Skip to main content

Reset your password

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

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

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

Registration complete

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

Close
Home » Page 1297

Do dark-matter discs envelop galaxies?

 

A new type of dark matter that could strongly interact with regular matter to form large discs that would overlap galaxies like our own has been postulated by a group of researchers in the US. There is believed to be at least four times as much dark matter in the universe as there is ordinary matter. But despite its great abundance, dark matter is generally thought to very weakly interact with conventional matter, causing it to form amorphous halos around galaxies that contrast with the richly structured galactic discs themselves. The new research suggests this view may be oversimplified, arguing that a substantial minority of dark matter might in fact interact strongly, and could be detected in cosmic-ray observations.

Dark complex

Much evidence has been accumulated to support the existence of dark matter, which, unlike normal matter, does not give off or absorb electromagnetic radiation. For example, the greater-than-expected rotational speeds of stars in the outer-lying regions of galaxies suggest that those galaxies contain more mass than can be accounted for simply by adding up all of the light.

However, scientists still do not know what dark matter actually is. They do know that much of dark matter interacts weakly with other matter and with itself. Among theorists’ leading candidates for dark matter are so-called weakly interacting massive particles (WIMPs) and axions, which rarely collide with one another. The existence of these particles is also suggested from work in other areas of physics – WIMPs being predicted by some forms of supersymmetry, while axions might explain why strong interactions obey charge–parity symmetry.

More than meets the eye

In the latest work, Lisa Randall and colleagues at Harvard University argue that such weakly interacting particles might not tell us the whole story. By considering the characteristics of the dark matter surrounding our own Milky Way galaxy, the researchers calculate that as much as 5% of that dark matter might not be weakly interacting. They also point out that this “double-disc dark matter” (DDDM), as they call it, would probably dissipate energy while retaining angular momentum from its motion about the galactic centre, causing it to form a thin disc just as ordinary galactic matter does. They work out that the dark and visible discs would have about the same mass, which would imply that the densities of DDDM and normal matter in the universe would be roughly equal.

“Our model isn’t proposed to solve any particular problems,” says group member Matthew Reece. “But we think it’s important to consider a wider range of possibilities for what dark matter might be. We are lucky to be living in a data-rich era, and we want to be sure that we’re not overlooking a dramatic discovery in all that data.”

According to the researchers, DDDM discs would contain the dark-matter equivalent of protons and electrons interacting via an analogue of electromagnetism, so creating dark atoms. The minimalistic model that they have considered in the current work, however, does not include analogues of the nuclear forces, so they have not predicted the DDDM spawning stars, as such. Reece says that he and his colleagues limited themselves to this simple model because it was relatively easy to analyse and work out how it could be tested, but explains that a more complex model, incorporating nuclear physics, could in principle be developed. In fact, he adds, “It doesn’t seem completely out of the question that there could be forms of life made of dark matter,” although he underlines that this speculative idea “isn’t very scientific since we don’t know a good way to test it”.

Detecting dark discs

The dark disc, on the other hand, might be detectable in the near future. Reece explains that evidence for its existence could come from the gravitational effect it has on the motion of the billion Milky Way stars that the European Space Agency’s upcoming Gaia mission will study. Alternatively, he and his colleagues write, annihilating DDDM particles would, given enough sensitivity, produce “strikingly different” signals from those of the ordinary dark matter that the space-based PAMELA, Fermi and AMS-02 detectors are looking out for.

Direct detection, however, would be more difficult. Physicists have built a number of experiments underground that look for any interaction between putative particles of ordinary dark matter and the material in the detectors. But as the Harvard team points out, these detectors would struggle to observe the DDDM variant since the dark disc that is home to this exotic material might not be aligned with our visible one. In addition, even if the two discs were aligned, their relative velocity would probably be too low to generate suitably energetic collisions within the detectors, the team says.

Daniel Hooper, an astrophysicist at Fermilab in the US, says that the Harvard group has put together “an interesting hypothesis” that he predicts “will receive a great deal of interest from many of us working on dark matter research”.

