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Metamaterial breaks refraction record

Researchers in Korea have created a new metamaterial with the most extreme positive index of refraction yet – a whopping 38.6. The metamaterial operates at terahertz frequencies and the team believes that it could find use in a number of applications including high-resolution imaging.

The refractive index of a material defines the angle through which light is bent when it travels between a material and the vacuum. Ordinary materials such as glass have refractive indices between one and three at optical frequencies, with a few materials like silicon approaching four. Over the past decade or so, physicists have been developing artificial materials with negative indices of refraction. These metamaterials bend light in the opposite direction to normal materials and can be used to make invisibility cloaks and superlenses.

While this new material has a positive index of refraction, its value is so large that it could lead to new terahertz technologies for security scanning and cancer diagnosis. The researchers also believe that the metamaterial could find use in invisibility cloaks.

Lattice of I-shapes

Created by Bumki Min of the Korean Advanced Institute for Science and Technology, the metamaterial is a polymer film inset with a pattern of thin gold or aluminium shapes. The team set the I-shapes into the polymer using lithography techniques. The I-shapes were slightly less than 60 µm tall and wide and repeated every 60 µm in a square lattice so that the individual shapes don’t touch each other (see figure).

The team found that the material achieves its peak refractive index of 38.6 at frequencies near 0.3 THz. The value drops away at other frequencies, but the refractive index remains above 20 for frequencies near 0.35 THz.

A material’s index of refraction is a function of two electromagnetic properties. These are its permittivity, or the ease with which it is electrically polarized, and its permeability, which is the ease at which it can be magnetized. In Min’s metamaterial, the permeability remained ordinary, while the metal patterning boosted the permittivity by a considerable amount.

Resonating with the gap

When linearly polarized light is shone through the material, the metal pieces become electrically polarized. The tops and bottoms of the I-shapes act as capacitor plates with their oppositely-charged neighbours, setting up an electric field in the gap between the shapes. The strength of this field indicates the material’s permittivity. At certain frequencies, the wavelength of the light resonates with the size of the gap, resulting in a stronger electric field. This increases the permittivity further and results in a high maximum refractive index.

By changing the size of the I-shaped pieces, and so changing their distance from one another, the researchers tested gaps between the metal edges of 30 µm down to 80 nm. This smallest spacing was achieved with aluminium, which was set into the polyimide with the help of more precise electron-beam lithography. The refractive index rose rapidly for gaps below about 5 µm, and the aluminium material, with its tiny gaps, had the highest maximum index.

The researchers also made thicker versions of their metamaterial, generating up to five layers of the gold pattern. In this material, each layer’s response is superimposed on the others, and as a result Min says it “exhibits a completely different bulk refractive index profile from that of a single-layer profile”.

For the five-layer material, the I-shapes were only 40 µm square, and the maximum refractive index of 33 was well above other measurements for gold. The high index was also maintained for a broader band of frequencies, remaining above 15 between 0.7 and 1.8 THz.

Skin cancer treatment

Jung-Tsung Shen of Washington University in Missouri calls the work “very significant”, noting that the Korean team’s high-index material is also flexible rather than rigid. “I believe their results could find potential applications in many situations where terahertz frequencies are used,” he says, citing security checkpoints and skin cancer diagnosis.

The metamaterial’s refractive index scales with that of the metal’s host. The researchers believe that higher refractive indices can be constructed by replacing the polymer with a material whose own refractive index is high, such as lead sulphide. Moreover, as demonstrated by the aluminium prototype, the thinner gaps between the metal pieces strengthen their capacitive behaviour – and hence the permittivity of the material.

And, Min says that the development of high-index materials may still lead to invisibility. “The broadened index spectrum will provide more design freedom in the path control of electromagnetic waves,” he says, potentially shrinking cloaking devices. “Positive high refractive index will also be useful for various applications such as high-resolution imaging.”

The work is reported in Nature 470 369.

How to trigger lightning

cow.jpg
Rocket-triggered lightning strikes a control tower

By James Dacey

In the same spirit as Benjamin Franklin and his famous kite experiments, Joseph Dwyer is a scientist willing to brave electrical storms to understand more about how lightning works. He carries out his work at Florida’s International Center for Lightning Research and Testing where he fires rockets into thunderclouds to trigger lightning.

