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Kirigami solar cells follow the Sun

 

The ancient Japanese art of kirigami, or paper cutting, has been used by researchers in the US to improve the efficiency of solar-panel tracking systems. The researchers cut a pattern in thin-film gallium-arsenide solar cells, which causes the cells to tilt when stretched. The system is an improvement on existing solar-tracking equipment, which is bulky, expensive and generally beyond the reach of household solar arrays. The team says that its new design could easily be deployed on individual houses as well as in larger arrays, and, as an added bonus, also improves the optical and mechanical properties of the solar cell.

Flat-panelled solar-cell arrays are most effective when sunlight is directly incident on their surface. Solar trackers are used to orient such arrays, along one or two axes, allowing them to follow the Sun as its position in the sky changes during the course of a day and throughout the year.

Cumbersome trackers

Depending on the geographic location of a solar array, and whether it has one or two tracking axes, a conventional tracker could boost yearly energy generation by 20–40%, compared with a static array. But despite these promising figures, such systems have not been widely implemented because of the high costs, added weight and additional space that they require. Indeed, the additional components required for tracking account for nearly 12% of the total cost of the system, and while this number increases by about 1% annually, the price of actual solar cells is dropping. Also, thanks to the tracker’s size, they cannot be used on the roofs of most homes.

To overcome these problems, Max Shtein and colleagues at the University of Michigan in Ann Arbor used a laser to cut a 2D pattern into gallium-arsenide solar cells. By stretching these patterned cells, the researchers can produce tilted solar-cell arrays in 3D. While the cell panel remains flat, the array elements pop up when stretched.

“All in all, we’re getting about a 30% improvement in the amount of energy harvested across the course of a simulated day, say in Arizona, for a given amount of semiconductor used, compared with stationary panels,” says Shtein. “It basically matches what conventional trackers can do in boosting energy production, but with considerably less bulk.”

Optimal harvest

While the cuts do reduce the area of the array available for sunlight harvesting, it is by a very tiny amount, and Shtein explains that the corners of the cuts are also rounded off to reduce stress in the structure, further reducing the area. By adjusting the strain on the stretched solar cells, the team was able to optimize the cells’ optical and mechanical properties. The researchers found that longer cuts that are spaced closer together made for less pulling effort, and that the degree of tilt is proportional to the amount of pull. From a practical point-of-view, these kirigami-enhanced cells could be placed within a double-pane enclosure to make them more weatherproof, and could be reinforced via tensioned support cables in large arrays to prevent them from sagging.

Although the team’s technique is still in the design phase and further research is needed, it offers a lightweight, scalable and cheap alternative to solar tracking, thereby maximizing the efficiency of such solar cells. Shtein told physicsworld.com that the kirigami approach could be extended to other thin-film or flexible solar cells. “That’s not to say there won’t be integration challenges – plenty of development to do there – but the basic idea should be the same,” he says. According to the researchers, their design opens up new markets for solar tracking, including widespread rooftop, mobile and space-borne installations.

The research is published in Nature Communications.

Physics logos

A recent controversy in the US about whether the Confederate flag should be displayed in public places has ignited a furious discussion about the meaning and purpose of symbols. The flag in question is one variant of several emblems of the Confederate states whose secession from the Union ignited the American Civil War. How is it possible for a logo whose elements are so simple – white stars inside a blue St Andrews Cross against a red background – to have such a variety of meanings?

Logos are everywhere, and a quick Internet image search turns up physics logos of all sorts. But what is their meaning and purpose? What aspects of physics do they promote? What is their value?

Many physics logos are simple and rather predictable, using a dot on a sweeping line to suggest an orb and orbit, either a particle or planet. These include the logos of the Brookhaven National Laboratory and the American Physical Society.

Other logos exploit the particle–planet ambiguity more imaginatively to incorporate aspects of physics or specific fields of research. The logo of physics and astronomy department at McMaster University in Canada, for example, also neatly suggests a specific planet, Saturn, while its yin-yang image invokes the complementarity of physics and astronomy.

Several physics logos suggest explosions or particle collisions, resembling a chandelier of the style that designers and architects call “mid-century modern”. A few logos manage to suggest both the microscopic and macroscopic nature of physics, such as one used by the engineering-physics programme at the University of British Columbia, which shows an orbit around a gear.