Meanwhile, Roberto Battiston, a physicist at the University of Trento in Italy and deputy spokesman of AMS-02, says that the peculiarly disc-shaped distribution of DDDM might be identifiable with his group’s experiment, if such a substance exists. But he cautions that there could be many competing theories to explain any dark-matter-like signal collected by the detector.

The research is published in Physical Review Letters.

Condensed-matter cosmology and spin wires

Waterloo, here I come (Courtesy: IQC)
Waterloo, here I come. (Courtesy: IQC)

By Hamish Johnston at the 2013 CAP Congress in Montreal

Yesterday morning I was back at the University of Montreal for more physics at the Canadian Association of Physicists Congress. I started off the morning with a bit of quantum cosmology and quantum gravity with a distinct hint of condensed-matter physics.

(more…)

Between the lines

Numbers
Sexed-up stats: Charles Wheelan’s new book aims to enliven a subject often considered deadly dull. (Courtesy: iStockphoto/Viorika Prikhodko)

Statistically speaking

Particle physicists use statistics to distinguish new particles from meaningless blips. Climate physicists need it to turn incomplete data into robust models. And pretty much everyone in science wheels it out to calculate error bars. Despite its usefulness, however, the field of statistics has a reputation for being dull and confusing; as a former classmate of your reviewer’s put it, “If I had one day left to live, then I would live it in stats class, because that way, it would last longer.” In his book Naked Statistics: Stripping the Dread From the Data, author Charles Wheelan concedes that his subject could do with a bit of a boost. The statistical sexing-up begins with the book’s front cover (which depicts a cartoon character holding a suggestive and strategically placed bar graph) and continues with the kind of writing that made Wheelan’s previous book, Naked Economics, into a bestseller despite its “dismal science” subject. In addition to clarity, Wheelan’s favourite tactics for keeping the reader hooked include a nice line in silly examples and a ready spoonful of wit to make the statistical medicine go down. In the second chapter, for example, he tells a story about a bar patronized by Bill Gates, a talking parrot and 10 low-paid workers. The point of this anecdote is that mean values can be distorted by outliers; when Gates walks into the bar with the parrot on his shoulder, the average annual income of the patrons leaps from $35,000 to about $91m. (The parrot, Wheelan writes, “has nothing to do with the example, but it kind of spices things up”.) He keeps the tone light even as the book moves on to more complex topics, such as the “garbage in, garbage out” problem of doing statistics with bad data and the various biases that can lead people to form erroneous conclusions. At the heart of the book is the central limit theorem, a pillar of statistics that, crudely speaking, states that a random sample of a given population will resemble the population as a whole. That might not sound terribly earth-shattering, but Wheelan shows that if properly applied, the central limit theorem can do some amazingly useful things – such as detecting when someone’s been fiddling experimental data. The theorem also allows us to infer that while most physicists won’t learn much from the first few chapters of Naked Statistics, nearly everyone will get something out of it by the end – a pretty good result, whichever way you slice it.

  • 2013 W W Norton/Wiley £18.99/$26.95hb 320pp

A classy bunch of particles

Following the success of Brian Greene’s The Elegant Universe, the word “elegant” has been somewhat over-used in popular-science writing. However, there is no other word to describe Jeremy Bernstein‘s A Palette of Particles: this is an elegant book, elegant in its writing and in its packaging, the sort of book that ought to be read with freshly washed hands and a glass of something cold and sophisticated. Its subject, broadly speaking, is the history of particle physics, and it is organized into three sections. The first of these deals with the electron, proton, neutron and neutrino, which Bernstein describes as the “primary colours” on nature’s palette. Next up are more exotic particles such as the pion and muon, antiparticles, strange particles and quarks; these are the “secondary colours”. The final section is devoted to nature’s “pastels”, beginning with the relatively vibrant Higgs boson and shading away to shadowy objects such as tachyons and gravitons. Bernstein is both a physicist himself and a veteran observer of other physicists; in many cases, he met or was personally acquainted with the people he describes, including Wolfgang Pauli, Emilio Segré and Sheldon Glashow. Perhaps as a result of this familiarity, the book sometimes reads as though it was written not for its stated audience (“a general reader with an interest in science”), but for insiders who, like the author, are already familiar with the main characters and their histories. It is, if you like, a bedtime story for quantum connoisseurs rather than a bracing wake-up call for newcomers – and like the best bedtime stories, it offers plenty for readers to think about. In Bernstein’s view, “nature [is] something like those Russian dolls, except in this case I think there may be no limit. The deeper we probe, the more will be left to probe.”