DACEY dwyer blog.jpg

I recently caught up with Dwyer to find out a bit more about his work in this exclusive interview for physicsworld.com. He is adamant that despite our familiarity with these spectacular natural lightshows, we really don’t know all that much about what is going on. He highlights three big outstanding questions:

How lightning initiates;
How it propagates, sometimes through miles of air;
And what happens when it smashes into the ground.

At the Florida centre, Dwyer and his colleagues are trying to get a handle on these questions by creating lightning in semi-controlled conditions. “When a thunderstorm comes overhead – and that’s fairly common here in Florida since we’re the lightning capital of the US – we measure the electric fields and we can tell when conditions are good for lightning,” he says. “The lightning is going to strike anyway some place, so why don’t we make it strike here and now?”

You can read the full interview here.

The physicist who tames lightning

We learn about lightning in school, I thought it was a closed case?
Well, we know lightning initiates up inside a thunderstorm but we’re not sure how it initiates – how it gets started. In fact there are still three big questions. The first is the initiation. The second is how does it propagate, sometimes through miles of air? And the third is, when it reaches the ground – how does it choose to strike this object and not the other object?

But isn’t it just an electrical discharge between thunderclouds and the ground?
In a sense, but the big problem is that to get a spark, air needs to break down. It needs to stop being an insulator and start being a conductor. We commonly experience this if you touch a doorknob and you get a spark between your finger and the doorknob. What happens is the charges get concentrated into your fingertip and you get a big electric field. Then, as your finger approaches, the conventional breakdown field is reached, which is about 3 million volts per metre – and then air sparks.

The problem is if you look up inside thunderclouds, the breakdown field that you need to make a spark is never found. People have been launching balloons for decades, they’ve been flying airplanes, they’ve been launching rockets…but the fields they record are not even close to this strength.

Photo of Joseph Dwyer
Joseph Dwyer

So this is the sort of question you’re asking at the lightning research centre in Florida?
Well, there are two kinds of research going on there. Most of the research done now is basic science – we’re just trying to understand how lightning works, with a long-term hope of maybe protecting people if we can understand it better. Then there are other areas of research that the University of Florida historically has done. They’ll test things for the power companies: they’ll trigger the lightning and have it hit a power pole and see what it does, sort of engineering things.

How do you create your own lightning?

The lightning is going to strike anyway some place, so why don’t we make it strike here and now?

When a thunderstorm comes overhead – and that’s fairly common here in Florida since we’re the lightning capital of the US – we measure the electric fields and we can tell when conditions are good for lightning. The lightning is going to strike anyway some place, so why don’t we make it strike here and now?

So we launch a small rocket with wire behind it. The rocket is around a metre tall and has a spool of Kevlar-coated copper wire attached to its bottom, and one end of the wire stays attached to the ground. So as the rocket goes up the wire comes off the spool so you’re left very quickly with several hundred metres of wire hanging there in the air.

Does this trigger a spark in the same way as normal lightning?
Yes and no. The only thing we’re missing is the very initial process that starts naturally inside the thundercloud. All the subsequent strokes are exactly the same for natural and for triggered lightning, there’s really no difference that we can tell.

If you think of the doorknob example, the wire is like the finger pointing up and that will get a spark going between the rocket and the thunderstorm. And that spark usually starts at the end of the wire where it joins the rocket, and starts travelling upwards on its own, until it finds the thundercloud charge and then it will bring it down to us. And then the lightning follows the channel back down.

Why did you become interested in X-rays produced by lightning?
Well, it was first suggested that lightning produces X-rays way back in 1925 by CTR Wilson. He was the Scottish physicist who won the Nobel prize for inventing the cloud chamber [used in particle physics]. But if you look through the literature there have been some hints and some ideas but nothing very definitive. It was kind of like the Loch Ness monster – someone might go out and take a fuzzy picture or someone claims to have seen it from a submarine, and in the end you don’t believe any of it!

X-ray camera in front of the rocket launch tower
X-ray camera in front of the rocket launch tower

Have you built a special X-ray detector for lightning?
Yes, I was actually hired in 2000 by Florida Tech as a space physicist and I was studying things like coronal mass ejections and interplanetary disturbances. I was also looking for other things to do. When you do space physics you mainly measure things with space instrumentation, and if you’re not actually building something that goes into space, you’re looking for other things to build. And I was thinking: what else can I build around here that I could use? And I thought there’s all this lightning going on in the background and so maybe I could look for X-rays from lightning.

