Cosmic Web filled with Dark Energy

Barren bubble-like zones could help untangle universe's mysteries.

 Sometimes nothingness can reveal a whole lot.

While the universe is mostly empty, it contains bubble-like voids that are even emptier, taking up most of the space in the cosmos. And new research shows that these voids all look similar regardless of size — a consistency that may help unravel some of the universe's biggest mysteries.

If you zoom way out, all the matter in the universe looks like a huge cobweb, consisting of an expansive network of filaments and wall-like structures that crisscross one another.

More than 80 percent of this matter is dark matter, the invisible and mysterious stuff that appears to interact only gravitationally with the regular matter that makes up stars and galaxies. Residing in these filaments and walls of dark matter are galaxies, and the densest regions — where the filaments intersect — are sites of massive clusters of hundreds to thousands of galaxies.

But between the filaments and walls of this cosmic web are vast cosmic voids. There are stray galaxies here and there, but overall, these bubble-like voids are about 80 percent less dense than the cosmic average. There can even be voids within a void, producing a nested structure.

Voids may be filled with nothingness, but researchers are now realizing that these empty spaces may actually contain clues for solving cosmic riddles like the nature of dark matter and dark energy, the puzzling force that's accelerating the expansion of the cosmos.

"Normally people look for the presence of something — and that's very important," said Joseph Silk of the Paris Institute of Astrophysics. "But sometimes it's also important to measure the absence of something. The absence of galaxies is just another way of looking at the universe."

Astronomers discovered cosmic voids along with the cosmic web in the late 1970s. Since then, most of the focus has been on studying the cosmic web itself. But that's been changing over the last few years, said Rien van de Weijgaert of the University of Groningen in the Netherlands.

Prompted by the discovery of dark energy, more astronomers are exploring these voids. They have combined recent observations that map the three-dimensional distribution of hundreds of millions of galaxies with increasingly better computer simulations.

"There has been an explosion of interest," van de Weijgaert said.

A new study now shows that, on average, voids have similar features regardless of their size. A team of astronomers led by Nico Hamaus of the Paris Institute of Astrophysics used a computer simulation to generate tens of thousands of cosmic voids, which were then analyzed to devise a model that describes their average characteristics.

Voids do contain small amounts of mass, and the model describes how this mass is distributed within them. The density is lowest at the center and increases farther out — reaching a maximum where the matter becomes part of the cosmic web — before dropping off again. According to the analysis, this model can be applied to all cosmic voids.

"I was surprised by the simplicity and universality of this simple prescription to model the voids," Hamaus said. The study was published in the June 27 issue of Physical Review Letters.

Other researchers have developed similar descriptions of voids, but the new study takes it a step further by generalizing the findings to all voids, van de Weijgaert said. And that means voids can serve as a powerful tool for probing dark energy.

"That's the hope," Hamaus said. "We haven't proven this yet, but we have a feeling that [this model] could be useful."

Dark energy is thought to be a kind of constant energy that's embedded within the vacuum of the universe. Because voids are so empty — and thus contain minimal gravity, for instance — the effects of dark energy should dominate, making these empty bubbles a pristine place to study the mysterious force.

The fact that voids seem to have consistent features means researchers can apply the model to observations of not just one or two voids, but as many as they can measure. By comparing the model to observations, researchers can then test theories of dark energy.

Some astronomers have even suggested that studying cosmic voids could be the most accurate technique yet for measuring dark energy, van de Weijgaert said.

The new model, Silk added, could also be a tool for testing theories of cosmic inflation — the idea that the universe expanded extremely rapidly within the first moments after the Big Bang. According to theory, the pre-inflationary universe contained tiny, random fluctuations. Thanks to inflation, these fluctuations ballooned to eventually become the voids, filaments, and walls of the cosmic web.

Theory predicts that because the fluctuations were random, there should be a similar number of regions of high density as there are regions of low density. The new model can help predict if this is really the case, Silk said.

As for why cosmic voids would all be so similar in the first place, no one's sure yet, according to van de Weijgaert. But, he suggests that it's likely related to the fact that the density fluctuations in the early universe were fairly uniform.

There's a lot more to learn about cosmic voids and what they may reveal about the universe, he said. "It's certainly not the end of the story."

New Technology Could Boost Solar Cell Efficiency By 30 Percent

 A decades-old discovery may be crucial to increasing solar panel efficiency.

