independent teams of scientists in the US, who have shown that arrays of tiny silicon wires have extremely good “thermoelectric” properties. The findings could lead to the development of cheap thermoelectric materials that boost the efficiency of both massive coal-fired generators and tiny solar cells.

Many methods of generating electricity — such as burning fossil fuels and nuclear fission — create vast quantities of waste heat. Thermoelectric materials that convert heat directly into electricity could someday be used to recover some of this energy, thereby boosting the efficiency of conventional power stations. Thermoelectric materials could also improve the effectiveness of solar cells and generate electricity from other sources of waste heat such as computer chips and refrigerators.

Low efficiency

Unfortunately, today’s thermoelectric materials are synthetic nanostructures that are very expensive to make and are nowhere near efficient enough to be used commercially.

Some researchers believe that silicon-based thermoelectric materials could overcome these limitations because silicon is easy and inexpensive to work with and has the right electrical properties to be a thermoelectric material. However, silicon is also a good conductor of heat. A thermoelectric material converts a temperature difference across a material into a voltage and silicon’s high thermal conductivity means that a large amount of heat is required to create a small temperature difference — making it a very inefficient thermoelectric material.

Factor of 100

Now, two independent teams in California have worked out a way to boost the thermoelectric efficiency of silicon by as much as a factor of 100. Both results are reported in the journal Nature.

Allon Hochbaum and colleagues at the University of California, Berkeley, created arrays of tiny silicon nanowires by dipping silicon wafers into an aqueous solution of silver ions. The nanowires were 20–300 nm in diameter and the team discovered that arrays have a thermoelectric efficiency about 60 times greater than bulk silicon at room temperature.

Rough surfaces

They believe that this rise in efficiency occurs because heat-carrying sound waves called phonons have a very difficult time moving along the very narrow nanowires, reducing their ability to conduct heat. One important feature of Berkeley nanowires is that their surfaces are rough, which the researchers believe contributes to their high thermoelectric efficiency, although the physics behind this remains unclear.

Meanwhile, at the California Institute of Technology, Akram Boukai and colleagues have seen a similar effect in even smaller rectangular nanowires with cross-sections of 10 × 20 nm and 20 × 20 nm. The team measured a 40-fold boost in efficiency over bulk silicon at room temperature. This increased to a 100-fold boost at -73 °C.

Intriguingly, their observations suggest that another phenomenon — called “phonon drag” — also plays a role in boosting the thermoelectric efficiency. Phonon drag occurs when phonons moving through the silicon collide with charge carriers such as electrons and drag them along.

Ekpyrotic cosmology resurfaces

A six-year-old controversial theory that told of a time before the Big Bang is undergoing a resurgence, only to be lampooned again by its original critics.

The theory of ekpyrotic cosmology was first put forward as an alternative to the standard “inflation” model of the universe. Inflation supposes that just after the Big Bang the universe underwent a brief period of rapid expansion. This amplified tiny density perturbations, which evolved into the stars, galaxies and galaxy clusters we see today. Although there is currently no way to prove that inflation ever occurred, the fact that it provides such a simple explanation for cosmic structure and the flatness of the universe has cemented it in cosmological doctrine since it was outlined in the early 1980s.

In ekpyrotic cosmology — which was proposed in 2001 by physicists Paul Steinhardt, Justin Khoury, Neil Turok and Burt Ovrut — there is no beginning of time. Instead, our visible universe exists on one of two four-dimensional “branes” floating in a five-dimensional space. These two branes are locked in an endless oscillatory motion in which they creep together, “bounce” through each other, withdraw and then creep together again (see animation: Branes collide). At each bounce, which is like a fresh Big Bang, ripples in the branes collide and liberate energy at different places to produce the initial density perturbations. And because energy conservation would favour a flat brane, the theory explains why our visible universe is flat too. “It’s like the antichrist to inflation,” says Khoury.

Past critics of ekpyrotic cosmology, however, have highlighted two particular problems. First, every bounce would be a point of infinite temperature and pressure known as a singularity, which is impossible to describe using conventional physics. Second, it was not clear that the density perturbations would be present on all length scales, as observations of the primordial radiation left over from the Big Bang — that is, the cosmic microwave background — indicate they should be.

‘From A to Z’

It’s like the antichrist to inflation Justin Khoury, Perimeter Institute

Now, Khoury and Evgeny Buchbinder, who are currently at the Perimeter Institute in Waterloo, Canada, with Ovrut, who is at the University of Pennsylvania in the US, say they have overcome these problems. Building on work performed in 2006 by Leonardo Senatore, Paolo Creminelli and others, the researchers say they can bring density perturbations “unscathed” through a bounce while leaving them scale-invariant on the other side. “For the first time we’re able to track the story from A to Z,” says Khoury.

Physicists have managed to get around the sharp bounce singularity in the past, but not without generating undesirable negative-energy particles. By merging ekpyrotic cosmology with a scalar field known as a ghost condensate, Khoury’s group claim to suffer no such complications. This field, which manifests as a fluid of massless particles in the same quantum state that permeate space with repulsive gravity, can smooth out the bounce.

On its own, though, a ghost condensate cannot keep the density perturbations scale-invariant. To do this, Khoury’s team invoke another scalar field. It turns out that the difference between the entropy fluctuations in this field and those the ghost-condensate field yields unambiguously scale-invariant perturbations (Phys. Rev. D 76 123503).

Stiff opposition

These ideas are barking up the wrong tree Nima Arkani-Hamed, Harvard University

The original criticisms of ekpyrotic cosmology were spearheaded by Andrei Linde of Stanford University in the US, and he has little praise for “new ekpyrotic cosmology” either. He claims that the theory is still plagued by negative-energy particles, which lead to a “catastrophic” instability in the early universe’s vacuum (arXiv:0712.2040v2). “The universe described by this theory instantly dies,” he told physicsworld.com. “Therefore it does not even come close to describing our world.”

Khoury insists that Linde’s argument is wrong because he does not acknowledge that particle physics theories are effective, in that they only ever work over a finite range of energy scales. “He’s predicting something by trusting the theory beyond the point where it should be trusted,” explains Khoury. “By contrast, at all times in our description of the bounce, the universe is at a sufficiently low energy that we can trust the ghost condensate description.”

Nima Arkani-Hamed of Harvard University, who first formulated the theory of ghost condensates, sides with Khoury’s group. Nevertheless, he thinks that new ekpyrotic cosmology is too complicated to rival inflation, and admits that he has “poked fun” at Senatore and Creminelli — his former post-docs — for their attempts to tackle the theory. “I believe these ideas are barking up the wrong tree,” he told physicsworld.com. “But they are definitely worth working with, they are interesting and clever.”

Copyright © 2026 by IOP Publishing Ltd and individual contributors