The Princeton Plasma Physics Laboratory has a modern, stylized logo. The graphic is ambiguous, and suggests an eye or a vortex of whirling plasma currents. It’s pleasing to look at and represents something relevant to physics.

Still other logos call attention to some specific and recognizable structural element instrument of the organization or the subject. The CERN particle-physics lab’s classic logo, which has a lot of lines but is nevertheless graphically simple, suggests an accelerator and manages to look almost 3D. The logo of the International Year of Light (IYL) communicates what it stands for, but almost too explicitly, with an image of the Sun surrounded by array of flags bearing colours of the spectrum.

The Sigma Pi Sigma (ΣΠΣ) society, which honours “outstanding scholarship in physics”, has an ambitious logo whose meaning is more opaque to outsiders than most – and surely even to many students, for the elements refer to century-old instruments. The overall shape of the logo resembles a voltmeter, which is “a symbol of the accuracy necessary for an experimental science”, according to a ΣΠΣ webpage, while the dynamo powering the light bulb inside “represents the creative energy needed to produce the illumination of knowledge”.

At the other extreme is that of the Institute of Physics, which publishes Physics World. It lacks any physics-related elements, confidently using a simple trio of only three sans-serif bold letters to suggest that the organization is distinguished enough not to need any representations.

The critical point

The logos I came across promote a variety of physics aspects: microworld and macroworld, theory and experiment, research and instrumentation. Still, I expected to find a wider range of elements used. Why aren’t there more pendulums, for instance, given that it’s a physics tool that has been in continuous service for over four centuries? Do so few measuring instruments turn up in physics logos because the simplest to represent – a pan and scale – has been co-opted by justice? Why aren’t there more logos incorporating light?

“Have a design contest” is rule number one of a website called “How not to design a logo”, which makes me reluctant to ask for new designs.

I am, though, interested to discover clever examples that I may have overlooked. Send them to me and say why you think they are interesting from a design and a physics perspective, and I’ll write about the results in a future column.

In fact, is it possible to encapsulate what physics is about in a single logo? Or does physics have so many meanings that this is impossible?

New laser sweeps out its own frequency

A laser that modulates its own frequency has been unveiled by researchers in the US. The device is a new and practical application of cavity optomechanics, and the team believes that it could be used to improve the resolution and cut the size, cost and complexity of LIDAR (light detection and ranging) systems, and also find application in retinal imaging.

In cavity optomechanics, the optical modes of a laser are coupled to the mechanical modes of an oscillating cavity via radiation pressure pushing on the cavity’s mirrors. Such set-ups have been used to investigate a wide range of fundamental physics and also to produce highly accurate atomic clocks.

In a conventional cavity-optomechanics experiment, the laser light is injected from the outside and the cavity simply responds to laser frequencies that resonate with its own modes of mechanical resonance. In the new research, Connie Chang-Hasnain of the University of California, Berkeley, and colleagues teamed up with the commercial company Bandwidth10 to use the cavity of the laser itself as the mechanical resonator. These mechanical oscillations therefore change the size of the laser cavity, which alters the frequency of the laser light.

Key alterations

To maximize this effect, the researchers made two key alterations to the laser set-up. First, they replaced the traditional laser – in which the light is emitted from one of the ends of the laser cavity – with a vertical cavity surface emitting laser (VCSEL). Light is emitted from either the top or bottom of the active region of a VCSEL. This means that the distance between the mirrors is much smaller than in a conventional set-up, and that the wavelength of the laser is much more sensitive to changes in this distance.

The second alteration involves replacing one of the mirrors traditionally used in VCSELs – the multilayer “distributed Bragg reflector” – with a different type of mirror called a “high-contrast grating”. This replacement mirror was first developed in 2004 by Chang-Hasnain’s group and comprises a single layer of bars that refract light. An important feature of this mirror is that it can be considerably lighter than other mirrors. The 20 μm × 20 μm mirror used by the researchers, for example, is just 130 pg, and this makes it much easier for the laser light to move it.

Under pressure

The researchers designed the mirror to be most reflective at the frequency of the VCSEL when the mirror is at rest. The light in the laser is therefore most intense at this frequency and exerts maximum pressure on the mirror. This pushes the mirror away slightly, decreasing the frequency of the laser and thereby reducing the mirror’s reflectivity. This, in turn, reduces the laser intensity and pressure, causing the mirror to retract. Because of a slight time delay in the response of the laser to a change in the length of the cavity, the pressure does not increase immediately and the mirror overshoots, which increases the frequency of the laser. When the pressure does increase again, the mirror moves back out and overshoots again. This cycle results in a periodic, rapid oscillation of both the frequency and the power of the emitted laser light.