 Scientists looking to boost the efficiency of solar panels are taking a fresh look at an exotic physics phenomenon first observed nearly 50 years ago in glowing crystals.

Called singlet fission, the process can enable a single photon of light to generate two electrons instead of just one. This one-to-two conversion, as the process is known, has the potential to boost solar cell efficiency by as much as 30 percent above current levels, according to a new review paper published in the Journal of Physical Chemistry Letters.

Singlet fission "was originally proposed to explain some weird results that were observed in fluorescent organic crystals," said the study's first author Christopher Bardeen, a chemist at the University of California, Riverside. "It received a lot of attention in the 1960s and 1970s, but then it was mostly forgotten."

But beginning around 2006, Bardeen and other scientists exploring new ways to boost the solar-energy conversion rates of photovoltaic panels began taking a renewed interest in singlet fission. In recent years, experiments conducted by Bardeen's group not only helped confirm that the phenomenon is real, but also that it can be highly efficient in a variety of materials. The hope is that singlet fission materials can be incorporated into solar panels to increase their energy conversion efficiency–the ratio of electrons produced to the amount of photons absorbed–beyond the current theoretical ceiling of approximately 32 percent, which is called the "Shockley-Queisser Limit."

"The efficiency of most commercial-grade PV panels, like the ones you would install on your house, are around 20 to 25 percent," Bardeen said.

Engineers have managed to overcome the Shockley-Queisser Limit through clever engineering to boost the efficiency of photovoltaic, or PV, panels up to 50 percent – for example, one technology, called multi-junction solar cells, involves combining two or more semiconductor panels. But such technologies are currently limited mostly to military and space applications due to their high costs.

"It may be possible to find a way to make [multi-junction cells] cheaply … Some companies are trying to do this, but without much impact so far," Bardeen said.

Many scientists believe the only way the next wave, or "third generation," of photovoltaic technology will surpass the Shockley-Queisser Limit while remaining inexpensive is if they make use of new physical processes such as singlet fission.

"First generation solar cells were based on silicon, and they were efficient but expensive. The second generation cost much less and was based on thin-film technology. The goal of the third generation is to keep cost down but get efficiency as high as possible," Bardeen said.

Currently, solar cells work by absorbing a photon and generating an exciton -- a bound electron with a negative charge and a positively charged "hole" -- which subsequently separates into an electron-hole pair. The electrons are then harnessed as electricity. In singlet fission, however, some photons -- those with higher energy -- get converted into two excitons, each of which can split to yield two electrons. Bardeen's team estimates that singlet fission can boost efficiency of solar cells by up to 30 percent, resulting in a maximum efficiency of above 40 percent instead of the current 32 percent.

Experts predict that it could be another 5 to 10 years before solar panels based on singlet fission technology are ready for commercial use. Before that can happen, scientists will need to gain a much better understanding of how singlet fission works, said Josef Michl, a photophysicist at the University of Colorado, Boulder, who helped revive interest in singlet fission several years ago. At the moment, the main challenge for researchers trying to create a singlet fission solar panel is "a thorough understanding of the underlying physics that should allow chemists to come up with more practical materials than the few that we now know to work well in the laboratory," said Michl, who was not involved in the study.

Michl called Bardeen's group a "key player" in the worldwide effort to develop the technology, and said that his team's experimental work has helped singlet fission shed its "reputation of an obscure and inefficient phenomenon."

The other primary hurdle toward a functional singlet fission solar panel will be one of engineering, Bardeen said. Once more materials that can undergo singlet fission are developed, they will still need to be incorporated into photovoltaic cells to convert solar energy into electricity. Researchers led by Marc Baldo at the Massachusetts Institute of Technology recently reported that they had proven that it was possible to create a solar panel that uses singlet fission, but the efficiency of their device was only 2 to 3 percent.

"Baldo's group showed that it could be done," Bardeen said, "but nobody's going to be putting those on rooftops tomorrow."A

How Quantum Mechanics Helps Us Breathe

New insights explain how respiration does not result in asphyxiation.

 Why don't we suffocate whenever we try to take a breath? An international team of scientists has used quantum mechanics – the science that usually deals with events at the level of the ultra-small – to solve this human-sized mystery.

Quantum mechanics has long proved its value in understanding such phenomena as the behavior of electrons and in classifying subatomic particles. But in recent years theorists have increasingly shown how it applies to all facets of life, large and small.