Was it easy enough to build?
The equipment was fairly straightforward to construct. I’d actually spent quite a few years making X-ray equipment, so that part wasn’t so hard. You basically just build a big, heavy aluminium box all welded together and you stick a detector inside with a battery – fibre optics are used to get the signal out. We had students build it and it took maybe three months to build, so compared with space instrumentation – which I was used to – it was a very simple thing to build. Within a month we had established that lightning emits X-rays.

Tell me about the X-ray images of lightning that you presented at the American Geophysical Union meeting back in December.

Oh yes, it’s a very exciting area to be in with a lot of so-called ‘low-hanging’ fruit.

For the first several years that we recorded X-rays from lightning, we measured important properties such as what energies the X-rays have and when they are produced. These measurements told us many things about how lightning emits X-rays, but we still were not clear about exactly what part of the lightning the X-rays were coming from as it propagated towards the ground. We eventually decided that we needed to make X-ray images of lightning, so that we could see where the X-ray emission was coming from. We were also curious what lightning looked like in X-rays. To do this we had to design and build a very specialized X-ray camera, a camera that is large, sensitive and very fast. Last year we built the first X-ray camera for measuring lightning – it is about the size of a refrigerator and weights 1500 lb.

So, it works well I presume?
This last summer we set it up next to our rocket launcher and were able to record several lightning flashes. The result was the first ever X-ray images of lightning, actually movies showing the lightning propagating towards the ground as seen in X-rays. Although the resolution of this first camera (30 pixels) was not great, it was good enough to clearly see the lightning racing towards the ground at 1/6 the speed of light. The camera recorded this at 10 million frames per second. We now have a brand new way of looking at lightning.

Do you think lightning research is a growing area?
Oh yes, it’s a very exciting area to be in with a lot of so-called “low-hanging” fruit. And just from going to conferences the last few years, there’s been a noticeable increase in activity. I came from a more mature field in space physics where this sort of exponential growth took place in the 1960s when it was first launching the satellites and discovering the Van Allen belt and stuff like that. So every field of science goes through this rapid growth. Then after you make the “easy” discoveries it levels off. That’s what’s completely different in lightning; we’re going through the exponential growth right now.

Spinning black holes twist light

Light passing near to the spinning black holes thought to reside at the centre of many galaxies becomes twisted, possibly offering a new way to test Einstein’s general theory of relativity. That is the conclusion of an international team of physicists, who say the phenomenon could be seen with existing telescopes.

The general theory of relativity (GR), put forward by Einstein more than 90 years ago, predicts few phenomena that can be easily tested. One example is gravitational lensing – that the gravity of stars and black holes can warp space–time enough to bend the passage of light. Another is time dilation, which makes clocks sitting in regions of lower gravity – say, at high altitudes – tick faster. Scientists are still trying to directly detect yet another general-relativity phenomenon called gravitational waves. These are ripples in space–time thought to be generated when large masses accelerate.

In 2003 Martin Harwit of Cornell University hinted that there might be one more testable effect to add to this GR toolbox. He was discussing a property of photons called orbital angular momentum (OAM). This is distinct from the more familiar intrinsic spin angular momentum of the photon, which is related to the circular polarization of light.

Interesting effects

A straightforward way to detect the OAM of photons in the lab had just been discovered in 2002 and, according to Harwit, there could be several astrophysical applications of OAM, including the investigation of spinning black holes. “A full theoretical investigation of such effects would be of interest,” he concluded. Now, in a paper published today in Nature Physics, a group of physicists led by Bo Thidé of the Swedish Institute of Space Physics in Uppsala have done just that.

The team performed numerical calculations of light passing spinning black holes, which are thought to account for most of the black holes in the cosmos. Around these ultra-dense objects, space–time becomes twisted in an effect called frame dragging. When light enters this region, say Thidé and colleagues, its normally flat wavefronts become twisted too, taking on a corkscrew shape and a change in OAM. The faster the black hole spins, the greater the change in OAM, say the researchers.