  • 2013 Harvard University Press £14.95/$18.95hb 224pp

The race to find the electric dipole moment

The University of Montreal actually has an ivory tower!
Congress HQ: the University of Montreal actually has an ivory tower!

By Hamish Johnston at the 2013 CAP Congress in Montreal

Yesterday I had lunch with Jeff Martin of the University of Winnipeg, who is a member of an international team that aims to measure the electric dipole moment (EDM) of the neutron at TRIUMF in Vancouver.

(more…)

Physicists design acoustically invisible walls

A rigid wall can be transformed from a total reflector of sound to an almost perfect transmitter by perforating it with tiny, regularly spaced holes covered by a thin elastic membrane, say researchers in Japan and South Korea. The discovery, an acoustic analogue to extraordinary optical transmission (EOT), could potentially be used in microscopes, noise filters, new types of windows, acoustic concentrators and many other applications.

EOT was discovered by Thomas Ebbesen of the University of Strasbourg and colleagues in 1998. It allows electromagnetic waves to pass almost unhindered through a lattice of sub-wavelength holes in a barrier that would otherwise be opaque in some metamaterials. In Ebbesen’s discovery, this was down to coupling between photons and electrons on the surface of the barrier.

Perfect transmissions

Following Ebbesen’s discovery, in 2006 a different team of researchers led by Nader Engheta at the University of Pennsylvania in Philadelphia discovered another mechanism that can lead to EOT. If the holes contain a material with a refractive index close to zero, the wavelength in the holes becomes extremely long, and thus the velocity becomes extremely large. The faster a wave travels, the more energy it carries, allowing the energy of the entire wavefront to squeeze through the tiny holes. Such materials are called epsilon-near-zero (ENZ) materials because the refractive index of a material depends on its permittivity – written as epsilon (ε) – a kind of electrical inertia that is representative of the resistance that is encountered when forming an electric field in a medium. An ENZ material offers almost no resistance to such displacement.

Same for sound

Now, Sam Lee of Yonsei University in Seoul, South Korea, Oliver Wright of Hokkaido University, Japan, and colleagues have produced an analogue of Engheta’s metamaterial for sound waves. Just as electromagnetic waves propagate as vibrations in a material’s electromagnetic field, sound waves travel as physical oscillations of the atoms. Sound waves cannot pass through a rigid barrier because the atoms cannot oscillate. Making tiny holes in the barrier will barely increase transmission. Lee explains that if, for example, the holes make up 3% of the volume of the barrier, “to ensure continuous volume flow across the barrier, the air in the holes has to move 30 times faster than the air outside the wall. The inertia of the air does not allow for the huge accelerations needed for motion of such amplitude.” To solve this problem, the researchers needed the air in the holes to have almost zero inertia – the acoustic equivalent of an ENZ material.

They achieved this, paradoxically, by covering the holes with a thin membrane of shop-bought kitchen cling film. With the tension tuned so that the membrane’s resonant frequency is the same as the frequency of the incident waves, the membrane’s resonance amplifies its oscillations. The resonance moves the air through the holes as though the air has no inertia, allowing it to move in response to even a small displacement and sucking almost all the energy of the incident waves through the barrier.

Invisible barriers

On the other side of the barrier, Huygens’ principle dictates that each hole produces spherical wavefronts. The separation between the holes is much less than the wavelength of the sound, which means the interference pattern of the waves reconstructs the plane wave in much less than one wavelength and the barrier is effectively invisible to the propagating waves.