The new research, led by Cédric Weber of Kings College, London and reported in the journalProceedings of the National Academy of Sciences, confirms that point.

"This work," said team member David O'Regan, a physicist at Ireland's Trinity College, Dublin, "helps to illustrate the fact that quantum-mechanical effects, which may sometimes be viewed as somehow very exotic or only relevant under extreme conditions, are at play in the day-to-day regimes where biology, chemistry, and materials science operate."

The conundrum elucidated by Weber's team stems from the way in which carbon monoxide reacts with proteins that carry oxygen around our bodies.

Those proteins, which contain iron atoms, transport oxygen molecules through the bloodstream to wherever the body needs them. According to conventional theory, the proteins should typically link up more often with molecules of carbon monoxide – from inside and outside the body – than with oxygen molecules.

If that happened regularly, it would result in asphyxiation, killing off humans and animals.

The small amounts of carbon monoxide naturally produced in our bodies would not be enough to displace oxygen fully, even if it had a greater binding ability. But we would be more vulnerable to poisoning by carbon monoxide in the atmosphere than experience shows to be the case. The fact that this doesn't happen means that oxygen molecules bind more effectively to proteins than theory forecasts.

"The problem that scientists have had is explaining how the proteins achieve this discrimination in favor of oxygen," said team member Daniel Cole, a chemist at Yale University in New Haven, Connecticut.

To do so, the team applied a computer simulation technique based on quantum mechanics to reactions of oxygen and carbon monoxide with myoglobin, the main oxygen-carrying protein in muscle tissue.

The technique, called density-functional theory, or DFT, won the 1998 Nobel Prize in Chemistry for its progenitor, Walter Kohn of the University of California, Santa Barbara. Since then it has become a bulwark of theoretical chemistry and physics.

"DFT has been the standard tool for simulating electronic properties of materials and molecules for a number of years," O'Regan said.

The team used the technique to study reactions between the iron atom inside myoglobin and a molecule of oxygen or carbon monoxide. These reactions involve electrostatics, the arrangement of electric charges in atoms and molecules. When the iron atom transfers negative electric charges to an oxygen or carbon monoxide molecule, it enables the molecule to attach itself to the entire myoglobin protein.

Unfortunately, the theory consistently predicted that carbon monoxide should bind to myoglobin much more readily than oxygen.

"Our previous DFT calculations had shown that there is a transfer of about half an electron to the oxygen molecule," Cole noted. "Although this provided some stabilization, it was not enough; the calculations predicted that carbon monoxide should be much, much more strongly bound than oxygen."

In response, the team employed two fresh approaches to the problem.

Because myoglobin molecules contain more than 1,000 atoms, the scientists used a special variety of DFT that, according to O'Regan, "is designed for dealing with larger systems without compromising on accuracy."

They also applied another extension of DFT, called dynamical mean-field theory.

"Using DMFT, we showed that, in fact, close to one electron is transferred to the oxygen molecule," Cole explained. "This provides much greater electrostatic stabilization than previously thought. It means that our estimate of the relative binding of oxygen and carbon dioxide is now in excellent agreement with experiment."

The analysis revealed that an effect called entanglement plays a critical role in binding oxygen molecules to the protein. Entanglement is a quintessential characteristic of quantum mechanics that links pairs of electrons so strongly that they no longer act independently. The process also involves Hund's exchange, another quantum-mechanical property that previous simulations had ignored.

"These effects strengthen the direct bonding between iron and oxygen, and also enhance electrostatic interactions with the protein," Cole explained.

The overall result: "We significantly improve the agreement of theory with experiment in terms of the relative tendencies to bind oxygen and carbon monoxide," O'Regan said.

The research has potential uses beyond understanding the molecular basis of breathing. According to Cole, the better understanding of how molecules bind to iron-containing proteins could help the drug-development process and possibly facilitate the design of artificial photosynthesis devices that would capture and store energy from the sun.

Remote Controlled Nanoscale protein motor !!

Zev Bryant, an assistant professor of bioengineering at Stanford, 

and his team are creating tiny protein motors that can be controlled remotely by light.

In every cell in your body, tiny protein motors are toiling away to keep you going. Moving muscles, dividing cells, twisting DNA – they are the workhorses of biology. But there is still uncertainty about how they function. To help biologists in the quest to know more, a team of Stanford bioengineers has designed a suite of protein motors that can be controlled remotely by light.