“This is a nice, and seemingly sound, piece of theoretical analysis couched in the framework of modern optical theory,” said Gary Gibbons, a theorist specializing in general relativity at Cambridge University. Marcus Werner at Duke University commented “This could be rather significant, since it would open up an entirely new observational method.”

Tantalizing possibility

To test the researchers’ prediction, astrophysicists would need to examine the phase of photons using radio telescopes such as the Very Long Baseline Array at Socorro in New Mexico, US. If the prediction is borne out in measurements, general relativity would be further reinforced as a theory. If it isn’t – a remote yet tantalizing possibility – there is the chance general relativity is not telling the whole story about space–time.

Martin Bojowald, at Pennsylvania State University, suggests the possibility that the OAM prediction could allow the direct detection of spinning black holes – a feat never accomplished despite widespread acceptance of their existence. Although nominally “black”, black holes are thought to emit a haze of photons called Hawking radiation, but this is so faint it is currently impossible to see over the universe’s background radiation. However, Bojowald believes the change in OAM might one day be just enough of a signature to filter out Hawking radiation for observation.

“New calculations of the quantum processes that generate Hawking radiation are required, but before one can address that, the twisting of light already opens the way to exciting new possibilities in black-hole physics,” he says.

The work is reported in Nature Physics DOI:10.1038/NPHYS1907.

Ultracold atoms could reveal the dark sector

Insights into dark energy and the expansion of the universe could come from an earthbound experiment involving ultracold atoms. That’s the claim of a team of physicists in the US and UK led by the Nobel laureate Martin Perl, which plans to drop caesium atoms through 1.5 m along two atom interferometers. Any “dark sector” forces on the atoms will result in a phase difference that the team hopes to measure by comparing results from the two devices.

Dark energy is the name given to the mysterious substance that is believed to be causing the expansion of the universe to accelerate by exerting a negative pressure that opposes the pull of gravity. But although dark energy appears to account for about 75% of the energy-mass content of the universe, scientists have no real idea what it is. Most attempts at understanding dark energy involve observations of distant supernovae and the cosmic microwave background.

But Perl and colleagues believe that we can glean new insights into dark energy from an atom-interferometer experiment performed in a laboratory here on Earth. The team hopes to detect any hitherto unknown “dark content of the vacuum” (DCV), which, they argue, could behave like dark energy on cosmological time and distance scales. The experiment, which is described in a paper on the arXiv preprint server, should be up and running by 2014.

Different space/time paths

According to Perl’s blueprint, each interferometer works by dropping a bunch of atoms through a 1.5 m tall vacuum chamber. A laser pulse is then fired at the bunch at the top of the chamber, putting each atom into a superposition of two quantum states. A second laser pulse would then be used to recombine the atoms at the bottom and then the interference between the two paths is measured to reveal the phase shift between states that accumulates during the fall.

The researchers plan to run the experiment using two identical interferometers, which will allow them to cancel out the effect of the Earth’s gravitational field. In the absence of any DCV, the phase shifts measured by each device would be the same. However, if the DCV density is different at each interferometer, the phase shifts would be different.

“We are now building the apparatus and expect to have one interferometer operating within two years and the full double spectrometer operating after three years,” Perl told physicsworld.com. “The major experimental problems are the reduction of noise from sources such as mechanical vibration and drifts in photonic components,” said Perl who is professor emeritus at the SLAC National Accelerator Laboratory in California.

According to the team, detection is based on two assumptions. The first is that the DCV exerts a force on matter – and that the force is not gravitational in nature. The second assumption is that the DCV field has a non-uniform spatial distribution. In other words, atoms in the two different interferometers would experience slightly different forces, leading to a small shift in the relative phase of the atoms – which would show up in the interference pattern.

Difficult to connect to dark energy

However, this second assumption contradicts the cosmological-constant model of dark energy, which describes dark energy as a property of empty space that is the same everywhere in the universe.

Indeed, if the experiment does detect hitherto unknown forces, it could be difficult to compare the results to other dark-energy measurements. “Most other dark energy searches (like microwave background analyses) are looking for dark energy on cosmological scales of time and space, whereas we are interested in dark energy density fluctuations on terrestrial scales, here and now,” says Holger Mueller of the University of California, Berkeley who collaborated with Perl on the design of the experiment. “Cosmology does not tell us what to expect on those scales.”