The researchers tested their acoustic-metamaterial design by placing an acrylic barrier perforated with four small holes in a tube. Loudspeakers producing waves of a single frequency were placed at one end and the researchers measured the intensity of the waves on either side of the barrier. The results were remarkable – they found, for example, that with the barrier perforated by bare holes, only 9% of the waves’ energy was transmitted. With a membrane placed over the holes, this proportion jumped to 81%. The metamaterial worked just as well when placed at an oblique angle to the incident wavefronts.

Ebbese, who was not involved in this work, views the result as a significant contribution to acoustics with numerous potential uses. “They’re talking about transmission, but in the same way you can also block waves at other frequencies by adding this membrane,” he says, listing noise filters as one application where this could be useful.

Novel lenses

According to the team, its method can be used over a range of frequencies such that it would work equally well for ultrasound. This, the researchers say, could be exploited to concentrate acoustic energy through tiny holes, forming novel lenses. They are currently working on the potential application of the idea to near-field scanning acoustic microscopy, where the properties of an object are studied by the sound waves it reflects. The researchers believe the concentration of the radiation’s energy into tiny holes during the process of transmission could allow them to achieve both very high signal intensity and spatial resolution.

The research is to be published in Physical Review Letters.

The June 2013 issue of Physics World is now out

Physics World June 2013.

By Matin Durrani

As physics has grown into a bigger, increasingly global and more connected endeavour, are there still any true physics hot spots? Are there any institutes, universities or regions that really are “the place to be”? Does good physics, in other words, depend more on who (or what) you know than where you are?

The importance of having the right people in the right location is well illustrated in this month’s issue of Physics World, in which science writer Brian Clegg looks at the role played by Manchester in the development by Niels Bohr of his model of the atomic nucleus 100 years ago.

What drew Bohr there were not so much the facilities at the University of Manchester’s physics department but rather its working environment and in particular the presence of the New Zealander Ernest Rutherford, with whom Bohr struck up a great rapport.

Our cover story this month concerns attempts to extract carbon dioxide from the air in the fight against climate change, while elsewhere in the issue we look at all the cool – and pretty fundamental – things you can do with ultracold neutrons.

(more…)

What happened to nuclear electrons?

By Hamish Johnston at the 2013 CAP Congress in Montreal

Sometimes I think that physicists can dwell too much in the past. Scientific papers, for example, often begin with a potted history of the field and it’s only in the second page that something new is mentioned.

(more…)

The first medical X-ray…

By Hamish Johnston at the 2013 CAP Congress in Montreal

Jean Barrette has one of the best jobs in the world as far as I am concerned. The retired nuclear physicist is curator of the McPherson Collection of physics instruments at McGill University here in Montreal.

Jean Barrette has a great job.

This morning in the “History of Physics” session at the Canadian Association of Physicists (CAP) Congress, Jean gave a talk that featured many of the beautiful experiments – lots of brass and polished hardwood – in the collection.

The collection was made possible by the Canadian physicist Anna McPherson, who left a sizeable sum to the university when she died in 1979.

One of the highlights of the talk was what is surely the first-ever medical X-ray, which was taken in 1896 just six months after X-rays were first discovered. Taken at McGill, it shows a bullet lodged in the leg of a shooting victim.

During his talk, Jean asked for help in identifying a mysterious piece of apparatus in the collection that so far he had not been able to identify. Jean is going to send me a picture and I’ll post it in an upcoming blog entry.

Is creativity as important in science as it is in art?

By James Dacey

Science-inspired art
“Parity Series, Far Infrared”. (Courtesy: Mehri Imani, Central Saint Martins)

The worlds of art and science came together yesterday in central London in a celebration of creativity across disciplines. A symposium at Central Saint Martins College of Art and Design was held to recognize the first group of students to complete the Art and Science MA course – the first course of its kind in the UK. Students taking this course are given the chance to explore the “creative relationships between art and science and how to communicate them”.