"Biology is full of these nanoscale machines that can perform complex tasks," said Zev Bryant, an assistant professor of bioengineering and leader of the team. "We want to understand how they can convert chemical energy into mechanical work and perform their specific tasks in cells."

Bryant's team, including doctoral student Muneaki Nakamura, designed blueprints for protein motors that would respond to light. Splicing together DNA from different organisms such as pig, slime mold and oat – the oat had the light-detecting module – the bioengineers created DNA codes for each of their protein motors.

The remote-controlled nanomotors are described by Nakamura, Bryant and their colleagues in a paper that appeared online Aug. 3 in Nature Nanotechnology.

When exposed to light, the new protein motors change direction or speed. "It's pretty fine spatial control; you can decide where the light is and where it isn't and control motors in this very exquisite way," Bryant said. Being able to control the motors in real time should be a boon for cell and developmental biologists trying to study forces and motion in living things.

"It's an entirely new project for us to be moving inside cells and organisms and to be working more closely with biologists," Bryant said. Now that he and his team have a basic blueprint, they will be able to customize these motors for biologists who are looking into specific tasks.

"In a future phase of our research, I hope that we can provide cell biologists with tools that allow them to change very specifically the properties of molecular motors in their cellular contexts," Bryant said.

There's also the possibility of using the controllable motors outside of biology, in diagnostic devices, for example. Bryant noted that researchers have worked on harnessing molecular motors to perform functions similar to their biological roles, "transporting molecules, sorting molecules, and concentrating molecules."

But first and foremost, Bryant is rebuilding these motors – incorporating features never before seen in biology – in an effort to illuminate their true nature.

"Evolution takes a basic design and makes motors that are fast and motors that are slow and motors that move long distances," Bryant said. "We've tried to build diverse motors and really challenge our understanding by pushing ourselves outside of what's already been done by evolution."

Transistor flipped by just a single photon

Blocked drain: one photon flips atomic transistor

Two independent teams of physicists in Germany have created the first high-gain optical transistors that can be switched using a single photon. Based on ultracold atomic gases, the devices make use of the "Rydberg blockade", whereby the creation of an atom in a highly excited state has a huge effect on the ability of the surrounding gas to transmit light. The research might lead to the development of all-optical logical circuits that could operate much faster than conventional electronics. The transistors could also find use in photon-based quantum-information systems of the future.

Communications and computing systems that use only light to transmit and process information have the potential to be faster and much more energy-efficient than those that use electronic signals. While optical-fibre communications is already widespread, the switching and processing of optically encoded data is usually done by converting light pulses to an electronic signal, which can then be easily processed. The electronic signal is then converted back to a light pulse.

Making photons interact

This time-consuming and energy-hungry process is necessary because photons do not readily interact with each other, which makes the design of all-optical components a major challenge that is currently being addressed by physicists and engineers. During the past few years, several research groups have made important breakthroughs in this area by showing that photons can be made to interact with each other in specially prepared samples of ultracold atomic gas.

Now, two independent teams led by Sebastian Hofferberth of the University of Stuttgart and Stephan Dürr of the Max Planck Institute of Quantum Optics near Munich have created devices in which a single "gate" photon can switch off a stream of as many as 20 photons. This gain of 20 is a huge improvement on previous attempts at optical switches, which either needed pulses of several gate photons to achieve gains greater than one or offered gains of much less than one for single-gate photons.

Both teams based their gates on gases of rubidium atoms that were cooled to temperatures below 1 mK. Normally, the gas is transparent to a beam of "source" photons, which can travel through the device and emerge via the "drain" – gate, source and drain being terms used to describe the control, input and output channels, respectively, of a conventional field-effect transistor.

Blocking the drain

When a gate photon is fired into the gas, it is absorbed by one atom, which puts that atom into a highly excited Rydberg state with one electron in an extremely large orbital. The large distance between this electron and the nucleus gives the atom a very large electric dipole moment, which shifts the energy levels of nearby atoms. This shift causes the gas to become opaque to light from the source, effectively switching the transistor off. The Rydberg state endures for about 1 μs, which is a surprisingly long time for an atomic system. This allowed Dürr and colleagues to use their transistor to switch off a stream of 20 source photons, while Hofferberth's team prevented 10 photons from reaching the drain of its device.