As a result, some cosmologists are sceptical about the experiment’s ability to tell us something about dark energy. “Atom interferometers are an exciting technological advance that will be an important tool in fundamental physics investigations,” says Eric Linder, who is also at Berkeley but is not involved in the work. “However, [cosmologists] have no expectation that they will reveal the cause of cosmic acceleration.”

Robert Caldwell, a cosmologist at Dartmouth College in New Hampshire, told physicsworld.com that would be “fantastic” if Perl’s team discovers some new phenomena in the laboratory. However, he warns that “it may be a hard sell to make the connection a posteriori to dark energy”.

The experiment is described in arXiv: 1101.5626.

Pulling power

By Louise Mayor

Love moves in mysterious ways, and try as we might to find one, there is no formula that will unlock the secrets of how to find and sustain romantic love.

So if you’re looking for foolproof tips on your love life this Valentine’s Day, I’m afraid they don’t exist. But you’ve come to the right place if you fancy watching three physicists from the University of Nottingham explain something even deeper – the four fundamental forces of attraction.

In some introductory comments to this new video, nanoscientist Philip Moriarty explains, “Generally, forces of attraction, when we’re talking about Valentine’s Day, mean people falling in love and what holds them together…Of course, I’m a physicist so I have to look into it a little bit more deeply than that.”

Ed Copeland, who is a particle cosmologist by trade, explains how three of the forces – the strong, weak and electromagnetic – can be beautifully described by quantum chromodynamics, or QCD. But he adds that gravity’s proving most difficult: “That remains the goal – to try and also bring gravity into this big picture.”

While the video might not explain the forces of love, it’s quite instructional if you fancy your chances at impressing the object of your affections with some good old-fashioned geek chic. Moriarty and Copeland, as well as Roger Bowley, are charmingly enthusiastic.

You can find plenty more videos from them, as well as other Nottingham University scientists, on the website Sixty Symbols.

Waltzing exoplanets

By Hamish Johnston

This visualization of exoplanet candidates is currently all the rage on the physics blogs. Exoplanets are planets that orbit stars other than the Sun, and those illustrated above are 1236 candidates that have been identified by NASA’s Kepler mission, which launched in 2009.

Further observations are required to confirm that all of these are actually exoplanets, but that hasn’t stopped Jer Thorp from creating a beautiful way of visualizing the Kepler data. Thorp is a Canadian artist and educator who describes himself as “a former geneticist, [whose] digital art practice explores the many-folded boundaries between science and art”.

To illustrate the relative size, temperature and orbit of the candidates, Thorp has all the exoplanets orbiting a single star – something they don’t really do.

Earth an Mercury are also included, making it crystal clear that astronomers have thus far only managed to discover planets larger than Earth. All of the candidates save one are in orbits tighter than Earth’s. The bias towards large planets in tight orbits probably doesn’t reflect all exoplanets in the universe, but occurs because such “hot Jupiters” are easier to spot using existing telescopes than are Earth-like planets.

Unfortunately there is little commentary to go along with the visualization, but it looks like Thorp explores several different ways of displaying the planets according to their size, orbit, orbital period and temperature. Not surprisingly, red is used to identify the hottest planets and blue is the relatively cool temperature of the Earth.

One thing Thorp does say is, “Two candidates are highlighted: KOI 326.01 and KOI 314.02. Out of all the candidates, those two may have the best chances of satisfying some of the ‘habitability’ criteria astronomers tend to use.”

While orbitals are a natural way to illustrate exoplanets, the concept can also be used to visualize other data – including the agricultural economics. Above is the “World Bank Orbital Comparison” of agriculture in the nations of the world. This work was done by James Grant, who is a digital art and design student in Tempe, Arizona.

Both visualizations were made using Processing, which is an open-source programming language.

The edge of physics: dispatches from the frontiers of cosmology

Physicists are searching for answers to some profound questions. What is the nature of dark matter and dark energy? What is the origin of mass? What happened to primordial antimatter? In this lecture, Anil Ananthaswamy talks about some of the telescopes and experiments at the cutting-edge of cosmology today, operating in some of the most extreme environments on Earth, including Lake Baikal in Siberia, an underground mine in northern Minnesota, the Atacama Desert in the Chilean Andes and even Antarctica.