(more…)

Physicists rethink celebrated Kelvin wake pattern for ships

Lord Kelvin may have been an accomplished sailor, but he might have missed a trick when he famously described the phenomenon of wakes fanning out at a constant angle of 19.47°, no matter the speed of the vessel. That is the claim of two French physicists, who have used satellite images and mathematical modelling to study narrower wakes associated with fast-moving boats.

Kelvin’s prediction is rooted in two key properties of gravity waves on the water surface: first, that those with large wavelengths travel faster than those with short wavelengths; and second, that the group velocity of a deep-water wave is exactly half its phase velocity. As a boat moves through calm water, it excites waves over a range of wavelengths, with the longest speeding away faster than the shortest and then dissipating. Constructive interference between the slower, shorter waves causes a pair of shock waves to form in a distinctive V-shape that emanates from the boat.

Kelvin showed that the angle that each arm of the V makes with the centre line is 19.47°, irrespective of how fast the boat is travelling. It is a universal pattern that even holds true for a duck traversing a pond.

But when Marc Rabaud of University of Paris-Sud in Orsay tried to demonstrate the phenomenon to a group of physics teachers at a local pool, he was stumped as to why his set-up produced much narrower wakes. A decade later, still puzzled, Rabaud and colleague Frédéric Moisy noticed that the photographs they used to illustrate Kelvin wakes to fluid-mechanics students were at odds with the theory they were trying to teach.

Kelvin or Mach angle?

Narrow wakes had been spotted before, of course, but scholars had come up with various ways of rationalizing their existence: the effects of shallow waters; superposing wave patterns; turbulence; and nonlinear interactions, to name a few. “But if you look at the pictures that apparently don’t work – they’re actually the most interesting ones because they really tell you something new,” says Moisy. He and Rabaud analysed images from Google Earth, using measurements of boats’ hull lengths and wake angles, as well as calculations of their velocities, to help them build a new mathematical model to describe narrow wakes.

What the researchers found was that at higher speeds, boats produce a smaller spectrum of wavelengths that tends towards the length of the boat itself. According to the duo, a boat cannot produce wavelengths longer than its hull, so as soon as the wake hits a limiting speed governed by this length, its waves all travel with equal speed through the water, much like sound waves through air. At this point, like the Mach cone associated with supersonic jets, the angle of the wake is suddenly governed solely by the speed of the boat – the faster it goes, the more its wake stretches and narrows.

Rabaud and Moisy’s numerical simulations were in strong agreement with their image analyses, which together allowed them to identify a Froude number (a relation between boat velocity and hull length) of roughly 0.5 as the transition point between the Kelvin and Mach regimes.

Reservations raised

Others in the field are sceptical. Yuming Liu, a marine hydrodynamicist at the Massachusetts Institute of Technology, has several misgivings about the work, including the fact that the numerical simulations consider only the “near field” region directly behind the ship, rather than the whole area over a distance of several wavelengths, which he says biases the results. Not only that, he is troubled by the researchers’ argument that a ship cannot excite waves longer than the hull. “You just can’t make an assumption like that,” he cautions.

Tarmo Soomere, a mathematician and wave expert at Tallinn University of Technology, agrees, saying that “the devil here could be the assumption” and worries – along with Liu – that the model has overlooked finite-depth effects that are well known to have a strong effect on wake angle. Rabaud and Moisy concede that most of their images necessarily come from shallower waters within a kilometre of shore, since Google Earth‘s resolution drops off sharply away from land. Yet they rule out a depth effect on wake angle within their data. “If we plot our data as a function of water depth, there is just a scatter of points with no correlation at all,” says Moisy.

Soomere cautions that photographic images can be deceptive, highlighting the steepest but not necessarily the highest wave components. “A much simpler explanation of their results is that a vessel wake consists of several components of different nature, and you never know which one of these is actually visible in an image,” he explains. “I deeply believe that the researchers have found a simple model for one of these components though…and this is truly fascinating.”

The research is published in Physical Review Letters.

Copyright © 2026 by IOP Publishing Ltd and individual contributors