"This effect should make it possible – at least in principle – to cascade such transistors to solve complex computational tasks," says Dürr. He also points out that the experiments offer physicists a new and non-destructive way of studying the physics of Rydberg states. The ability to operate at the single-photon level also means that the transistors could find use in quantum-information applications such as secure quantum-communication systems or powerful quantum computers.

Another interesting aspect of the devices is that the gate photon is re-emitted by the gas when the Rydberg states decay – an effect that has been observed in other experiments. In principle, this means that the transistors could also be used as storage devices for quantum information.

Both experiments are described in separate papers in Physical Review Letters.

Silicon Nanorods bend light in new direction

Swirling nanorods: an axicon lens made of silicon

Ultrathin coatings that arbitrarily manipulate the phase and polarization of electromagnetic waves have been created by researchers in the US. The coatings are made from silicon nanorods using a technique that is compatible with industrial processes such as photolithography. The researchers say that the coatings could be used in new types of optical components that are much less bulky than traditional lenses. The technique could even be used to bend light in ways not possible with conventional lenses.

Fermat's principle – the rule that light travels along the path of least time – says that electromagnetic waves travel along the path on which they accumulate the least phase. In a medium of higher refractive index, the wavelength shortens and so a wave accumulates more phase across the same distance. A wave therefore bends towards the normal to reduce the distance travelled in the medium and the phase accumulated.

Manipulative metasurface

In a conventional optical component such as a lens, phase accumulates continuously as the wave propagates and this determines the nature of the wave that emerges from the lens. However, if the phase of a wave could be changed discontinuously at a surface (called a metasurface), then the wave could, in principle, be manipulated in ways not possible with conventional optics.

While this is straightforward in theory, the challenge facing physicists is how to create such a phase discontinuity using real materials. In 2011 researchers at Harvard University led by Federico Capasso and Zeno Gaburro covered a surface with V-shaped gold antennas so that the surface could be used to introduce any desired phase shift to optical waves passing through it. While this allows the arbitrary redirection of visible light, there are two major problems with this approach. First, the metallic nature of the surface means that most of the visible light is lost as it travels through the surface. Second, thin layers of metal are very difficult to work with and incompatible with the complementary metal-oxide semiconductor (CMOS) process used to make modern electronic devices.

In the new research, Mark Brongersma and colleagues at Stanford University in California use lossless silicon optical antennas. When illuminated by a particular frequency of light (which can be selected by varying its diameter), the antenna will resonate strongly. This causes the light wave to pick up a phase shift that depends on the relative orientations of its polarization axes to the antenna. By appropriately tailoring the orientations and distances between the antennas, the surface can impart any desired phase shift to the light. This allowed the researchers to reproduce the functions of a bulk lens with a single layer of nanorods just 100 nm thick.

Axicons and Bessel beams

The team was able to create various types of "lenses" using this technique. These include traditional focusing lenses and an axicon. The latter is a specialized type of conical lens that transforms an ordinary laser beam into a Bessel beam – a ring-shaped beam used in optical tweezers and eye surgery.

Optics expert John Pendry of Imperial College London is impressed. "If anyone in the electronics or photonics game wanted to use a material, it would have to be silicon," he explains. "You can lay down silicon extremely flat and shape it very precisely. Metals are nowhere near silicon in terms of the precision and the control you can exert over them; so, if you can translate a technology like metasurfaces into a silicon environment, you're on to a real winner because you can hook on to this bandwagon that's been rolling for half a century now."

I think that Intel or other companies based on CMOS technology can implement such a metasurface now

Erez Hasman, Technion-Israel

In the experiment, the metasurfaces were fabricated by electron-beam lithography, but team member Erez Hasman, now at the Technion-Israel Institute of Technology in Haifa, says that commercial companies could produce large quantities using industrial processes such as photolithography. "I think that Intel or other companies based on CMOS technology can implement such a metasurface now," he says.

"The theoretical concept is not surprising at this point, but the fact that they built it and it works is interesting," agrees Andrea Alù, an expert on metasurfaces at the University of Texas at Austin. He looks forward to the development of optical components that are not possible with normal lenses. Hasman suggests that one of the first such uses might be to interface waveguides with free space. "In general, the modes of a laser resonator or a waveguide are very complex and different from the modes of free space," he says. Coupling the two together to allow signals to pass between them, he explains, is very difficult using a lens or a prism but should be no problem using the 2D metasurface.

The research is published in Science