Date: Thursday 10 February 2011

Speaker: Anil Ananthaswamy, consultant, New Scientist magazine
Anil Ananthaswamy is a consultant for New Scientist magazine. A former software engineer, he trained as a journalist at the University of California, Santa Cruz. Since 2000 he has worked for New Scientist in various capacities, including as physics news editor and deputy news editor. He is also the author of physicsworld.com‘s Book of the Year 2010: The Edge of Physics: Dispatches from the Frontiers of Cosmology.

Moderator: Margaret Harris, reviews and careers editor, Physics World

The video runs for approximately 60 minutes.

New nanostructure combines memory and logic elements

A new way to make complete and programmable logic circuits that combine both memory and logical processing in a single structure has been unveiled by physicists in Germany. Such structures could lead to smaller, faster and more energy-efficient integrated circuits and their development has been one of the main goals in spintronics – a relatively new technology that exploits the spin of an electron as well as its charge. The researchers believe that the structure could lead to a new way of designing logic circuits in which the memory and logic elements are no longer separated.

“Single spintronics devices have been demonstrated for quite a while now but we have demonstrated a complete and programmable logic circuit – and that’s new,” said team leader Laurens Molenkamp at the University of Würzburg. The new circuit is made of a ferromagnetic semiconductor containing a mix of gallium, manganese and arsenic ((Ga,Mn)As) that combines read/write capability with a logical process.

Spintronics devices exploit the fact that the spin of an electron can point in an “up” or “down” direction and switching between these two directions is a property that could be used to store and process information. Such circuits would be smaller and more efficient than conventional silicon-based electronic circuits – which rely on switching charge alone – because, in principle, switching spins from up to down can be done using very little energy. Spintronics devices would carry or manipulate information via a “spin current”, which consists of electrons with opposite spins moving in opposite directions.

Strain relaxation

When deposited as a 2D layer, (Ga,Mn)As is most easily magnetized along one of two directions – both in the plane of the layer and at right angles to each other. The Würzburg researchers had already discovered that if they patterned such layers into narrow wires, a phenomenon known as “strain relaxation” causes these wires to have only a single magnetization axis – along the length of the wire.

Now, the team has observed that in a structure where such a wire contacts a wider area of (Ga,Mn)As, passing an electrical current through the wire causes the wide (Ga,Mn)As area to become magnetized in the same direction as the wire. Attaching wires from north, south, east and west directions to the wide area can transform the structure into a simple logic gate.

Polarized spin current (a charge current with different numbers of up spin and down spin electrons) from a north–south or east–west bar magnetizes (or “writes”) the central wide area in either a “0” or “1” state respectively. The device can then be read by measuring the resistance across a tunnel junction connecting a bar and the wide area. This resistance is higher for 1 than for 0 and combining two such “1-bit” devices will allow the researchers to build complete XOR logic gates – something that the team is busy working on at this very moment.

Programmable gate

“The other nice thing is that the actual mode of operation of the gate is programmable – it depends on what wires are actually used for imputing the logic bits and how they are initialized,” Molenkamp told physicsworld.com. For example, the logic operation performed by the device depends on whether the north–south or east–west oriented bars are defined as 1 or 0 in each of the two bits of the element. “In this sense our circuit functions as ‘field programmable gate areas’ in the present integrated circuit (IC) technology.”

In conventional computers, the output of a logic gate is short-lived and so it has to be stored in a memory (such as RAM) that lies in another part of the computer. Since the wide areas in Molenkamp and colleagues’ structures are made of a ferromagnetic material, the logic state (or magnetization) is much longer lasting. It therefore does not need to be transferred elsewhere but can be exploited straight away.

The layout could thus lead to a fundamentally new information-processing paradigm Laurens Molenkamp, University of Würzburg

“Our circuit unites logic and memory functionality in a single, very compact, layout and makes the standard procedure of separating logic and memory redundant,” explained Molenkamp. “The layout could thus lead to a fundamentally new information-processing paradigm,” he added, “and I hope that our work will help convince the electronics industry of the potential of implementing spintronics in next-generation information processing systems.”

That said, there is a downside for (Ga,Mn)As itself – the material is only ferromagnetic at temperatures below around 150 K. However, the concept presented by the Würzburg team should apply to other common ferromagnetic materials that are magnetic at room temperature. Now, that is good news.