The researchers believe that this “self-sweeping laser” will have many practical applications. One is LIDAR, which is an imaging technique with a wide range of uses including manufacturing, remote sensing and self-driving vehicles. LIDAR involves firing light pulses at an object to determine its shape and distance based on the reflected light. To determine the time at which a particular reflected pulse was emitted, the power and frequency of the light are swept constantly. Sweeping is also used in optical coherence tomography (OCT) when surveying the back of the eye to check for signs of disease.

Energy-efficient

Today, swept lasers for LIDAR and OCT use a mechanical motor to move one of the mirrors, which can make the systems large and power-hungry. “[Cavity optomechanics] is heading in a new direction that’s relatively little explored,” says Albert Schliesser of the Niels Bohr Institute in Copenhagen. “The engineering applications are something that make this paper really interesting because it indicates that there are possibilities in that area.”

The research is published in Scientific Reports.

Friction between the sheets

Physics helps to explain why it is so hard to pull apart two interleaved phone directories. (CC  BY SA Mark Longair)

By Michael Banks

Ever tried – and duly failed – to pull apart two interleaved phone books? Well, a team of researchers from France and Canada, led by Héctor Alarcón of the University Paris-Sud, has now studied why this is such an impossible task.

(more…)

How can we detect gravitational waves?

Gravitational waves were predicted by Albert Einstein in 1916 as a consequence of the field equations of his general theory of relativity. These ripples in space–time cause space to stretch in one direction perpendicular to the line of travel while simultaneously compressing it in the other. In this video from our 100 Second Science series, Nergis Mavalvala of the Massachusetts Institute of Technology describes how interferometry is used to search for the effect of gravitational waves here on Earth.

Almost a century after gravitational waves were predicted, the hunt to detect them directly is hotting up. In a feature article in the September issue of Physics World, science writer David Appell explains how a major upgrade to the Laser Interferometry Gravitational-wave Observatory (LIGO) may soon bring this hunt to a successful close.

  • Members of the Institute of Physics (IOP) can get immediate access to the September issue of Physics World on desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full digital access to Physics World.

Levitating diamonds could improve motion sensors

By levitating a tiny, nano-sized diamond using light, physicists in the US and Finland have created a controllable quantum system that has optical, mechanical and spin degrees of freedom. Based on a single “nitrogen vacancy” (NV) defect in the diamond, the system could be used in devices that measure extremely weak forces – or even to create “Schrödinger’s cat states”.

NV defects occur in diamond when two adjacent carbon atoms are replaced by a nitrogen atom and an empty lattice site. One type of NV (NV–) is of great interest to physicists building quantum devices because its spin state (–1, 0 or +1) can be determined very easily using light. Furthermore, NVs are well isolated from their surroundings, which means that their spin states – unlike those of most other solid-state systems – keep their quantum nature for relatively long times.

Multiple vacancies

This is not the first time that nanodiamonds have been levitated – back in 2013 Levi Neukirch, Nick Vamivakas and colleagues at the University of Rochester did so using an optical trap. In their experiment, which was done in air, laser light at another wavelength was used to determine the spin states of NVs in the diamond, which was tens of nanometres across. This was an important first step towards creating a “hybrid quantum system”, but the air meant that the nanodiamond could not be put into its lowest energy mechanical state, while the presence of multiple NVs meant that it could not be used as a single spin state either.

Now, however, the Rochester team has joined forces with Eva von Haartman at Åbo Akademi University to address both problems by levitating the single-NV nanodiamonds in a vacuum and showing that the mechanical motion of the diamond can be tracked using the spin of the NV. The team used irregularly shaped diamond nanoparticles about 40 nm across that were coated with silicon oxide to make them more spherical – which makes optical trapping easier. The particles were then trapped using light from a near-infrared laser focused onto a narrow region in a vacuum chamber at just 1 kPa – about 1% normal atmospheric pressure.

Red light, green light

The team trapped a nanoparticle with just one NV and read out its spin state by shining green laser light on it. Some of this is absorbed by the NV electron before being re-emitted as red light through photoluminescence. This process depends on the spin state of the NV, which makes the technique ideal for measuring NV spin.

When a nanoparticle is held in the optical trap at atmospheric pressure, its motion is random because of collisions with air molecules. However, as the air is pumped out of the chamber, the nanoparticle oscillates with simple harmonic motion at a frequency of about 250 kHz. By monitoring the intensity of the photoluminescent light, the team showed that the nanoparticle was indeed oscillating back and forth across the focal region of the green laser – with the photoluminescence at a maximum when the particle was at the centre of the focal region.

The team was also able to control the spin state of the NV using electron spin resonance (ESR), which involves firing a microwave signal at the nanoparticle and causing a transition between spin states. This transition involves a peak in the absorption of the microwaves at the transition energy, which was also observed by the team. Finally, the team studied the effect of an applied magnetic field on the NV spin. In the absence of an applied magnetic field, the +1 and –1 spin states have the same energy. However, when a magnetic field is applied, one state gains energy while the other loses it. This causes a splitting of the absorption peak into two peaks, which was also seen by the team.

Opposing forces

Having learned how to monitor the motion and spin state of the nanoparticle, the team now wants to find ways of manipulating the quantum properties of the system. This could involve, for example, applying a magnetic field such that an NV in the +1 state would feel a force in one direction, while an NV in the –1 state would feel a force in the opposite direction. Under the right experimental conditions, these opposing forces would put the entire nanoparticle into a quantum superposition of two mechanical states – analogous to the famous Schrödinger’s cat, which is in a superposition of being both dead and alive.

Another possible application for the system is an accelerometer that could detect tiny external forces by their effect on how the nanodiamond oscillates. But before the team can try such experiments, they must tackle the problem of the trapped nanoparticles surviving barely a minute or two in a vacuum before being completely degraded. The team believes that this happens because the particles are heated by the laser light but are unable to get rid of the heat through contact with air. The researchers had thought that the silicon-oxide coating would improve the robustness of the nanoparticles, but this was not the case and more work on this is needed.

The research is described in Nature Photonics.

Do general anaesthetics trigger a phase transition in the brain?

The process of losing consciousness under a general anaesthetic could involve a phase transition in the brain. That is the conclusion of scientists in the US, who have developed a new mathematical model of how the brain’s neurons interact with each other. The model shows how a small reduction in information transfer between neurons can bring about a sudden loss of consciousness, and reproduces many of the changes in the brain’s electrical activity observed during anaesthesia.

Anaesthetics are used routinely during medical procedures, so it might come as a surprise that scientists do not fully understand how they cause a patient to lose consciousness. Monitoring a person’s level of consciousness while they are being given an anaesthetic generally relies on measuring that person’s brain waves. Those waves are generated by the many electrical impulses fired between neurons, and create a measurable voltage on the scalp that can be recorded via an electroencephalogram (EEG). The correlation between waves recorded on opposite sides of the head indicates the level of consciousness, and this information is used by anaesthetists to vary the dose of anaesthetic given to the patient.

Universal phenomenon

However, this approach is very much an empirical one, and to understand how anaesthetics bring about unconsciousness, scientists have developed computer models to try and capture the underlying changes in neuronal activity. In 1997 Jamie Sleigh of the University of Auckland and Duncan Galletly of the University of Otago used a fairly simple 2D model to show how loss of awareness correlates with reduced efficiency of neural synapses. “Anaesthesia is universal for all animals with nervous systems,” says Sleigh, “which suggests that it is a very generic universal phenomenon that can be modelled at quite an abstract level.”

The latest work improves on the earlier research by modelling how information is transferred between different layers of neurons. Developed by the physicist Yan Xu and colleagues at the University of Pittsburgh, the model reproduces an electrical signal arriving at a node (either a single neuron or a group of neurons) within the thalamus region of the brain (responsible for sensory input), and then simulates the resulting signals induced in successive layers of the cerebral cortex, which is key to conscious awareness. Nodes are connected to one another across and within the layers, with the probability of information flow between them determined using percolation theory. This theory describes, among other things, how hot water flows through coffee grounds.

Ease of communication

To test their model, Xu and co-workers compared its output with EEG waveforms from patients who had undergone general anaesthesia. Regulating the amount of anaesthetic in the model meant varying the value of a single parameter – p – between 0 and 1, which changed the weighting, or the ease of communication, between connected nodes. Doing so, the researchers found they could indeed reproduce several of the main changes to the waveforms of anaesthetized individuals, including a shift to lower frequencies and higher amplitudes, as well as more synchronization between waves in different areas of the cortex and more power deposited by waves in the front of the brain.