The circuit is described in Phys. Rev. Lett. 106 057204.

Future of cosmology looks bright in a dark universe

Cosmologists can rejoice: they’ll still be able to do their jobs a trillion (1012) years from now – even after the universe’s expansion has pushed nearly all galaxies out of sight. That’s the conclusion of an astronomer in the US, who argues that the giant black hole at the centre of our galaxy will eject stars into the void beyond, providing objects that future cosmologists can use to trace the universe’s expansion.

Ever since the late 1990s, when astronomers used supernova explosions in distant galaxies to discover that the universe’s expansion is accelerating, the far future of cosmology has seemed bleak. Within roughly 100 billion years, nearly all other galaxies will be so distant that their light won’t reach us. As a result, future observers won’t know that the universe is expanding. Furthermore, the cosmic microwave background – which is the Big Bang’s afterglow and a key clue to the universe’s origin – will be attenuated below the threshold at which it can be detected.

Standard scenario is ‘wrong’

In October 2010 Abraham Loeb gave a public talk at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, in which he recounted these difficulties. “People were very intrigued and came to me afterwards,” says Loeb, who is professor of astronomy at Harvard University. “Someone said, `Why don’t you write an article about that?’ And I said, `Well, I’ll think about it,’ and then when I thought about it I realized that it’s wrong: there will be a way in the distant future to verify the standard cosmological scenario that we now have.”

The key to Loeb’s proposal are so-called hypervelocity stars. In 1988 Jack Hills, then at Los Alamos National Laboratory in New Mexico, said that if a double star skirted close to the giant black hole at the Milky Way’s centre, one star in the pair could fall toward the black hole. This star would lose an enormous amount of energy, and by the principle of conservation of energy its mate would gain enormous energy and fly away at high speed.

‘Hypervelocity stars will save the day’

Then, in 2005, Warren Brown of the Harvard-Smithsonian Center for Astrophysics and his colleagues announced the discovery of the first hypervelocity star. Astronomers have since spotted more than a dozen others. “These hypervelocity stars will save the day,” says Loeb. That’s because, even a trillion years from now, the black hole at the galaxy’s centre will still be ejecting stars. These stars will probably be red dwarfs, dim suns that can live for trillions of years. By tracking the motions of these stars after they leave the galaxy, astronomers can deduce the universe’s expansion.

Long before then, Loeb expects the Milky Way to merge with the Andromeda Galaxy, 2.5 million light-years away, producing an even greater galaxy he calls “Milkomeda”. As a hypervelocity star leaves Milkomeda, the galaxy’s gravity first slows it down. But eventually the accelerating universe speeds the star up. “By monitoring the motion of these stars, a future cosmologist could infer the existence of the cosmological constant,” explained Loeb. The cosmological constant represents the repulsive force of empty space; it’s what’s causing the cosmic expansion to accelerate.

The more mass Milkomeda has, the farther is the distance at which the universe’s expansion will manifest itself. If Milkomeda has 2 trillion times the mass of the Sun, Loeb calculates this transition distance occurs 4.4 million light-years from us. If instead Milkomeda has 10 trillion solar masses, then this transition distance is 7.5 million light-years.

Not bright enough?

However, Lawrence Krauss at Arizona State University in Tempe, who had earlier considered the potential of hypervelocity stars, is skeptical. “Right now, with supernovae – the brightest objects in the universe – we’re barely able to discover the existence of a cosmological constant,” says Krauss. “To propose that you can use individual stars to measure the expansion of the universe – it may be barely physically possible, but is it likely?” Krauss thinks not, that a civilization a trillion years from now would probably consider Milkomeda and its satellite galaxies to be the entire universe, surrounded by a static void, and would have no incentive to spend vast sums of money tracking subtle changes in the velocities of a few dim and distant stars.

Loeb counters that these stars will be a thousand times closer than today’s farthest supernovae and that a civilization will have a trillion years to develop a giant telescope to study the escaping stars.

Loeb wrote his paper in January, after a major snowstorm hit New England. “It allowed me not to be interrupted, not to be distracted,” he says. He has since submitted his work to Physical Review D. A preprint is available on arXiv.

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