The team also demonstrated the phase shift in sensory perception brought about by a tiny change in p. It did so by modelling how a digital image of a famous picture of Albert Einstein is transmitted through the network. The researchers showed that at p = 0.32 the light intensity recorded at the output nodes leads to a barely discernible head and crop of hair, whereas at p = 0.38 the unmistakable image of Einstein is clear.

“Master parameter”

“What is remarkable is that by changing just one master parameter, you can reproduce the most essential features of the transition from a conscious to an unconscious state,” says Xu, a professor of anaesthesiology who trained as a physicist. He proposes that experimentalists put his group’s model to the test by establishing whether – as the model predicts – unconsciousness affects which neurons are involved in the process of relearning. “We ignore a lot of biological detail but those details probably aren’t critical for consciousness to occur,” he adds. “We try to search for those universal rules that govern the emergence of consciousness.”

Peter McClintock, a physicist at Lancaster University in the UK, praises the “interesting and surprising” results, saying that although the research will be of practical benefit only if a way can be found to measure p, “improved understanding must bring us closer to better measures of depth of anaesthesia”. But he does not believe it will change our fundamental understanding of consciousness much. “I don’t think we are getting very much closer to solving the mind–body problem,” he says, “although we orbit around it ever closer as new knowledge and ideas accumulate.”

Sleigh agrees. The modelling by Xu and colleagues, he says, “clearly reflects anaesthesia–aesthesia transitions, but I am not sure that it solves the ‘hard problem’ of (human) consciousness”.

The research is reported in Physical Review Letters.

Whisky in space, methane-capturing coffee, conference disasters and more

 

By Hamish Johnston

Fancy a wee dram while you are orbiting the Earth? With the growing interest in space tourism, travellers could soon be enjoying a sip or two of whisky in space. To make such tipples as enjoyable as possible, the Scotch whisky maker Ballantine’s has developed a special “space glass” that works in the free-fall conditions of Earth orbit. The firm is also developing a special blend of whisky to be enjoyed in space.

Created by Ballantine’s master whisky blender Sandy Hyslop and James Parr from the Open Space Agency, the new glass was filled with Scotch and tested in free-fall at the ZARM drop tower in Bremen, Germany. You can find out more about how one’s palate changes in space and the challenges facing the glass designers in the above video. And if you want to know if the glass passed the free-fall test, there is a second video called “Space Glass Project: the microgravity test”.

(more…)

Protein pulling reveals a new way that molecules resist external forces

A new mechanism that keeps a protein complex mechanically stable when stretched has been discovered by researchers in Germany, the US and Israel. The mechanism channels forces along paths perpendicular to the “pulling” axis in the structure and its discovery could lead to a better understand of how proteins and other large biological molecules resist external forces. The work may ultimately see the development of artificial mechano-active systems that might be used as scaffolds for tissue engineering or as components in engineered nanomaterials or protein-inspired machines.

Mechanical forces are fundamentally important in biological systems. Cells sense and respond to mechanical cues in their environment and react, for example, by modulating gene-expression patterns. Forces also play an important role in how cells join together to create larger structures such as tissues and organs. At the molecular level, such behaviour is governed by mechanically active proteins, which can sense and react to external forces by changing their shape and modulating their function in a number of ways.

Extremely stable

The researchers, co-led by Hermann Gaub of Ludwig-Maximilians University in Munich, used experimental and numerical methods to study mechanical properties of a multi-domain cellulosome protein complex. This complex is of great interest to researchers because it is known to be extremely stable when subject to mechanical forces.

Constantin Schoeler and colleagues in Munich used an atomic force microscope (AFM) to pull on a protein complex while monitoring how forces travel through the structure. Meanwhile, Klaus Schulten and Rafael Bernardi of the University of Illinois performed steered molecular dynamics (SMD) simulations using state-of-the-art supercomputers to calculate how forces propagate through cellulosome. After comparing their findings, the teams were able to identify the most probable paths that applied forces take through the molecules.

“Our results show that this mechanically stable complex uses an architecture that exploits simple geometrical and physical concepts from Newtonian mechanics to resist external forces,” say the researchers. “The analytical framework we describe provides a basis for developing a deeper understanding of how various mechano-active proteins function.”

Non-parallel routes

“As far as we know, this is a new concept in the biophysics field,” explains co-team-leader Michael Nash of the Center for Nanoscience at Ludwig-Maximilians University in Munich. “Our results imply specific force-propagation routes non-parallel to the pulling axis make the protein complex more mechanically robust.”

“Based on this work and other work from our group, we have developed a ‘toolbox’ of molecular modules based on cellulosomes that we could now use in a variety of biophysics experiments,” adds Nash. “We are now further investigating the fundamental properties of these remarkable molecules and looking into how we can exploit cellulosome proteins in diverse fields, from biomedicine to bioenergy.”

A better understanding of how proteins respond to external forces could provide important information to scientists who are trying to develop scaffolds for tissue engineering. These are artificial structures that provide a template for living cells to create tissues or organs. Such scaffolds must have very specific mechanical properties to create the desired tissue or organ.

The research is described in Nano Letters.

Galactic shape-shifting signals a decrease in star formation across cosmic history

Most of the stars in the universe were born within spiral galaxies like the Milky Way but now find themselves inside “dead” elliptical galaxies, according to new analysis of data from the Hubble and Herschel space telescopes. Astronomers led by Steve Eales of Cardiff University, UK, have shown that 83% of stars in the universe were born in spiral galaxies, but today only about 49% of stars exist in spirals. According to the team, this means that many spiral galaxies have somehow transformed themselves into elliptical ones.

When two spiral galaxies collide, astronomers believe that they will merge into a single elliptical galaxy, which is a giant, amorphous spheroid of stars. The merger process uses up all of the spare star-forming gas in the colliding galaxies, and this means that elliptical galaxies have no gas left with which to form new stars. However, there was no quantifiable evidence that there has been a widespread transformation of spirals into ellipticals, until now.

Galactic energy

The latest research is based on a survey of 10,000 galaxies in the nearby universe selected from the Herschel Astrophysical Terahertz Large Area Survey and the Galaxy and Mass Assembly Survey. It also includes galaxies in the early universe, as seen in Hubble’s Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey project. These data allowed Eales’s team to measure how much energy is coming from stars in disc galaxies and elliptical galaxies, respectively, at different times in the history of the universe.

“By measuring the total amount of energy from a particular patch of sky, we could work out how many stars have formed to generate that energy,” Eales told physicsworld.com. “Then we looked at the fraction of the energy associated with disc galaxies and the fraction associated with elliptical galaxies.”

The further afield one looks, the smaller and more indistinct galaxies become, until it becomes difficult to determine what type of galaxy they are. To address this, Eales’s team applied the Sérsic profile, which measures brightness across a galaxy. Spiral galaxies are distinguished by having different brightness profiles to ellipticals.

Stellar milestone

If Eales’s team is correct, then the universe has now reached a milestone, with more than half of its stars existing in elliptical galaxies and star formation continuing to dwindle across the universe. Indeed, ellipticals comprise cooler, redder, longer-lived stars with little star formation and are often described as being “red and dead”.

Team-member Dave Clements at Imperial College, London, adds: “The star-formation rate is certainly dropping, which means that the amount of energy being produced by stars is also declining.”

However, not everyone agrees with the team’s conclusions. Richard Bower, an astrophysicist at Durham University, UK, is concerned that our understanding of the total energy output of star-forming galaxies in the very distant universe is too uncertain to be able to come up with a firm figure of 83% without numerous assumptions being made.

“I’m not convinced that you can determine those things as accurately as you would need to in order to come to a strong conclusion,” he says.

Not dead yet

Furthermore, Bower’s own work with Durham’s Evolution and Assembly of Galaxies and their Environments project, which models the formation of galaxies across large volumes of the universe, suggests that galaxy evolution need not stop at “red and dead”.

“We see galaxies forming stars, stopping, turning red, but then something will happen to rejuvenate them and they’ll go back to forming stars, possibly at a lower rate,” says Bower. Indeed, in 2012 astronomers used Hubble to identify clouds of cool and potentially star-forming gas around 16 “dead” elliptical galaxies.

Eales agrees that sometimes galaxy evolution can run backwards. “There are a lot of processes that could cause galaxies to move in the opposite direction, but the dominant process still seems to be this disc-to-elliptical transformation,” he says.

The research is described in Monthly Notices of the Royal Astronomical Society.

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